THE ECOLOGY AND CONTROL OF RICE PESTS
vV l t o the Intensity and Synchrony of Cultivation
by Michael Eliezer Loevinsohn, B.Sc. August 1984.
A thesis submitted for the degree of Doctor of Philosophy of the University of London.
Centre for Environmental Technology, Imperial College, London SW 7.
1 ABSTRACT
The population biology of pests under intensifying Asian rice cultivation is investigated in theoretical and experimental terms and the insights gained applied to questions of control.
The response of specialist pest populations to changes in the duration, extent and synchrony of cultivation is first investigated in a model. Predictions are then tested, relying in particular on new methods for the analysis of seasonally fluctuating light trap data:
(i) From historical records, the extension of double cropping under irrigation is shown to have been central to the recent increase in pest densities. The key dynamic feature of several species is density independent carryover between seasons coupled with undercompensating regulation within seasons.
(ii) In an area of intensive cultivation, the additional time made available by asynchrony between farms is shown ♦ to result in increased populations of most pests. The response is conditioned by generation length and dispersal range.
Intensity and asynchrony are then examined as selective forces moulding pest life histories. Variation in several vital parameters, notably longevity, is found among stemborers originating from different cultivation regimes. It is concluded that (i) a more sophisticated environmental classification than assumed by current theories is required to account for the results and (ii)
2 the evolutionary response of pests is unlikely to be sufficient to undermine efforts to synchronise cultivation.
The origins of asynchrony and the possibility of reducing it are investigated in Nueva Ecija province, Philippines. Techniques adapted from operations research show that farmers in the normal course of events act so as to minimize asynchrony. However, inputs not under their control, notably irrigation, cause significant delay. After one year, a synchronous planting proposal gained farmers' active support. Success was limited by acrimony between them and public institutions, while the interests of landless workers were not effectively represented. The prospects and organizational requirements for future such schemes will depend on social, economic and administrative conditions. However, some pest losses may be reduced at negligible cost by introducing synchrony considerations into, in particular, routine irrigation management.
3 To Mrs Kulisap
and her neighbours
in Nueva Ecija :
... sa tao ang gawa".
4 TABLE OF CONTENTS
page
List of Figures ...... 8
List of Tables ...... 12
List of Abbreviations ...... 14
Chapter I Introduction ...... 15
1.1 The Intensification Crisis .... 15 1.2 Pest Control After the Crisis ... 24 1.3 The Scientific Problem ...... 29
Chapter II Limitation by Time: A Model and its Predictions ...... 38
2.1 Introduction ...... 38 2.2 The Biological Context ...... 39 2.3 The Structure of a Relevant Model ...... 44 2.4 The Basic Model ...... 50 2.5 Models of Passive Dispersal .... 55 2.6 Insect Abundance and the Locality ...... 58 2.7 Increasing the Extent of Cultivation ...... 63
5 2.8 Increasing the Number of Crops ...... 68 2.9 The Impact of Asynchrony ...... 76 2.10 Changing Maturity of Varieties ...... 86 2.11 Conclusions ...... 87
Chapter III Population Dynamics Under Intensification ...... 90
3.1 Introduction ...... 90 3.2 Intensification at One Site .... 90 3.3 Comparisons Between Sites ..... 117 3.4 Discussions and Conclusions .... 140
Chapter IV The Ecology of Asynchrony in Nueva Ecija ...... 144
4.1 The Origins of Intensive Cultivation ...... 145 4.2 The Study Area ...... 151 4.3 The Sequence of Cultivation .... 154 4.4 The Components of Asynchrony ... 160 4.5 The Irrigation System and Asynchrony ...... 185 4.6 Asynchrony as a Function of Distance ...... 188 4.7 Variation of Intensity ...... 199
Chapter V Population Dynamics Under Asynchrony ...... 201
5.1 Introduction ...... 201
6 5.2 Pest Damage in Relation to Delay ...... 201 5.3 The Kerosene Light Trap Network ...... 214 5.4 Variation among Traps ...... 218 5.5 Trap Catches and Pest Damage ... 220 5.6 Trap Catches and Asynchrony .... 224 5.7 Measuring Dispersal Range Under Natural Conditions ...... 248 5.8 Conclusions ...... 262
Chapter VI Evolution and Agricultural Change ...... 267
6.1 Introduction ...... 267 6.2 Methods ...... 269 6.3 Results ...... 272 6.4 Discussion ...... 284
Chapter VII Implementing Synchronous Planting ...... 292
7.1 Introduction ...... 292 7.2 Synchrony and Scale ...... 293 , 7.3 Possible Consequences of Synchronisation in Nueva Ecija .. 297 7.4 Promoting Synchronous Planting .. 305 7.5 Implications of the Project .... 323 7.6 Implementation on a Wider Scale ...... 330
Chapter VIII Conclusions ...... 337
7 Acknowledgements ...... 342
References ...... 345 LIST OF FIGURES
page
1.1 Rice yields in Indonesia ...... 17 1.2 Changes in pest abundance at IRRI ...... 21 2.1 Density dependence of stemborer mortality ...... 49 2.2 Probability of host location in relation to spatial intensity ...... 62 2.3 Impact of spatial intensification on pest dynamics ...... 65 2.4 Impact of multiple cropping on population carryover ...... 71 3.1 Changes in farming practices in Laguna .. 92 3.2 Double cropping in Laguna ...... 93 3.3 Features of a seasonally fluctuating population ...... 95 3.4 The increase of yellow stemborer at IRRI ...... 98 3.5 Density dependence of YSB growth ...... 100 3.6 Carryover from the dry to the wet season ...... 101 3.7 Carryover between wet seasons ...... 103 3.8 Abundance of stemborers in North Krian .. 107 3.9> Density dependence in N. Krian ...... 108 3.10 Carryover of yellow stemborers in relation to double cropping ...... 110 3.11 The length of the wet season for green leaf hopper ...... 112 3.12 Carryover of GLH in relation to double cropping ...... 113 3.13 Predicted and observed catches of YSB at IRRI ...... 116 3.14 Sites of the Cropping Systems Programme . 119
9 3.15 Pest abundance in relation to cropping intensity ...... 126 3.16 Density dependence of pest increase .... 133 3.17 Density dependence of carryover ...... 136 4.1 Insecticide use in Central Luzon ...... 150 4.2 The study area in Nueva Ecija ...... 153 4.3 Factors influencing the timing of cultivation ...... 155 4.4 Delays along a canal ...... 157 4.5 The components of asynchrony in the dry season ...... 165 4.6 Compensation in cultivation ...... 167 4.7 The components of asynchrony in the wet season ...... 173 4.8 Calculating asynchrony as a function of distance ...... 190 4.9 Asynchrony as a function of distance .... 194 4.10 Variation in asynchrony ...... 195 4.11 An area of chronic asynchrony-...... 196 4.12 Yield in relation to asynchrony ...... 198 5.1 Stemborer damage in relation to delay ... 204 5.2 Caseworm damage in relation to delay .... 206 5.3 Leaffolder damage in relation to delay .. 207 5.4 The significance of delay for caseworm .. 211 5.5 The significance of delay for yellow stemborer 212 5.6 The kerosene light trap ...... 217 A 5.7 Stemborer damage in relation to trap catch ...... 223 5.8 Tungro infection in relation to trap catch ...... 224 5.9 Stemborer abundance in relation to asynchrony...... 227 5.10 BPH abundance in relation to asynchrony...... 227 5.11 The number of YSB males in relation to
10 asynchrony ...... 220 5.12 Asynchrony versus distance for YSB and BPII ...... 229 5.13 Caseworm abundance in relation to asynchrony...... 231 5.14 Nymphula fluctosalis numbers in relation to asynchrony...... 231 5.15 Nephotettix species abundance in relation to asynchrony...... 235 5.16 Leaffolder abundance in relation to asynchrony...... 236 5.17 YSB numbers in relation to asynchrony in the dry season ...... 242 5.18 Green semi-looper numbers in relation to asynchrony...... 242 5.19 BPH catches as a function of distance from a source ...... 250 5.20 Cumulative dispersal of BPH ...... 254 5.21 Catches of 3 pests as a function of distance from a source...... 256 5.22 Cumulative dispersal of 3 pests ...... 259 6.1 Yellow stemborer development times in 2 environments ...... 273 6.2 YSB fecundity in 2 environments ...... 277 6.3 YSB development times in 3 environments . 279 7.1 The probability of typhoons in Nueva Ecija ...... 302 7.2 Posters used in promoting synchronous planting ...... 311 7.3 Irrigation fee payment and yield ...... 326 7.4 Indicative synchronisation plan ...... 332 7.5 An asynchronous schedule in Nueva Ecija . 334 7.6 An asynchronous schedule in Laguna ..... 335
11 LIST OF TABLES
page
1.1 Suggested causes of the BPH outbreaks .. 23 2.1 The major insect pests in the Philippines ...... 40 3.1 Population parameters through intensification - YSB in Laguna ...... 104 3.2 Parameters of cropping intensity at 10 Philippine sites ...... 122 3.3 Yield loss and pest abundance ...... 125 3.4 Pest abundance and parameters of intensity ...... 130 3.5 Density dependence of population increase ...... 135 3.6 Population parameters in relation to cropping intensity ...... 139 4.1 Compensation in cultivation - transplanted fields ...... 169 4.2 Compensation in cultivation - direct seeded fields ...... 171 4.3 Timing of cultivation and method of establishment ...... 172 4.4 Compensation in cultivation - wet season ...... 174 4.5 Timing in relation to source of tractor 175 4.6 Timing in relation to source of labour . 177 4.7 Timing in relation to source of credit . 179 4.8 Timing in relation to drainage difficulties ...... 180 4.9 Timing in relation to irrigation problems ...... 182 4.10 Timing in relation to tractor and credit
12 source - dry season ...... 183 4.11 Variation in planting through an irrigation system ...... 187 5.1 Stemborer damage and delay - analysis of variance ...... 20 5 5.2 Leaffolder damage and delay - analysis of variance ...... 208 5.3 Correlation of catches within sites .... 219 5.4 Equations relating pest abundance and asynchrony - wet season ...... 232 5.5 Response to asynchrony in relation to the rate of increase ...... 239 5.6 Equations relating pest abundance and asynchrony - dry season ...... 243 5.7 The threshold of response to asynchrony . 245 5.8 Comparison of results - wet and dry seasons ...... 246 5.9 Analysis of variance of dispersal experiments ...... 258 5.10 Estimated dispersal distances for 4 pests ...... 261 6.1 YSB development times in 2 environments - analysis of variance ...... 274 6.2 Survival rates of YSB in 2 environments . 276 6.3 YSB fecundity in 2 environments - analysis of variance ...... 278 6.4 YSB development times in 3 environments - analysis of variance ...... 280 6.5 Survival rates of YSB in 3 environments . 282 6.6 Summary of life history studies ...... 282 7.1 Sources of subsistence after a typhoon .. 300 7.2 Awareness of synchronous planting among farmers ...... 321 7.3 Benefits of synchronous planting cited by farmers ...... 322
13 ABBREVIATIONS USED IN THE TEXT
ACES Agency for Community Educational Services BPH Brown planthopper Nilaparvata lugens CW Caseworm Nymphula depunctalis GLH Green leafhopper Nephotettix spp. IRRI International Rice Research Institute LBP Land Bank of the Philippines M-99 Masagana 99 credit program NF A National Food Authority NIA National Irrigation Administration RGS Rice green semi-looper Naranga aenescens RLF Rice leaffolder Cnaphalocrocis medinalis SARF Special Agricultural Rehabilitation Fund SFOP Small Farmer Organization Project UPRIIS Upper Pampanga River Integrated Irrigation System WAT Weeks after transplanting YSB Yellow stemborer Scirpophaga (=Tryporyza) incertulas
14 C H A P T E R I
Introduction
1.1 The Intensification Crisis
Under a range of pressures, demographic, commercial and political, agricultural systems in many parts of the tropics have undergone marked changes in recent years. The term "intensification” has been used in a number of contexts to refer to different aspects of this transformation:
- an increase in the proportion of land devoted to arable cultivation;
- an increase in the number of crops grown per year; and
- an increase in the use of labour or purchased inputs.
Following Geertz (1963) and Ruthenberg (1976), one can distinguish two processes at work under different conditions. "Involution" is said to occur in situations in which agrarian change yields declining returns to labour, as in parts of sub-Saharan Africa, where under population pressure the traditional bush-fallow cycle is progressively shortened and farmers return to the same plot before natural fertility has regenerated (Lagemann et al. 1976, Gleave and White 1969). The classic
15 instance of involution occured in Java, where a restriction of the land available for subsistence rice farming by the expansion of sugarcane cultivation for colonial markets led to a concentration of labour in lowland rice and a hard-won increase in yield per unit of land (Geertz 1963). "Evolution” is said to occur where production increases faster than the input of labour and is the more usual where new resources, of whatever sort, are being brought into use.
Rice cultivation in South and Southeast Asia has within the past 15 years undergone widespread evolution: in several countries, notably China, India, Indonesia and the Philippines production has grown faster than population (USDA 1979). Figure 1.1 illustrates the trend of rice yield in Indonesia, where by 1981, for the first time in many years, more rice was produced than consumed (IRRI 1982). A marked upturn in yields occurred in the region after the release in the mid-1960's and the subsequent rapid adoption by farmers of fertilizer- responsive semi-dwarf varieties first bred at the International Rice Research Institute (IRRI 1975). Increased use of agrochemicals, principally nitrogenous fertilizer and insecticides, accompanied the new varieties and was often encouraged by government- sponsored credit and extension programmes, such as "Masagana 99” in the Philippines and "Bimas" and "Inmas" in Indonesia. Important increases in production have also been brought about by the expansion of irrigation, making possible dry season cropping on previously rainfed land, stabilizing yields in the wet season, and increasing the productivity of fertilizer and other inputs (Taylor 1980). Less important overall in Asia has been the role of increase in the cultivated area (Herdt 1982, Colombo et al. 1978), though opportunities do exist in certain, if often marginal environments (Conway
16 LU a > (TONS/ HA") 2.0 2 2.8 3.2- 3.6-| . 4 1960 - - t — " t - i i T- - "t i" iue . Yed o umle rc i Idnsa From Indonesia. in rice unmilled of Yields 1.1 Figure IR 1982). (IRRI 64
------1 " i r i i ■ i i ■ i i - i i - i i ■ i i ■ i i r i " 1 17 68 72
76 i i" " r " i " i i 80 et al. 1983).
The social and economic impact of these changes has been massive and a subject of considerable controversy (Palmer 1976, Farmer 1977, Lipton 1978, Griffin 1979). By no means has it been clear to all observers that rural communities have benefitted from the increases in production. From surveys of farmers in Central Luzon, Philippines, Cordova et al. (1981) found that despite a 180% increase in the proportion of land double-cropped and a 55% increase in yield per season, net farm income declined nearly 20% in real terms between 1966 and 1979. The growing landless sector of the rural population is likely to be more seriously affected by this trend, particularly in areas where the potential for expanding cultivation or for absorbing labour in other industries is limited (Chambers and Farmer 1977).
Ecological consequences of intensification have been no less profound. The impact of the growing agricultultural use of pesticides on fish and other aquatic life (Kok 1969, Moulton 1973), on the mosquito vectors of malaria (Chapin and Wasserstrom 1981; but see Bruce-Chwatt 1981 and Sharma and Mehrotra 1982) and on human health directly (Jeyaratnam et al. 1982, Loevinsohn in prep.) are of major concern, as is the spread through irrigation of snails that are the intermediate hosts of schistosomiasis (van der Schalie 1969). Yet arguably the most significant effect has been the increase in crop losses due to insect pests and d iseases.
Particularly devastating have been the outbreaks in the early and mid-70's of brown planthopper (BPH) Nilaparvata lugens (Stal) in many parts of Asia (Dyck et al. 1979, Mochida et al. 1977, Oka 1979). BPH emerged
18 as a principal determinant of yield, threatening in some instances the viability of rice cultivation itself, yet at a symposium in 1964 (IRRI 1967) it had been considered of only secondary significance. A number of insect-vectored virus diseases also caused widespread destruction. Previously of localized and sporadic concern, tungro virus, spread by the green leafhopper (GLH) Nephotettix spp, affected large areas in the Philippines in 1970-71, Indonesia in 1969 and 1972, and India, Bangladesh and Malaysia in 1969 (Ling 1976, Lim 1972). Two diseases vectored by BPH grew to significance during the period of intensification. Grassy stunt virus, first recognized in 1960, caused widespread damage in 1971 in Central Java and 1973-4 in Laguna, Philippines, Kerala, India, and Amparai, Sri Lanka (Ling 1977). Ragged stunt virus, identified only in 1977, has caused significant damage in parts of the Philippines and Indonesia, and is now known in 8 countries (Hibino 1979).
But it appears that many of what have traditionally been considered the principal insect pests of rice have also increased with intensification. The gall midge Orseolia oryzae (Wood-Mason) has expanded its range in Thailand and Indonesia and has increased in the severity of its attacks in these countries, as well as in India and Bangladesh (Hidaka 1974, Hidaka and Yaklai 1980). Kiritani (1979) and Pathak and Dhaliwal (1981) cite evidence of widespread increase in the damage caused by whorl maggot Hydrellia philippina, white backed planthopper Sogatella furcifera (Horvath) and leaffolder (RLF) Cnaphalocrocis medinalis (Guenee). The yellow stemborer (YSB) Tryporyza (=Scirpophaga) incertulas (Walker) was previously an important economic pest, possibly the dominant one, from Afghanistan to the Philippines (Grist and Lever 1969), and there is
19 evidence that it has caused increasing damage in recent years in many parts of its tropical range including Indonesia (Oka n.d., Bernsten 1981) and India (Singh 1968, Kalode 1974). Data presented by Lim and Heong (1977; fig. 3) indicate that in North Krian, Malaysia the light trap catch per crop of YSB has increased through the 1960's to mid-70's, though the authors do not draw this conclusion in their text. Pathak and Dhaliwal (1981) contend however that it has declined in significance in many areas, but the evidence on which this contention is based is questionable.
One of the few long term records of pest abundance comes from the light traps on the IRRI farm in Laguna province, Philippines. Figure 1.2 illustrates the trend of the logarithm of the annual catches of BPH, GLH and YSB from 1965 to 1981. Though the curves show considerable independent behaviour in recent years, some of the causes of which will be considered below, in each there is evidence of significant increase in the late 1960's to the mid-1970's, . a period of major change in the cropping systems of the surrounding farms (Kikuchi et al. , 1982). Similar trends have been noted in other areas, including N. Krian, Malaysia (Lim and Heong 1977) and various locations in India (Kalode 1983), where long series of light trap records are also available.
The rapid and widespread change in the status of rice pests with intensification has constituted a major challenge to entomologists and ecologists, indeed in many ways an intellectual and professional crisis comparable to that North American scientists faced some years earlier when pesticide resistance and the effects of insecticides on wildlife forced a reconsideration of the prevailing dependence on chemical insect control (Perkins 1982). In Asia the crisis involved fewer
20 o < I- 12 X o (LOG 14 0) *13 13 15 11 11 10
rp a te RI xei na fr. . yellow A. farm. ental experim IRRI the at traps tmoe; . re lahpe; . brown C. leafhopper; of green courtesy Data B. planthopper. stemborer; iue . Ana ttl o rc pss n light 3 in pests rice of totals Annual 1.2 Figure —i 95 7 9 71 69 67 1965 ------1 ------i i i 1 ------1 ------r- 21 —1 1— V.A. 75 Dyck. i ----- 77 1 ----- i i ----- 79 1 ------i -----
81 r
A C
workers, drawing on a narrower base of research, yet a far larger number of producers and consumers, much closer to the subsistence minimum w The causes of the shifts in pest incidence were a major concern of entomologists in the region. At a conference held at IRRI in 1977 on the BPH problem (IRRI 1979b), several papers detailed the ravages in particular countries and suggested possible causes. What emerged was a list of farming practices and aspects of agro-ecosystem structure (Table 1.1) that had changed in many areas at more or less the same time and that were often consequences of the same underlying process of intensification. Several factors, for example the use of higher levels of nitrogenous fertilizer, had over the years been shown through experiments in field plots to affect pest populations. Those acting at the landscape level were suspected of contributing to the problem, yet for the most part on no more than anecdotal evidence. What is perhaps most remarkable is that, despite the significance of the intensification crisis for the long term productivity, profitability and sustainability of rice cultivation, so little research was conducted to determine the responsibility of individual factors and how they interacted. This failure has meant that the control tactics proposed to cope with the consequences of intensification have been essentially opportunistic and poorly coordinated, and that it has been impossible to move with confidence beyond mere coping to confront the causes of increased pest damage. I suggest and seek to show in the study that follows that a deeper understanding of the population biology of rice pests is 22 Table 1 .1 Agricultural changes linked with outbreaks of the brown planthopper in Asia. Level Factor Supposed mode Quality of Reference of action evidence 1 decreased favourable E (p) Dyck et al. 1979 plant micro MacQuillan 1974 spacing environment 2 improved favourable and E (p) Bannerjee et al. 1973 Within water stable micro- Dyck et al. 1979 management environment field increased a. favourable E (p) Cheng 1971 use of N micro Kalode 1976 fertilizers environment b. improved plant nutrit ion . pesticide resurgence E (p) Chelliah and Heinrichs 1980 misuse due to: Kenmore 1980, 1980a a. destruction of natural enemies b. stimulation of population growth at low doses suscepti favours Mochida and Suryana 1975 bility of population modern increase varieties Between 6. increased a. increased double time for fields cropping population Sogawa 1979 growth O Fernando 1975 Kalode 1974 7. increased Oka 1979 asynchrony b. reduced between fallow farms E (p) - Based on experimental results from field plots where only the factor concerned was varied. O Based on uncontrolled observation. crucial for this. Yet an effective response to the crisis clearly requires more than these insights. Tothill (1958), considering the origins of earlier crises in other crops, contended that "We have changed the environment once; we have it in our power to change it again...". Applied to rice in Asia the suggestion is surely naive. Rice cultivation in the aftermath of the process of agrarian change known as the Green Revolution is a far more complex affair than previously, involving a larger number of actors, linked in economic and social relations many of which existed in only embryonic form 15 years ago (Goodell 1983). Any change in the distribution and abundance of rice or in the manner in which it is grown is likely to have manifold impacts beyond altering the potential for pest population development. It is clear that if entomologists and ecologists wish seriously to examine the potential for pest suppression through environmental modification, that is, if they wish to apply ecology, they will have to take on or participate in a more comprehensive analysis, that considers more than merely the biotic consequences of such action. I will return to this point more than once in what follows; I wish first however to examine briefly the present state of pest control in rice and of its scientific underpinnings. 1.2 Pest Control After the Crisis The insect and disease outbreaks that accompanied the general intensification of cultivation led to more insistent calls for the development of integrated pest management (IPM) strategies for rice. Many workers envisaged these as drawing on a range of tactics including judicious use of pesticides based on economic 24 thresholds, the deployment of resistant varieties, and the implementation of "cultural" controls, notably synchronous planting, post-harvest plowing or burning of the stubbles,t the planting of trap crops, and moderate application of nitrogenous fertilizer (Oka 1979a, Heinrichs et al. 1978, Reddy 1979, FAO 1982). In practice however, the greatest share of research effort has been directed to the development of insect- and disease-resistant varieties and to the refinement of insecticide application methods and procedures. Based on data provided by Pathak and Dhaliwal (1981), of the scientific papers published world-wide between 1976 and 1979 dealing with rice insect control, 50% concerned insecticidal methods, 25% varietal resistance, 19% biological control, 5% autocidal and other chemical means, and 2% cultural methods. Applied research may be more concentrated than these figures suggest for, as Pathak and Dhaliwal note, much of the work on biological control is of a taxonomic or descriptive nature and of little immediate utility. Reliance on varietal resistance and the generally prophylactic use of insecticides have come to characterize small farmers' crop protection practices (Litsinger et al. 1981, Sadji 1981) in the absence of effective, feasible and demonstrated alternatives. The planting of wide areas of closely related resistant varieties has added a new dimension to the intensification syndrome, creating uniform selection pressures for virulence and the potential for rapid build-up once such a phenotype appears. Varieties have been bred at IRRI and at national institutions that incorporate major gene resistance to BPH, GLH, gall midge, rice blast Pyricularia oryzae (a fungal disease) and bacterial leaf blight Xanthomonas campestris pv oryzae: in each case virulent strains have been observed 25 some time after widespread adoption by farmers (Khush 1980, Pathak and Saxena 1980). The limitations of varietal resistance as a control tactic are evident in the case of BPH. The release and rapid adoption of IR 26 and related varieties with the Bphl gene allayed the immediate crisis of the 1972-3 outbreaks in the Philippines and Indonesia. Within two to three years however, virulent phenotypes, referred to by some workers as "biotypes", came to dominate the BPH population and cause widespread damage. The cycle was repeated with the release of varieties such as IR36 incorporating the bph2 gene. Though the resistance of these remained effective for substantially longer than that of the previous generation of varieties, by 1981 there were indications of renewed virulence in large areas of northern Sumatra, Indonesia (Sogawa 1983) and Mindanao, Philippines (Khush, pers. comm.). Concern has been expressed by a number of observers over the current narrow base of resistance and the uniformity of its deployment (Oka n.d. , 1979a, Ministry of Agriculture 1980, Hargrove et al. 1979). In 1979 2/3 of the lowland rice area in Central Luzon, Philippines was planted to two sister lines, IR 36 and 42 (Cordova et al. 1981), while in Bali IR 36 covers 3/4 of the planted area and more than 1/3 in Indonesia as a whole (Ministry of Agriculture op. cit.). Varietal resistance represented in one sense the most conservative research response to the intensification crisis: it demanded no institutional adjustments beyond that in breeding and testing the lines. Insect control was wrapped in the seed and the earlier rapid adoption of susceptible high-yielding varieties had shown that farmers would make use of improved seeds when these were available. Varietal resistance was seen as compatible 26 with the existing pattern of rice cultivation and thus attracted the lion's share of research into novel control tactics. Perkins (1982) makes a similar point, in greater depth, in his consideration of recent trends in North American entomology. Entomologists, he claims, were guided by a shared appreciation of the social and economic context in which research results would be applied, a highly competitive and, increasingly, a technologically intensive form of agriculture. A premium was therefore placed on control tactics that could be adopted by individual, progressive farmers. I believe that much the same process was at work in Asia, indeed the links between institutions, and movement of personnel and information make it of little use to consider the developments in rice research in isolation. The result of this is that, currently, several novel methods of insect control requiring coordination over a wide area and/or greater institutional support face two sorts of constraint: firstly, a shortage of fundamental research to determine their technical demands and effectiveness, and secondly a lack of understanding of how they might be integrated into the rice production system. For instance, burning of the stubbles or ploughing soon after harvest has been suggested as a means of breaking insect and disease life cycles (Litsinger et al. 1978, Khan 1967): regrowth from the stubble sustains low populations of GLH and serves as a reservoir for tungro virus, while stemborers survive the fallow period as late instar larvae in the base of the stems. Yet as Oka (1979a) points out, burning results in a loss of nutrients, particularly N, and in many areas the cropping schedule does not allow sufficient time for the stubble to dry. Personal observations in the Philippines indicate that the ratoon crop that would be lost by ploughing provides a supplemental food source 27 for the landless, and for the community as a whole after natural disasters have destroyed the main crop. None of these drawbacks can be confronted until the extent of the benefits from the innovation and its precise requirements (how large an area must be treated, and how soon after harvest) have been determined. Similar objections have been brought against synchronous planting, which may drive up tractor and labour hire rates through concentration of demand, and depress the price for the harvest by creating a market glut (Bernsten 1981). The necessity for coordination among farmers and with government agencies that provide crucial support services raises questions of how such a scheme is to be organized and where the initiative should come from (Goodell, in press, this study). Yet these problems depend again on the scale of the intervention that is envisaged: how many hectares need be planted synchronously, and within how many days? Despite the fact that synchronous planting figures prominently in many recipes for integrated pest control in rice (Oka 1979a, Reddy 1979, FAO 1982, Brader 1979, Litsinger et al. 1978), it is striking that, with but one partial exception considered below, no more than anecdotal evidence exists for its effectiveness (Fernando 1976, Oka 1979, 1979a). None provide the degree of ecological insight that would be required if the balance of impacts of synchronization is to be evaluated, even if in only qualitative fashion. If lack of understanding of the population biology of rice pests has been one of the factors that has impeded the adoption of novel control tactics, it has also prevented ecological insights from influencing the evolution of agricultural systems, so as to ensure that the potential for pest build-up is minimized. 28 . Intensification is encouraged and shaped by a number of policies of both governments and international agencies: - Subsidies, loan programmes and extension efforts. In many countries, these have encouraged the adoption by farmers of the basic elements of intensive farming: the new seeds, agricultural chemicals, and machinery. - Support for research on cropping systems in national and international institutions. This has aimed at finding combinations of crops and crop management techniques that permit intensification of production on a given unit of land (Zandstra et al 1981), combinations that may subsequently be promoted through government credit and extension projects. - Public investment in large-scale irrigation projects. Over much of Asia, the area on which double- and even triple-cropping is possible has been markedly increased in recent years. The Trilateral Commission estimates that further investments of $53 thousand million are required in Asia over the next 15 years to meet rising demand for rice (Colombo et al. 1978), and its proposals would result in an increased concentration of production in lowland irrigable areas. Taken together and seen from an ecological perspective, these policies contribute to a massive alteration in the abundance and distribution of the host plant of rice pests and diseases, yet seldom do pest suppression considerations enter into the design and execution of intensification programmes. 1.3 The Scientific Problem 29 Changes in particular factors, whether at the individual farm or landscape level, have been implicated in increases in pest incidence in many other crops and environments; from a voluminous literature, one might point to Nakasuji (1974) and Hirano and Kiritani (1976) with respect to temperate zone rice, and Hambleton (1944), Pearson (1958) and Barducci (1972) dealing with tropical cotton. As concerns tropical rice, two recent contributions to the literature stand out as having taken the intensification crisis seriously as a scientific problem, susceptible to analysis if not always to controlled experimentation. From studies on the IRRI research station and in farmers' fields, Kenmore (1980, 1980a) showed that lycosid spiders responded numerically to BPH population increase and suggested that these generalist predators were important in the natural regulation of the pest. Drawing on his own and a growing body of other work, he showed that insecticides disrupted this control, particularly those chemicals markedly more toxic to spiders than to BPH, and those, such as methyl parathion and diazinon, that stimulated the pest's reproduction at sub-lethal doses. Survey data indicated that Philippine rice farmers' use of resurgence-inducing chemicals had increased substantially in the years immediately prior to the first outbreaks of BPH, and that the dosages typically applied were such as to stimulate reproduction. Kenmore considered critically a number of the other factors that had been put forward to account for the outbreaks. The high-yielding varieties were as a group no more susceptible than the traditional ones they replaced, he argued, and nitrogen fertilizer had been found to significantly affect BPH density only at levels considerably above those farmers applied. In his view, no mechanism had been demonstrated to implicate 30 irrigation or double-cropping independently of the impact of the concurrent increase in pesticide use. Pesticide-induced resurgence offered the only experimentally verified explanation, he contended. Kenmore's hypothesis led to falsifiable conclusions: it suggested that where widespread adoption of pesticides lagged behind the other elements of intensification, so should have the outbreaks. By the same token, were the thesis correct, BPH damage should have been most severe where the use of resurgence- causing insecticides had been the greatest. To my knowledge, no rigorous test of Kenmore's hypothesis has yet been attempted. From the perspective of the present work however, one limitation is that it restricts attention to BPH to the exclusion of the other pests that increased during the period of intensification, few of which have been shown to resurge after pesticide treatm ent. Lim and Heong (1977) considered the impact of asynchrony of cultivation between fields on the population levels of YSB and GLH in Malaysia. Though quantitative data on asynchrony wgrc lacking, they suggested that it had increased during the period of transition from one crop to the two made possible by irrigation. Seasonal light trap catches of GLH had risen during the period, while those of YSB had declined. They reasoned that the increased time during which rice was available in a locality due to asynchrony permitted continued population growth of GLH, while on the other hand the natural enemies of YSB were benefitted by the continuity of the interaction with their host. Lim and Heong concluded that "cultural" controls such as synchronous planting could have undesirable consequences for those pests previously well regulated by their 31 natural enemies. Despite their reliance on an unmeasured parameter as the independent variable and the ad-hoc nature of the hypothesis adduced to account for the difference in behaviour of the two species, Lim and Heong’s work represented an advance over the largely speculative previous work in that it considered pest density as a measurable attribute of a locality, affected by factors beyond those of individual crop management. Lim and Heong's work brought into focus a fundamental question that had been posed by Southwood and Way (1970): should habitat modification seek to make conditions difficult for pest increase by reducing the abundance and apparency of the host plant, or should it rather increase the permanence of the crop so as to enable natural enemies to remain in contact with the pest? Several workers have pointed out thattropical rice approaches the ideal of a perennial crop in terms of the potential for biological control (Pantua 1979, Kenmore 1980a, Greathead, 1982). The crucial applied question is whether this potential should be exploited: should cultivation be synchronized or not, should further increases in cropping intensity be opposed or supported, should rice stalks be destroyed or conserved? An initial theoretical consideration of the problem suggests that the answer turns on the relative vagility and rate of increase of the pest and its natural enemies, as well as on the duration and the spatial distribution of the crop. Elaborations of models of predator/prey interactions in heterogeneous environments (Levin 1974) may be useful to clarify thinking here. The approach followed in the present work however is to test hypotheses generated from a simpler model that restricts attention to the dynamics of pests and host 32 plant, assuming that under prevailing conditions natural enemies are incapable of fully regulating pest density. The utility of this model is ascertained by examining experimentally the response of pest populations to marginal increases in crop permanence through multiple cropping and asynchrony. This study seeks a unifying explanation for the correlated increases in a number of the principal insect pests and diseases of rice during the past 15 years and then attempts to apply this understanding to current problems of control and of ecosystem design and management. It is based on the assumption that processes acting at a spatial scale greater than the length of one field and a temporal scale longer than one crop season have been key to the emergence of the intensification crisis. Yet at present there is a dire lack of theory to guide experimentation at this level and to make sense of the reams of data over time and space that quickly flow from monitoring efforts (Taylor 1979, Stinner 1979). In what follows, I attempt to derive substantial and falsifiable ' hypotheses regarding the impact of agrarian change on pest population biology. It is vital I believe for the health of ecology and in order to effectively respond to the demands of application that we construct and seek to test such hypotheses. A recurring theme throughout this thesis is the potential for profitable stimulation of theoretical by applied ecology. Darwin was not the last to recognize that agriculture is perhaps ecology's best laboratory, yet there is a danger that the continuing sub specialization of the discipline will shut it off from this challenge and opportunity. Much of the present research was catalysed by a simple and provocative question from community organizers working in Nueva 33 Ecija, Philippines: how many farmers must plant synchronously, and within how many days? In Chapter II I argue that the transient nature of the rice crop and the use of agrochemicals that disrupt natural enemy control have created a simplified environment in which pest numbers are limited largely by the duration of rice availability. The nature of lowland rice ecosystems, in particular their extent and lack of ' vegetational diversity, suggest that oligophagous pests should predominate. Together, these considerations make simple mathematical models appropriate and simple control measures of potential use against a range of pests. A model is developed of pest dynamics within what I term the locality - the area within which the majority of dispersal occurs - and that considers the complete annual cycle of crop and fallow. In Chapter III, I test hypotheses regarding the response of pests to intensification, using data from the Philippines and Malaysia. Employing a simple analytical procedure for seasonally fluctuating populations, I show that, in all cases considered, the greatest change has been in the numbers colonizing fields at the beginning of the season, rather than in their rate of increase through the season, suggesting that the shortening of the fallow due to multiple cropping and asynchrony has been central to the increase in pest densities of recent years. Population growth is found in general to be density dependent but undercompensating, while carryover between seasons is largely independent of density. This combination can be shown to account for the detailed population response to intensification in the past and gives hope of being able to predict the impact of future such change. 34 The following two chapters explore in some detail the phenomenon of asynchrony of cultivation and of pest response to it. Though asynchrony might best be considered a consequence rather than an element of intensification, the impact on time-limited pest populations should be similar, as elaborations of the models in Chapter II suggest. Furthermore, because it is not an inevitable consequence of intensive production, asynchrony may be the more easily manipulated. Chapter IV describes the 20,000 ha study area in Nueva Ecija where double-cropped rice is the predominant land-use pattern. Though spatio-temporal intensity and input use are shown to be relatively constant across the landscape, asynchrony, measured by the standard deviation of planting date within a specified radius, varies markedly and systematically. The natural history of asynchrony is considered, examining the origin and impact of delays of several of the requisites of cultivation, including water, labour, and tractors or water buffalo. Defining what are referred to as the "components of asynchrony", I show that it is possible to partition responsibility for variance in the date of transplanting. Under present conditions, variation between farms in the arrival of irrigation water is shown to be the most important factor, and one not under farmers' direct contol. Chapter V describes a network of farmer-operated kerosene light traps which was maintained in the study area for more than 16 months. These inexpensive lights are shown to provide a useful means of assessing local insect pest density: results are consistent between nearby traps and reflect patterns of infestation in adjacent fields. Most importantly, catches over a season are found to correlate significantly with the asynchrony of cultivation for most of the species considered. The 35 distance over which pests appear to respond to asynchrony is roughly constant over 2 seasons and is consistent with estimates of dispersal range obtained from natural experiments. Furthermore, the quantitative response to asynchrony is found, as hypothesized, to be proportional to pests' potential rates of increase. The importance of these findings for the design of synchronous planting schemes as well as for assessing the impact of unintended irrigation delays is considered. The changing distribution of host plants in space and time is likely to constitute a major selection pressure on specialist-pest life-history parameters: intensification and asynchrony affect not only population dynamics but population genetics as well. In Chapter VI experiments are described in which yellow stemborers from 3 environments were reared under the same conditions. Significant differences were found in generation length, fecundity and survivorship, in a pattern predictable only in part on the basis of current theory. The results suggest the need for a more sophisticated classification of environments and environmental change; they also indicate that, though deliberate synchronisation for pest control is likely to evoke an evolutionary response from pests, this is unlikely to be sufficient to counteract the direct population dynamic effect of a reduced availability of plant resource. Finally in Chapter VII, I consider the implementation of synchronous planting as a control tactic. Several potentially deleterious effects, of an economic and social nature, are considered. In particular, I suggest that the impact of synchronization on the ability of a community to withstand natural disasters and on the 36 income security of landless labourers may be the most significant disadvantages, and ones which may be overlooked by researchers. Yet these effects are likely to be scale-dependent and the results of the previous chapters are vital therefore in suggesting the minimum degree of synchrony required for purposes of pest control. In a one year pilot project that promoted synchronous planting in Nueva Ecija, progress was found to be limited by the strained relations that existed between farmers and support institutions, though widespread support at many levels was found. Future efforts, if they are to make significant headway, must take account of the divergent interests of those directly involved in rice production; conversely, in other social and economic environments the prospects for implementation may well be different. 37 C H A P T E R I I Limitation by Time: A Model and its Predictions 2.1 Introduction In this chapter I develop a simple mathematical model of pest population development within agricultural landscapes. My concern is with the impact of changes in the timing and distribution of rice cultivation over areas commensurate v/ith the dispersal range of the species, rather than with the effect of crop management practices within the field. Favouring here models that, in Levins' (1966) terms, emphasize generality and realism at the expense of precision, I attempt to derive testable and ultimately usable conclusions regarding the consequences of increasing (i) the proportion of land devoted to cultivation, (ii) the number of crops per year, (iii) the degree of synchrony with which they are planted, and (iv) the duration of the varieties grown. The first two and the final trends are elements of what I refer to as spatio-temporal intensification, whereby the amount of space and time devoted to cultivation is increased. Asynchrony, as I have suggested, is not itself an aspect of intensification, but may be affected by many of the developments that make intensification possible and its impact on pest populations may be described by similar models. Though my concern is primarily with insect pest dynamics, the model I develop is to some extent applicable to plant diseases and in what follows I point out where essential biological differences make separate treatment necessary. Furthermore, though I focus on Asian rice cultivation, similar questions have been posed by workers in other agricultural contexts. Stinner (1979; see also the 38 subsequent comments by M.J. Way) for example sketches the requirements for constructing a "regional life table" for a pest in a diverse crop mosaic. As I hope to show, many of the principles and methods needed for such an enterprise may be most readily developed and tested in the ecologically simpler context of a rice monoculture. Indeed, several of the phenomena considered occur as well in natural environments: in particular, the synchrony of rice cultivation and masting in tropical forest trees (Packham and Harding, 1982) may have similar population consequences for crop pests and seed predators, respectively. 2.2 The Biological Context Lowland rice cultivation is practised on some 80% of Asia's rice area and produces more than 90% of the annual harvest (Barker and Herdt 1974). Regardless of the source of water, whether from rainfall or irrigation, the pattern of farm operations is similar. Seedlings are raised in a small nursery area and, when 20-40 days old, are transplanted into saturated and puddled soil after it has been plowed and harrowed several times (De Datta 1981). Direct-seeding of pre-germinated seeds is increasing in popularity in many areas of Asia but is still less widely practised than transplanting. The modern varieties that are now planted on majority of lowland farms in Asia (Herdt and Capule 1983) are generally photoperiod-insensitive, and mature in 100-135 days, regardless of season. For example, IR 36, possibly the most widely grown cultivar of any crop (IRRI 1982), has a growth duration of approximately 110 days, or less than 90 days in the field after transplanting. These general features of lowland rice cultivation have a number of consequences for rice pest ecology. 39 Tabic 2.1 Characteristics of the major insect pests of rice in the Philippines Species Approximate generation Stage of crop Host References length (days) attacked plants Yellow 45 - 55 V, Re, Ri rice Bannerjee and stemborer Pramanik 1967 (YSB) This study Brown planthopper 20 V, Re, Ri rice Sogawa 1979 (BPH) Green leafhopper 20 V, Re rice Misra 1980 (GLH) grasses Caseworm 20 V rice IRRI 1981 (CW) grasses Leaffolder 35 V, Re, Ri rice Velusamy and (RLF) grasses Subramaniam 1974 Rice green semi-looper 20 V rice Pantua and (RGS) grasses Litsinger 1984 V - Vegetative, Re - Reproductive, Ri - Ripening 40 (1) Firstly, the water-saturated chemically-reduced soil environment is incompatible with all other major crops save taro Colocassia spp. Rice therefore is grown almost exclusively as a sole crop within each field. By the same token, the risk of damage to upland crops in adjacent fields through seepage makes a diverse mosaic of crops in a landscape an infrequent occurrence. This fundamental agronomic incompatibility is reinforced by the attractiveness of rice to farmers due to the relative stability of its yields where the water supply is adequate, the assured market for production in Asia, various government programmes and subsidies, and farmers' preference for planting a subsistence rather than a cash crop. The resultant vegetational uniformity suggests that lowland rice insect and disease pests should be predominantly specialists, as the abundance of other plants, save certain weeds, is low. In Table 2.1 I summarize salient biological characteristics of what are generally considered the principal insect pests of rice in the Philippines, with which I will be concerned in much of this study, all of which are important as well in other parts of Asia. In particular it should be noted that both BPH and YSB, arguably the two most serious pests of rice in Asia, are entirely monophagous. Others attack no other cultivated crop save rice and are found on alternative weed hosts for the most part only during the fallow period. In several cases these insects have been found to prefer rice in free choice experiments (Pathak 1983, with repect to green leafhopper) or to exhibit faster rates of increase when confined to rice (IRRI 1981, with respect to caseworm). This contrasts to the situation in upland cultivation where rice is grown in unsaturated soils and more often in proximity to other crops. Here polyphagous life histories appear to be more frequent (Litsinger et al. 1977). In Kenya, Ho and Kibuka (1983) found a shift in the species composition of the stem 41 boring guild from upland to lowland irrigated rice environments: Chilo partellus and Sesamia calamistis, pests of sorghum and maize predominated in the uplands, the monophagous Maliarpha separatella in the lowlands. The availability of alternative hosts may as well go some way towards explaining the distribution of stemborer species within areas of lowland cultivation. Surveying the composition of the guild in major rice growing areas of Luzon island, Philippines, Cendana and Calora (1967) found the polyphagous Chilo suppressalis dominant on rice farms in Laguna province, while in Central Luzon, some 100 km to the north, the monophagous YSB was encountered almost exclusively. The authors sought to account for this pattern on the basis of climatic differences. I suggest however that more to the point may be the fact that rice cultivation in Laguna is largely restricted to the narrow plain surrounding Laguna de Bay, and upland crops such as maize, an alternative host of Chilo, are planted nearby on higher land. In contrast, over much of the flat alluvial Central Luzon plain, rice is the only crop grown. Shifts in species composition at a site over time may also be tied to changes in the relative abundance of host plants. Figure 1.1 illustrated the trend in recent years of 3 of the principal insect pests in light traps at the IRRI farm in .Laguna province. As mentioned in the introduction, the period beginning in the mid to late 1960's was one of major change in rice farming. In Laguna, of particular importance has been a marked extension of double cropping, from 39% of the rice area in the vicinity of IRRI in 1969 to 99% 2 years later (3.2.1). In the next chapter I consider in detail the impact of such change on the major insect pests. In terms of the present discussion however it should be noted that, comparing 2 periods, the first from 1965 when year-round light trapping began until 1970, and the second from 1971 to 42 1979, the mean annual catch of YSB increased 176% but that of Chilo only 64%, indicating a shift in relative proportions towards YSB. As there does not appear to have been any significant increase in the abundance of the alternative host plants of Chilo, a pattern such as this is to be expected if it is assumed that monophagous pests are more efficient exploiters of a host plant than their polyphagous guild-mates (Roughgarden, 1972). That the degree of polyphagy of pests in an area is linked to the diversity of host plants appears self-evident and for all that may be no less true. In practical terms, the supposition suggests that agrarian change resulting in an increased density of rice, particularly if concentrated in areas where rice cultivation is already well established, will have its greatest impact on specialist pests. It is to these, relatively simple life histories that the model developed in this chapter relates. For the most part I will assume constant the essential bionomic characteristics; in Chapter VI I consider intensification as a selection p ressure. (2) The second feaure of the lowland rice environment of significance to the ecology of rice pests is the rigorous tillage that the soil is subjected to each crop cycle. All plants, whether rice (stubble) or weed are destroyed and the field becomes uninhabitable. (Note that this need not be so for sedentary pests such as nematodes, of importance generally only in deepwater and upland cultivation (Grist and Lever 1969, Castillo et al. 1977), and for those pathogens able to survive tillage as spores.) If the interval between the destruction of host plants and the time at which the new crop becomes suitable for oviposition exceeds the egg-laying period of the adult, then the first generation in the new crop cannot be directly descended from the last generation in the previous crop within the same 43 field. Data to be presented in Chapter IV indicates that in a mostly mechanized irrigated area of Central Luzon, Philippines, the mean interval between plowing and transplanting in one season was 36.7 days, with a standard deviation of 13.5 days. Save in the extreme lower tail of the distribution, this exceeds the adult life span of the insects considered in Table 2.1, and implies that population development depends on colonization from other rice fields or uncultivated areas of alternative hosts. Area-wide processes such as spatio-temporal intensification and asynchrony, if they are to have any impact on pest densities within a field, must act by altering the level of immigration. The colonization of short-duration crops has in other contexts generally been studied at the level of the individual field (Hokyo and Kuno 1977, Gutierrez et al. 1974); I diverge from this in what follows by considering an area large enough to include the origin of the majority of immigrants to a field. Immigration is treated therefore as a largely endogenous rather than exogenous variable. 2.3 The Structure of a_ Relevant Model The evolution of the agricultural landscape that we are considering here entails changes in the number of habitat patches (fields) existant at any one time, in their isolation and in the frequency with which they are cropped .and fallowed. There are I suggest 3 principal approaches in examining the impact of these factors on the abundance and distribution of rice pests. (1) The first focusses on stochastic processes. Through an increase in the total population size and in the probability of successful dispersal between patches, the likelihood of random extinction is decreased and the number of potential sites that are colonized at any one time increased (Lewontin and Cohen 1969, Roff 1974). Stenseth (1980) considers the 44 utility of different control strategies to affect these two parameters for a pest whose dynamics are dominated by stochastic fluctuations. I suggest that a model of this sort is inappropriate for the insect pests of rice; these typically maintain dense populations, disperse over relatively large distances and have high rates of potential increase, all factors which reduce the likelihood of extinction (MacArthur and Wilson 1967). (2) More relevant to the ecological attributes of the organisms with which we are concerned and to the applied questions being posed are deterministic models that consider pest population dynamics in a set of patches of specified geography, duration and synchrony and linked by dispersal. This is the approach adopted in the following sections. For the most part our concern is with pests whose generation time is short relative to the duration of the crop (T/H approaching 1 in Southwood's (1981) terminology), which may multiply for from 1 to 3 or 4 generations in one field. Damaging infestations typically build-up within the crop rather than invading from outside. The response of such pests to landscape-level changes in the distribution of rice might be termed numerical, as contrasted with the functional response of longer-lived and polyphagous pests such as birds, mammals or even locusts that individually disperse i^ito and out of a field and need not breed within it. In most of what follows I ignore the impact of weather on population growth though of course I do not deny it. Such an approach is justified given the objectives of the model; spatio-temporal intensification and asynchrony are no more likely to occur in climatically marginal areas, nor are they likely to be associated with significant climatic modification (though admittedly irrigation may create a more equable micro-climate for some pests). The effects we are 45 concerned with are average ones over sites and years, and for this reason I believe it reasonable to consider the impact of weather on population growth as random. I return to the details of the model below. (3) A further level of realism would be added by considering explicitly the dynamics of natural enemy populations. In what follows I will resist this temptation for a number of theoretical and practical reasons. (a) There are I suggest few firm indications that, under field conditions, natural enemies are able to regulate rice pest populations at levels significantly below the carrying capacity of the habitat. Possibly the most detailed study of predator/prey interactions under tropical rice farm conditions is that of Kenmore (1980), who showed that spiders (Lycosa spp.) increased numerically, apparently in response to the growth of BPH populations in farmers' " and experiment station fields. Yet finding a positive correlation between the population growth of a predator and the density of its prey, as Kenmore did, is not sufficient to demonstrate that effective regulation occurs. Indeed, as both spiders and BPH invade the young crop at relatively low levels and build up within the field for a few generations until harvest, a pattern such as Kenmore observed is to be .expected, even were there no interaction between them. Nor is it conclusive evidence that the pest population increases markedly when natural enemies are excluded or reduced by pesticides, again as Kenmore was able to show. What is required is evidence that the mortality attributable to natural enemy action increases proportionately with initial pest density to an extent sufficient to contain further increase (Varley et al. 1973, Southwood 1978). Experimental demonstration of this requires detailed study in a number of seasons or sites. To my knowledge the necessary data has 46 not been gathered. (b) The capacity of natural enemies to regulate pest populations is necessarily limited. Examination of theoretical models of parasitoid/host and predator/prey dynamics suggests that pests can escape natural enemy control when immigration exceeds a certain critical value (Hassell 1978, Beddington et al. 1975); indeed, this is implicit in the definition of a local equilibrium. This fact is central in the present context, for both intensification and asynchrony tend, as will be shown, to increase immigration levels to a field. (c) Our concern here is with the population dynamics of insect pests under production, not experimental conditions. Since the advent of the Green Revolution, insecticide use has increasingly become a factor in rice agro-ecosystems. Several studies have shown the capacity of insecticides to disrupt natural enemy activity (Kenmore 1980, Carino et al. 1982), giving rise to population dynamics resembling those of a simple insect/plant interaction. The pattern of pesticide use has not generally been analysed in the same fashion as have natural mortality factors, but there is some evidence from surveys in the Philippines that it is not applied in a density-dependent fashion, at least when considered between farms in a given area and under moderate levels of infestation (Marciano and Flinn 1981). The prevalence of calendar-based, as opposed to threshold application schedules among farmers (Litsinger et al. 1981, Sadji 1981) supports this assumption. Taken together, these factors argue that there is no compelling reason to consider explicitly natural enemy dynamics in theoretical or experimental terms. This is certainly welcome, for the analysis of multi-species spatio- temporal models is notoriously complex, and in the field one 47 would be at a loss as to which natural enemies to monitor, for it is often unclear which are the most important. Similarly, the evidence suggests that limitation of pest populations by intra-specific competion is unlikely to be a common occurence. Even under outbreak conditions, peak populations are generally encountered in the final generation prior to harvest or senescence, suggesting that time and the deterioration of the habitat may be limiting growth. Stemborer infestations have been known to reach 50- 80% "whiteheads" (severed panicles) (Rothschild, 1971), though under current conditions in the Philippines, damage to the crop exceeding the cost of chemical control occurs at some 2-5% (V.A. Dyck, pers. comm.) Yet even these lower levels are often not exceded under farm conditions (Chapter V). Rothschild (op. cit.) studied the population dynamics of stemborers in Sarawak, Malaysia and prepared partial life tables over 4 seasons for 3 species occurring together in the crop: YSB, Chilo suppressalis and Sesamia inferens. From the data he presents, it is possible to examine the degree of density-dependence in mortality due to all causes from egg to pupa. In Fig. 2.1 I plot the logarithm of numbers entering the pupal stage against the logarithm of the number of eggs. Taking all 3 species together, the least-squares regression gives a slope close to 1, suggesting mortality is independent of density (Southwood 1967)1. None of the individual species appear to differ substantially from the overall trend. As explained in the next chapter, the true slope is likely to be to some extent underestimated by least-squares regression. Taking account of this fact reinforces the interpretation that positive density dependence is lacking. 48 LOGe EGGS Figure 2.1 Numbers of 3 stemborer species entering the pupal stage in relation to numbers entering the egg stage, both as estimated per 200 hills. The least-squares equation is: In Y = 1.14 In X - 5.42 n =12. The slope differs from 0 (t = 12.2, P .001), but not from 1 (t = 1.45, NS), suggesting density independence. a Tryporyza incertulas , ^ Chilo suppressalis , □ Scsamia inferens. Data from Rothschild (1971). 49 Models of density-independent population development have been proposed for BPH in Japan (Kuno and Hokyo 1970), where it does not overwinter, reproducing each year apparently from long-distance migrants. In the United States, Butler et al (1974) report that the annual build up of bollworm and cabbage looper was adequately described by exponential equations. I suggest that the prevalence and extent of density dependence in the development of the pests with which we are here concerned is an empirical question; the approach adopted in what follows is to construct firm null hypotheses, based on the assumption that population growth under the conditions of the farm environment is unregulated by intra-specific processes or natural enemy action. The adequacy of this assumption will be tested in the experiments reported in Chapters III and V. 2.4 The Basic Model Though the generations of tropical rice pests are generally thought to be less well synchronized than is typical of temperate insects, neither is there co.mplete overlap. The seasonal nature of rice cultivation, even where irrigation is available, ensures that the major source of immigrants to a field, other rice fields, are only periodically present. Moreover, there may be abiotic cues •that synchronize population development. It has long been known that the emergence of many tropical aquatic insects is correlated with the phase of the moon (Johnson 1969), and there are indications that this may be true as well in the case of YSB. Unpublished personal observations suggest that its emergence and oviposition in the field may be entrained by the phase of the moon, and Dr. P.S. Beevor (pers. comm.) reports that under laboratory conditions its emergence can be controlled by altering light levels. Reviewing the literature on the population dynamics of BPH in the tropics, 50 Dyck et al. (1979) contend that it is generally possible to discern population peaks in field infestations corresponding to the development of successive generations. In this situation, a model expressed in difference equation form would appear to be the most appropriate, and this will be adopted in what follows, though I will attempt to indicate where results would differ were the growth of the population considered to be continuous and were its age structure taken into account. I describe first a general model applicable to a range of insect pests and diseases, and then consider some of the compromises their divergent characteristics dictate. Within a field, the numbers of a given stage, say the dispersive, can be described by: ( 2 . 1 ) where pj. is the net rate of increase within a field, that is, exclusive of dispersal losses in generation t, f^ the fraction of the population not emigrating, xj the number immigrating to the field, and p'^ their within-field' net rate of increase, which is often found to be less than that of non-migrants (Johnson 1969). The non-dispersing fraction, f, may be a function of time and possibly of density, as in BPH where the proportion of macropterous dispersive adults has been found to increase towards crop maturity (Kenmore 1980). The net rates of increase, p{ and p^ , are assumed to be time-dependent, likely decreasing as the crop matures due to reduced photosynthesis as senescence sets in, lower free nitrogen content in older plant tissue, and increases in natural enemy attack (though, as suggested above, this may be a result of increase in their, numbers with time, rather than a true response to pest density). Where the field is fallow, net rates of increase are fractional, approaching zero for those pests, such as BPH, that do not become 51 quiescent, enter diapause or that cannot subsist on wild hosts. Consider an extensive agricultural area consisting of s fields (here assumed of equal dimensions) and sufficiently isolated from other such areas that dispersal between them is much less than the average interchange between fields within the area. I refer to this as a "region"; within it, pest numbers can be expressed as a column vector whose elements are the individual subpopulations in each of the s fields, nj ^ • When the area considered is large enough to include the source of the greater part of migrants to a given field, the x* ^ terms can largely be replaced by an endogenous variable, that is, one dependent on the population dynamics in the source area. In what follows, I consider explicitly as sources only rice fields, consigning to a residual exogenous variable input from stubble, volunteer or wild rice, and, for the non-monophagous pests such as GLH and case worm, that from weed hosts". This variable includes as well immigration from outside the region. The vector can then be expressed as: ( 2 . 2 ) Here, all the capital and underlined letters represent vectors over the s fields of the same quantities as in equation 2.1. Where population growth and dispersal are density-independent processes, and Fj. can be considered vectors of random variables with a given mean and standard deviation. Ej is a vector of immigrants to the area from outside the set of fields explicitly considered, and assumed to be small in relation to those originating within the region. is a matrix of coefficients, m - , representing the proportion of individuals dispersing from field i that 52 reproduce in field j. This matrix representation of dispersal in a sub-divided habitat is a more general formulation than that employed by Usher and Williamson (1970), which took account of movement from a source only to the 2 nearest patches in each dimension. The "locality" around a field j may be defined as the area that includes the fields i from which individuals disperse to j with a probability mjj^ exceeding some arbitrarily small value. In later sections I consider this concept more closely and discuss problems involved in determining its dimensions experimentally. Equation 2.2 can be simplified. I suggest that in generations subsequent to the first, the contribution of immigrants from outside the region E{, can largely be ignored. They are however crucial to population development at the beginning of the crop, for as suggested in section 2.2, nQ = 0 for all the pests here considered. Solved recursively, the basic equation then becomes: This has not taken us very far however. In practical terms, solution of 2.3 is likely to be thwarted for a number of reasons, perhaps chief among which will be that the dispersal coefficients m.l .jt . are unknown, difficult to measure and likely variable over generations. In some instances it may be possible to make use of empirically derived relationships of the numbers reaching a point from a defined source as a function of distance. R.A.J. Taylor (1978) has reviewed a large number of studies of insect dispersal and has found that a flexible equation of the form 53 Nx = exp(a + bX^) , (2.4) (where Nx are the number moving a distance X from the source and a, b and c arc constants) provided a good fit to most, though as a minimal concession to reality it would seem logical to consider separately those organisms not dispersing, for physiological or anatomical reasons. Yet even if one restricts one's attention to those animals that are potential migrants, there is no assurance that an equation found to describe the distribution of distances they move under one set of conditions will remain applicable under another. In particular, I suggest that specialist herbivores will alter their dispersal patterns as the distribution in space of their host plant changes. Even among insects that are generally thought to disperse in a relatively passive manner, such as aphids, there is considerable opportunity to influence the distance travelled and the likelihood of locating a suitable host. Johnson (1969) refers to what German workers have called the "befallsflug" of certain aphids, in which they retain sufficient fat reserves after landing on an unsuitable host to launch themselves on at least one more dispersal attempt. Kennedy et al.'s (1961) studies of the response of aphids in flight to the wavelength of radiation reflected from vegetation suggest an ability to distinguish appropriate .host plants at a distance, however crudely. Among insects that are primarily active dispersers, there is clearly more opportunity to influence the outcome of movement. An excellent demonstration of this is provided by Johnston and Heed (1975), who reanalyse the results of a number of workers who have studied the dispersal of Drosophila spp using release-recapture techniques. The usual experimental procedure in such studies is to place baited traps at fixed distances within a pasture or grassland that provides the insects with little additional food. Johnston 54 and Hood find that the mean-square dispersal distance tends to increase with the separation between traps, suggesting that Drosophila fly further the sparser the distribution of its food. Under more natural conditions, Johnston and Heed (1976) observe that the oligophagous D. nigrospiracula disperses significantly further in areas where its cactus host is relatively rare. Though perhaps not surprising, these results nonetheless should give us pause when considering the "dispersal range" of a given species and how we should go about measuring it. In the context of the present work, Johnston and Heed's findings suggest that as intervening fields are created by additional land being brought under cultivation or, within one season, through asynchrony, the proportion of dispersers moving between 2 given fields is likely to decrease. 2.5 Models of Passive Dispersal Nevertheless, there are some pests of rice whose dispersal characteristics may be considerably simpler. I am referring here in particular to fungal and bacterial plant pathogens. The movement of wind-dispersed spores is considered an entirely passive affair by plant pathologists (Zadoks and Schein 1979, Waggoner 1962) dependent on weather patterns, and likely to be little affected by the density of the crop in the landscape. There is as well no indication that the population density of pathogens affects the probability of dispersal, which is usually considered a simple function of distance (Van der Plank 1975). For such pests, the model as expressed in equation 2.3 may be of some use and an analytical solution is possible. Where the individual dispersal coefficients, mjj, and the number of fields remains constant over time, and the rate of increase of migrants and non-migrants is assumed to be the same (a not unreasonable assumption for organisms that 55 contribute nothing to their displacement), equation 2.4 reduces to: A / f i ~ (2'5) Here the vectors ^ { + 1 and E q refer to the numbers of dispersive spores immediately after landing and the fraction f not dispersing is incorporated in the mj j coefficient. Additional simplifications are possible by making use of matrix theory. M can be assumed to be a positive definite matrix (Searle 1966, Seneta 1980) as its elements represent the finite though possibly very small probability of dispersal between two fields. The Perron-Frobenius theorem (Searle op cit. , Pielou 1977) states that for any positive definite matrix, there exists a dominant root, X*, larger than all others; it is itself real and is associated with a dominant vector, all of whose elements are positive and real. When the within-field rates of increase, p. , are taken to be constant over fields and generations, equal to a mean value p^, in the asymptote, the numbers in each field will grow at a net rate pm X* and be found in proportions given by the dominant vector. A number of models based on essentially these assumptions have appeared in the plant pathology literature. Kiyosawa (1976) considered in strategic terms the spread of disease within fields of mixed susceptible and resistant (where p=0) varieties; despite the difference in scale from the present work, the basic population equation was as given in eq'n 2.5 with constant p. Plants were assumed to be distributed in a regular grid and dispersal between them to be governed by a simple exponential decay with distance. Kiyosawa however employed a brute-force simulation approach, calculating in each generation the numbers dispersing between pairs of plants. A simulation of growth over 3 generations for a grid 56 of 200x200 plants required more than 10 hours of computer time and had to be replaced by one 75'o smaller. A much more economical solution is possible making use of the dominant root and vector of the dispersal matrix, which may be calculated by very efficient routines available at many main-frame installations (NAG 1981). Zadoks and Kampmeijer (1977) have described a model of disease spread within a grid of fields, in order to assess the impact of changes in their average isolation, dimensions and the number of varieties planted. Again, their basic population equation is as above, save that they consider self-limitation of the population. Many of their results however pertain to conditions early in the development of an epidemic and the assumption of a pure exponential growth should make little difference. Pielou (1977) has also shown that at least one method of ammending the conceptually similar Leslie projection matrix to take account of density- dependent processes results at equilibrium in a distribution of organisms proportional to the dominant vector of the density-independent case. Like Kiyosawa, Kampmeijer and Zadoks perform a full matrix multiplication at each iteration, and complain that a simulation of one season for a grid of 40x40 fields requires more than 3 hours. Here again, calculation of the dominant root and vector would have saved considerable time. But even further simplification is possible under Kampmeijer and Zadoks' assumptions. Where the distances between fields in either direction in the grid are constant, and where dispersal is given by a gaussian diffusion equation, it can be shown that the dominant root is the product of the roots of two much smaller matrices, those governing dispersal between and within rows, reducing further, in the case where the two distances are equal and the grid square, to the square of the root of either matrix alone. The problem in 57 this case is that of finding the root of a Kroneker product of two matrices (Bellman I960, Scarle 1966). In both cases considered here, the use of matrix methods yields no additional insight into the processes at work; it provides merely a more efficient solution of the specified growth equation. In doing this however it may help to free scientific skill from concern with getting an answer to consideration of the form of the model and the validity of its assumptions. One point in particular that is emphasized by employing a formulation as in equations 2.3 and 2.5 is that the distribution and abundance of host plants affects not only pest numbers at equilibrium, a level that I suggest they may seldom reach, but perhaps more importantly the rate of population increase, by altering the probability of successful dispersal. I will return to this point below. 2.6 Insect Abundance and the Locality As I have suggested above, the dynamics of most insect pests within a region are not likely to be well represented by a model that assumes the probability of dispersal between two points to be a constant function of distance. The simplifications of equation 2.5 that are reasonable for organisms that disperse passively may not be in the case of the oligophagous insects with which we are most concerned. ‘This suggests that, under conditions of changing host plant distribution, it may be necessary to give up hope of determining explicitly the numbers in each field, though I believe it is still possible to say something about population dynamics within an area as a whole. To some extent, one is free to draw the boundaries around the area as widely or narrowly as one choses: one can speak of the numbers of a given species in an area of 1 hectare or 10,000. However, when the area is very small in relation to 58 the typical dispersal range of the pest, its population there will be poorly correlated with the probability of host location, the within-field growth rates or the level of colonization measured within the area. The approximation that we have made use of to this point, as in eq. 2.3, of ignoring immigration from outside the area subsequent to the colonizing generation becomes difficult to sustain. Conversely, if one considers an area much wider than the distance the pest moves, the numbers at the centre are essentially unaffected by conditions at the periphery. In what follows, I consider, rather than the numbers within an entire region, the density at the centre of what I have defined as the locality. This change in emphasis to point density will enable us to derive expectations for the behaviour of populations within large continuous tracts of cultivated land without needing to worry about edge effects. Density at a point is also readily measurable with monitoring tools such as light traps, whereas determining the numbers within an area requires a more onerous sampling effort. The question remains however of the size of the locality. Population genetics has confronted a similar problem in considering the breeding structure of mendelian populations. Wright (1978) has defined the genetic neighbourhood as an area with radius twice the standard deviation of the distance organisms move from their place of birth to where they reproduce. A large number of release-recapture experiments, involving Drosophila in particular has subsequently been carried out to determine neighbourhood size. A similar rigorous definition of locality might be adopted here and the experimental methods of population genetics borrowed. However, as pointed out earlier, release- recapture techniques are unlikely to be of use where the distribution of rice varies within and between seasons. An 59 alternative method that I will make use of in Chapter V is to seek correlations between the point density of pest populations and parameters of intensity and synchrony measured within increasing radii from the point. Beyond a certain distance defined as the bounds of the locality, these parameters would be found to add nothing further to one's ability to explain variation in point density; a more sophisticated variant might assume that the influence of the parameters declined monotonically with distance, as does dispersal, and would set the bounds of the locality at some conventional level of influence, say 5% of the maximum. Compared to release-recapture techniques, this approach has the advantage of determining, if indirectly, the distance organisms move under the conditions they actually confront during population development. I will return to this question in a later section and again in Chapter V; for the present, it is sufficient to assume that the dimensions of the locality can in principle be determined. In what follows, I use similar symbols to those in the preceding equations, though the definitions have been somewhat alterred. The density at the centre of a .locality in generation t+1, N^+-j , can be described as: ( 2. 6) where Eq» F, P and P' represent mean values over the area, possibly weighted by proximity to the centre, of the density of colonizers, the proportion not migrating and the within- field net rates of increase for dispersers and non dispersers respectively. Here E0 is measured before the insects have settled, i.e. it is an aerial density and they must locate a suitable field before reproducing, whereas in the case of pathogens considered in eq. 2.5, Eq was the vector of spore densities in the fields. Similarly, the 60 density at the centre in subsequent generations is measured during dispersal. Initially, I will assume the fields to be in approximate synchrony, so that the rates of increase P and P' are means calculated over conditions of similar host age, but in a later section this stipulation will be relaxed. Lp is the mean probability for those individuals that disperse of successfully reproducing in a field within the locality. This parameter is closely related to K * in equation 2.5. One of Brauer's (1962) theorems states that the dominant root of a positive definite matrix is greater than the smallest row sum and less than the greatest. For the matrix M, the sum of the elements 3 mjj represents the probability that females dispersing from field i oviposit in any other. Hence lies between the largest and the smallest probabilities of successful dispersal within the locality, as does the mean value, L. Largely for convenience, eq'n 2.6 can be simplified as: t (2.6a) A/t+, - £> T~ O 3'- In what follows, I refer to Q _S' as the local net rate of increase in generation g. Its value is affected by the within-field net rates of increase of migrants and non migrants, by the probability of dispersing and by the probability of locating a host for those that do. In practice however it will prove difficult to unambiguously assgn individuals censused to one generation A or another. This difficulty is compounded in the experimental work that follows which relies on light traps with a catching bias related to the phase of the moon. One way to avoid this problem is to consider the density of insects over a season that spans several months. I return to the operational definition of the parameter in the next 61 100 1 •/ / / / o\Np cn 50 - 100 AREA DEVOTED TO RICE % Figure 2.2 Probability of successfully locating a field, Lg, during a given generation in relation to the area devoted to rice cultivation, A. Both variables expressed as per cent of maximum attainable values. The dashed line represents a 1:1 ratio. to chapter; in theoretical terms however, SN, the seasonal density at the centre may be expressed as the sum of the density of adults produced in all generations: (2.7) where t' is the number of generations produced during one season. In many cases, conclusions drawn with respect to the numbers in a particular generation can be shown to apply equally to the seasonal total, the sum over all generations. Where their behaviour differs, I will derive expectations for both N and SN. S 2.7 Increasing the Extent of Cultivation It will be helpful here to define the parameter "A", the proportion of land under rice cultivation within a locality. The question I pose in this section, and one that will be important as well in later considerations, is how Lg changes as a function of A. I suggest that in general the relationship will be of the form illustrated in Fig .2.2, that is, the probability of successful dispersal, considered as a percentage of its maximum value when all land in the area is under cultivation, is at every point greater then the proportion actually cultivated and rises at a continually decreasing rate. Several factors will contribute to a relationship of this form: (1) Among aerially dispersing organisms, only those that fall out in an essentially rain-like fashion, as might pathogens from a uniform spore cloud, will land on rice fields in the same proportion that the fields occupy in the landscape. If dispersal is to any extent a declining function of distance, propagules will land in greatest measure on land near the source and in particular in the 63 field in which they originated. For this reason alone, Lg will bulge above the 45° line. This is a point that has been made by Kiyosawa and Shiyomi (1972) and Kiyosawa (1976), and applies to passive as well active dispersal. Moreover, as the expansion of cultivation is generally dependent on factors such as soil, water and proximity to settlements that are not randomly distributed, fields are likely to be aggregated in space. The probability of a passively dispersing organism landing in a field is thus greater if it originates in one than if it is distributed equiprobably over the whole area. (2) Even for an inefficient disperser that moves from a source in a pattern described by a random walk, the probability of locating another field approaches a maximum value when the mean distance between fields is greater than zero, that is to say, where the fields are not contiguous. Though a complete proof of this is not available, I believe it can be shown to follow from models of a random walk in two dimensions with absorbing barriers (Prabhu 1965). If this is so, it suggests that Lg approaches a maximum before A reaches 100%. (3) Among oligophagous insects however movement in search of a host is unlikely to be a random process: host- locating insects make use of distant cues such as odour (.Jones and Coaker 1977) and reflectance (Kennedy et al. 1961). Furthermore, their probabilities of turning appear to be adapted to the spatial distribution of the host. Murdie and Hassell (1973) showed in laboratory studies that houseflies turned in such a way as to efficiently locate an aggregated food source, resulting in a relationship as in Fig 2.2. Figure 2.2 closely resembles the shape of simple enzyme- substrate kinetics in biochemistry, where the x-variate is 64 □ A C D AREA DEVOTED TO RICE % Figure 2.3 Expected impact on pest dynamics of change in the area devoted to rice cultivation, A. a. The probability of successfully locating a field and the net rate of increase during the first generation. b. The net rate of increase in subsequent generations. c. Pest numbers during a given generation, g. d. Pest numbers during a given generation, taking account of threshold effects. 65 generally expressed in units of substrate concentration and the y-variate in those of the speed of the reaction (Lehninger 1975). There the degree of bulging above the 45° line is determined by the parameter K of the Michaelis- Menten equation, with low values indicating high enzyme affinity for the substrate. The analogy to host-specificity of herbivorous insects or indeed other consumers is close. Though the measurement of specificity is likely to prove difficult in practice, in the present context I will rely merely on assumptions about the general shape of the curve. Given a relationship between Lg and A as illustrated in Fig 2.2, what will be the impact on population in a given generation, Ng? In the first generation after transplanting, all adults are migrants and the local net rate of increase can be expressed as Q ,= LiP, . Plots of L1 or Q1 as a function of A would differ from each other only in scale (Fig 2.3a). In later generations however, when there is a resident population and dispersal beyond the field is not essential, Qg has a higher intercept determined by the values of Fg and Pg, the proportion of non-dispersers and their net rate of increase (Fig. 2.3b). In all generations the form of the relationship of Qg with A is broadly similar, its shape determined by the change of Lg with A. I have argued that the first derivative of Lg with respect to A is everywhere non-negative (increasing the proportion of land cultivated will make host-location easier, up to a maximum), while the second derivative is everywhere non-positive (the advantage to the pest decreases with increasing A). Given these minimal conditions, it can be shown by the product rule of differentiation that N{, determined as in equation 2.6a by the product of t-1 Qg values, will itself have a non-negative first and a non 66 positive second derivative. If all variables arc expressed as a proportion of the maximum they take when the landscape is filled with rice, it is also clear that at any given value of A, N t will be at least as small as the smallest Qn y and lie at least as close to the 45° line (Fig. 2.3c). A further factor however will influence the behavior of N^: at low values of A, Qg and in particular Q1 may be insufficient to permit the pest to become established in the area. If during the first generation, P1 < l/L^, the small number of migrants to the crop will suffer decline. In later generations, when a non-dispersing population is established, the level of A required to avoid population decline is reduced. These considerations suggest that in Fig 2.3c will be shifted somewhat to the right, as illustrated in Fig 2.3d. Even this limited knowledge about the form of the relationship of and A may be of some use. It suggests that a given increase in the proportion of land devoted to rice cultivation will result in a less than proportional increase in the local numbers of oligophagous insect, pests, and the increase is reduced at higher levels of A. Though there have been efforts to bring large uncultivated areas into production, as in the tidal swamplands of Kalimantan, Indonesia (Conway et al. 1983), expansion of cultivated area .in Asia appears to be for the most part incremental, occurring within landscapes already predominantly occupied by rice. It should be noted that this discussion pertains to density-independent growth. Where pests are well regulated by natural enemies, a given increase in the area cultivated is expected to produce a proportional increase in the local population: doubling the habitat when the equilibrium density is unchanged will result in a doubling of numbers. 67 2.8 Increasing the Number of Crops As discussed in Chapter I, major increases in Asian rice production have occurred due to the construction and extension of irrigation facilities, making possible dry- season cropping. A lesser contribution has been made by the introduction of short duration, photoperiod-insensitive varieties that allow farmers to attempt a second crop under rainfed conditions where the season is long enough and sufficiently reliable. To understand the impact of the introduction of a second crop on the abundance of pests, we require a population model that considers the entire annual cycle, making explicit the relationship between pest development in one crop period and that in another. Initially in what follows, I consider the case where the increase in rice cropping index ,fCI", the area planted to rice per year in a locality divided by the area so planted at least once, is integral, that is all farms are planted at least twice, thrice and so on. This condition will be relaxed in a later section. In the last generation of the first crop season, the point density at the centre of the locality will be: ( 2 . 8) and the seasonal density (2.9) The density at the end of the next crop, whether the next year in a single-crop area, or the next season where multi cropping is practised, can be expressed as: 68 ✓ and the seasonal density / ( 2. 11) where are the generation-specific net rates of increase in the next crop and 9 is the carryover coefficient, the ratio of the number of adults immigrating to the second crop to the number in the final generation of the first crop. My use of the term is similar to that of Kiyosawa (1972) and Kiyosawa and Shiyomi (1976) who considered year-to-year changes in the abundance of rice blast disease in Japan. As suggested earlier in this chapter, the rigours of cultivation and the longevity of the common insect pests ensure that no individual female oviposits in the field from which she emerged; carryover is possible only at a spatial scale larger than the field and depends on ."fallow strategies", combinations of survival, dispersal, and low- level multiplication on secondary hosts that are unique to each pest. . A central question is how 0 varies as the length of the fallow period decreases with the introduction of a second or third crop in the year. Though sufficiently accurate long term records are perhaps nowhere available, I assume that where one crop per year was grown, populations remained more or less constant over time. Were this the case, 0 would fluctuate around -i (TT Qg) r c 69 implying that increase during the crop was on average balanced by mortality through the fallow. One hypothesis regarding the impact of multiple cropping is that 0 is invariant with time, that as the fallow decreases from 6-9 months in one crop environments to 2-3 months where two are grown, carryover remains at approximately the same level. Such perfect regulation might be due to a scarcity of alternate hosts or sites to survive the fallow, or to an efficient natural enemy that attacked at this point when, its rate of increase at a minimum, the pest would be most vulnerable. If the potential for increase in the two seasons is similar, that is, if the Yg terms are approximately equal to the Qg, regulation in this manner by resources or natural enemies will produce a density in the second crop equal to that in the first and a yearly total in a twice-cropped area merely double that in a single-cropped locality. However, a stronger conclusion from this reasoning, one that requires no assumption about the behavior of populations in different seasons, is that density in the wet season will remain constant as the number of crops per year increases. Though the precise shape of the relationship between 0 and the length of the fallow is unknown for the major insect pests of rice, this hypothesis appears the least likely possibility. More plausible I suggest is a function as illustrated in Fig. 2.4. For an insect like YSB that survives the fallow as a quiescent late-instar larva, mortality can only increase with time. In particular, it is known that dessication of the rice stubble, which is progressive in an unirrigated field, adversely affects its survival (Khan 1967). Pests such as caseworm that subsist on alternate hosts on which their rate of increase, inclusive of dispersal losses, is negative will also be characterized by a monotonically decreasing © with length of the fallow. (That their rate of increase on these hosts is in fact negative is indicated, as will be shown in later chapters, 70 CARRYOVER h fallow. the ewe saos 0 o rdcin n h lnt of length the in reduction of 0, seasons, between iue . Epce ipc o pplto carryover population on impact Expected 2.4 Figure 0 UAIN F ALW MONTHS) (M FALLOW OF DURATION 3 7 1 6 9 by the fact that the numbers that reinfest a second crop arc much less than the final numbers in the preceding one.) BPH, being strictly monophagous and apparently without diapause or quiescent stages, must during the course of a prolonged fallow colonize a succession of rice hosts. With the exception of sparse wild or volunteer plants, locating these will require dispersal into areas of different cropping schedule. Consider a wet season crop immediately prior to the introduction of double-cropping. The numbers in the final generation in this first year are: ( 2. 12) In the subsequent dry season, numbers in the final generation will be: / ' (2.13) where 01 is the increased carryover between the two crops. Similarly, in the next wet season (season 1 in year 2): where 02 is the carryover between the dry and wet seasons. In general, final wet season numbers in any subsequent year x can be expressed as: 72 /\lt ' M - A/t' (>. 0(e, e^TT Y3 JT ^) x'; (2.15) implying that populations are increasing at an annual rate determined by the carryover coefficients between the seasons and the seasonal rates of increase. It can be shown that the seasonal total and the yearly total increase as well at this same exponential rate. Records do exist of population change over time at sites that have undergone intensification, including the light trap series at IRRI illustrated in Fig. 1.2 and that for YSB in North Krian, Malaysia given by Lim and Heong (1977). In both, there is evidence of exponential increase over a number of years. Were independent estimates available of seasonal rates of increase and of carryover coefficients for the different species, it would be possible to verify whether the yearly rates of increase were as predicted by the model. A simpler, yet I believe more telling test of the model is possible. It is assumed here that the shift in the annual balance between population growth in the crop and decline through the fallow has occurred due to reduction of the latter. This conclusion is testable with the available data. Note that of all the hypotheses that seek to account for the greater pest abundance in the wake of intensification (Table 1.1), only an increased period of rice availability is expected to result in such a pattern. All the others presume that the critical changes have occurred while the crop is in the field. Where adequate records exist however, it appears that the exponential increase of pest populations has abated in recent years. At which stage of the annual cycle is regulation most likely to occur? I suggest that as carryover 73 between crops is affected largely by the physical factors of weather, distance and time, particularly in the case of BPH and YSB, it is unlikely that this will show evidence of density-dependence. Within the crop, on the other hand, there are many more opportunities for adjustment, both by natural enemies and by man (who applies insecticide and plants resistant varieties). A second hypothesis then, weaker than the first, is that, where population levels have stabilized after intensification, the balance between increase during the crop and decline through the fallow will be reestablished by a reduction of the former. It should be emphasized that this need not imply perfect density- dependence in the seasonal rate of increase, such that higher numbers in the first generation are prevented from translating into higher numbers in the final generation. What is required is merely that the accounts for the year be in balance, that on averagelT C ITY' be reduced to equal 1/©102» which is possible when density-dependence within the crop is under-compensating. I have restricted my attention thus far to cases where the increase in rice cropping index, Cl, is integral. , Though change of this sort has occurred in some areas, for example where gravity irrigation schemes serve wide areas, in many instances multiple cropping has come about in piece-meal fashion, on those farms for example that benefit from .shallow-well pumps, or that are situated on heavier, water- retaining soils. What will be the effect on pest numbers of the introduction of a second crop on, say 50% of the rice land in a locality? I suggest that the problem is similar to that of the effect of changes in A, considered above. When only a few scattered farms plant a second crop, the probability of dispersing insects locating them is low, thus reducing the Y^ values and the numbers generated within the crop. At levels of Cl just greater than 1.0, Yg may be so low as to prevent a population becoming established during 74 that season. As the proportion of land double-cropped rises, the probability of an insect locating a rice field rapidly approaches a maximum, and as argued earlier, should follow in roughly parallel fashion. A relationship as illustrated in Fig. 2.3 may be anticipated. On this reasoning, it is clear that the behaviour of pest populations governed by density-independent proceses will be unreliably related to the rice cropping index, Cl. Consider two localities of equal A. In one, 50% of the land is planted just once and 50% three times, while in the other all land is cropped just twice. Both will have Cl = 2, but the argument advanced above suggests a greater capacity for increase in the first case. A more useful index would consider separately the number of crops and the area planted during each. I will return to this pointin the next chapter. A final matter. To this point, I have not considered the evolutionary changes that may be rquired to enable a pest to exploit a host that is suddenly present at a time of year when it was previously absent. If seasonal factors are involved in changing dispersal patterns or breaking diapause, the pest may initially gain no advantage from a newly-introduced second crop. Those individuals however that make use of cues more reliably linked to the presence of fice will be increasingly well represented in future in the population. In general there appears to have been little work done on the cues that govern critical events such as the emergence of stemborer adults from rice stubble, though in that particular case there are indications that moisture may be important (Rothschild 1971; Goot 1925, quoted in Khan 1967). It appears however that, at least within tropical Asia, natural selection has uncovered sufficient variation in phenology to enable population increase to occur at whatever time of year the rice host becomes available. To 75 the extent that a pest makes use of inappropriate seasonal cues upon the introduction of the new crop, the increase in numbers that has been considered above will be delayed until other phenotypes emerge within the population. 2.9 The Impact of Asynchrony Consider an area growing a given number of rice crops per year and with a given proportion of the land devoted to rice cultivation. Assume that varieties of similar maturity are grown on each farm, and that for a given pest these provide a duration D that it can exploit for population growth. Within any one field, a maximum of D/c generations can be produced, where c is the generation length of the pest. If however one considers only complete generations to the adult stage, then clearly only the integer part of that quotient will develop. The same obtains for a perfectly synchronized locality. Now assume that a proportion v of the fields in the area are delayed in planting by just more than c(l-f'), where f' is the fractional part of the quotient. The area as a whole will then be able to produce a further generation. Beyond that one however, additional generations will require delays in increments of c. Yet delays of more than c(l-f') in planting imply that the .colonizing generation invading the area confronts a reduced density of fields of (l-v)s (where s is the number of fields in the locality) and the additional generation emerging after the early planted fields are no longer habitable a density of v(s). Qg will hence be reduced to some extent in each generation. What will be the net effect of these 2 factors, an increased number of generations but reduced crop area? A precise answer is hampered by the lack of an expression comparable to the Michaelis-Menten equation to describe the relationship suggested in Figures 2.2 and 2.3 between Lg and Qg on the one hand and A on the other. A 76 partial answer that yet provides some insight is obtained as follows: Assume that Qg increases with A along the 45° line in Figs. 2.3a and b, which it was suggested underestimates the host-locating abilities of oligophagous insect pests. Then whereas a perfectly synchronized locality will produce a pest density in the final generation of / , ? £ 0TTQa (2-16) t a=o J ? an area with sufficient delay in planting to give rise to an additional generation will yield hia. (2.17) t + i The delay results in increased numbers if Rn > 1 where Rn /Ns^z , which entails that ( i-v) v Qt'+, > 1 (2.18) This is ensured by a large v: the product v(l-v) reaches a maximum of .25 when 50% of the fields are delayed. Thus Qt+1 must exceed 4 under these conditions for asynchrony to result in an increase. In general, the larger , the greater Rn, the impact of delay. The conditions are less stringent if one considers the seasonal density. Where a perfectly synchronous locality will give rise to £ ^ ^ (2.19) *c#v.iV|:VJ 77 delay sufficient to produce an extra generation results in / -t'+i t' t'~i /\j^ ^ bo l I TT~ f 7 T $j t i r Q j — #o) (2.20) An increase occurs again if R§ > 1 where Rs SNa/SNs, implying J _ -L- _i_ i + Q t ' + Qt ™-t-i • " 7r ( 2 .21) (l-V) Where the generation net rates of increase are greater than 1, the numerator on the right side of the equation will be close to 1, and when v is small, need itself be little greater than 1. However, Rn and Rs will be greater than calculated from the above equations, and the conditions for increase under asynchrony more easily met if the pest shows some aptitude for locating its host. In that case, (1-v) and v would be replaced by factors a0 and a^ ^ nearer 1, representing the degree to which Q0 and are depressed below the maximum were all s fields planted. Similar results can be shown to obtain as delays arise .sufficient to allow 2 or more additional generations. In general we can refer to the depression of the net local rate of increase in the gth generation as ag. If these values are close to 1, that is if the areas delayed are not so small and scattered as to make their location difficult, and further, if the local rates of increase in the additional generations Q^+2 *'* are s^mdar to (measured when the same number of fields are habitable), then an exponential increase of density in the final generation, d ’ *s exPec^ed with d, the delay. The seasonal density 78 however, under these conditions can be shown to increase slightly less than exponentially: where is constant, the increase is SN^' | = SN^'Q + Q (SNj . ^ 1 is the seasonal density over t'+l generations). This rapidly becomes indistinguishable from an exponential trend the larger is SN{ and Q. What would be the expectation were the pest well regulated by its natural enemies? If regulation occurred at densities near those at which it immigrates to the crop (E0), then delay in planting would not allow any greater population development. Regulation that became effective at some density greater than that or only after a period of several generations would be reflected in a relationship of ^ and SNt+j with d that was less than exponential. An ecologically meaningful index of asynchrony would take into account both the length of delays in planting, expressed in terms of generations, and the probability of the pest locating the areas so delayed. However, besides the fact that the host-location functions suggested in Figures 2.2 and 2.3 are unknown for all rice pests, the derivation of a useful measure of asynchrony is complicated by the non- random spatial distribution of planting dates. Delays are likely to be associated with geographically localized factors such as irrigation, soil or possibly labour and Credit shortages. (These aspects are considered in detail for a specific area in Chapter IV.) A delay of more than a generation is likely to have a greater impact on pest densities in the immediate vicinity than on those at a considerable distance relative to the pest's dispersal capabilities. Essentially, this is to bring into question the size of the locality for the pest. What is the area assumed on the abcissa of the host-location functions? How large is it necessary to draw one's frame when measuring asynchrony, or for that matter any of the other aspects of 79 intensification which we have considered, the area under cultivation, or the number of crops per year? The index of asynchrony that I will make use of in Chapters IV and V is the standard deviation of planting date evaluated within a defined radius from a sampling point. Details of its calculation will be considered there. It will be shown in Chapter IV that the distribution of planting dates is often approximately normal and hence that from their standard deviation it is possible to determine the proportion of the rice area that is delayed beyond a given number of days. If, as has been assumed here, population growth is largely density independent, then it is clear that for a given standard deviation measured in days rather than in generations the increase in the logarithm of final or seasonal density should be proportional to a pest's intrinsic rate of increase, r. The ability of the different pests to locate and exploit delay will also vary. I suggest that the most important differences are in their flight range and hence that the population of a vagile pest will be correlated with the standard deviation measured within a wider radius than that of a more sedentary one. Indeed this provides an operational means of assessing the size of the locality, though confirmation should be sought from more conventional .dispersal studies. However, it is also conceivable that pests will differ in their ability to make use of the tails of the distribution of planting dates, that in an area lying within the flight range of all pests some will be more responsive to the odd, isolated field they pass over. The impact of a given standard deviation within this area will depend then not only on r, but on the pests' "affinity" for rice as well. 2.9.1 Population Distribution within the Locality 80 Thus far we have been concerned with the aerial density of pests, either in a given generation or over a season, at the centre of a locality, without considering the number of fields from which they have emerged. Given fairly unrestrictive conditions, it can be shown that if there is sufficient delay to sustain a further generation, density per field at a given stage of crop development will be higher in late-planted than in early-planted fields, and will increase at a faster rate. This is demonstrated as follows: th In the t generation after the early fields have been planted a total of fc. o IF Q N a J ( 2 . 22 ) adults will be produced and if 1 < t < t', it may be assumed that these have emerged from the full area of s fields, hence with mean density N^/s. A generation later, the population will be A/a t+, = / ^ t Qt (2.23) again distributed over s fields. The slope of the increase • in density is then h ~ ^<1 (®t + l ^ (2.24) ear/y ------ (Clearly if all the Qg = Q, the slope is that of a discrete generation exponential growth model, and b - ^ (2.24a) C 81 When fields delayed in planting by a generation reach t, that is t+1 generations from the time the first fields were planted, the aerial density in the locality is as given in equation 2.23, and, if t+1 < t', the mean density in the field will be N^/s. A generation later, the numbers will have become (2.25) and the density N^^/s. Hence over the same span of time after transplanting, infestation in the late-planted fields increases with slope (2.26) C and by , \ b / where (2.27) for + i ^ 1. If the net rate of increase can be assumed constant over generations the condition is met for all Q } 1; where it varies, equation 2.27 implies it need under no circumstances be greater than 2 to ensure that the slope of . the time trend of infestation be steeper in late-planted fields. If the growth stage considered occurs towards the end of the crop so that when late planted fields reach that point the number of fields is reduced to v(s), the pests' overall numbers in the locality are reduced by the factor a^^'. However because a^-j' is greater than v for an efficient host-locating pest, the mean density will be greater than were the full area planted. Hence the slope of infestation will rise even faster in late-planted fields if calculated up to this final generation. This simple, yet not overly simplistic analysis rests on the assumption that younger fields are no more attractive to dispersing adults than older fields still capable of sustaining growth, and that the rate of increase is more closely tied to calendar time than to physiological time in the crop. If adults do oviposit by preference in younger fields, and if at any moment their rate of increase there is greater, the above patterns will be accentuated: infestation at a given time after planting will be even greater in delayed fields than in those planted earlier and will increase at an even faster rate. An experimental test of these predictions will be described in Chapter V. Before leaving the subject however I wish to draw attention to some of the social consequences of the above conclusions. As will become clear in Chapters IV and VII, delay in planting is often associated with poor access to and inadequate levels of crucial factors of production such as irrigation, credit and labour, and hence with reduced yield and net farm income. If in addition the impact of increased pest populations is greatest in, those fields, asynchrony may be expected to exacerbate economic inequalities within farming communities. 2.9.2 Further Considerations Several points require clarification. Firstly, the above analysis does not apply if the delay in planting is so great as to create what is essentially another season. The definition of "season" must be one that is relevant to the pest; I suggest that when the period of suitability "D" in two fields within a locality no longer overlaps, to the extent that the gap created exceeds the oviposition period of the female, then provided there are no fields planted at intermediate dates, the fields can be said to be in 83 different seasons. In this case, the individuals that invade the late-planted field are the progeny of insects that have reverted to "fallow strategies" and the discussion in section 2.8 becomes relevant. Secondly, it is important to consider to what extent the conclusions arrived at earlier are dependent on the possibly constraining assumptions that have been made. In particular, it has been argued that the response of a population to increasing delay will be step-like, with the length of the step given by the generation length of the pest (save in the first additional generation where it is only (l-f')c). The implications of this are important, for it suggests that delay of less than approximately one generation will have no impact on pest populations and that efforts to synchronize planting to within much less than this are unnecessary. How robust is this conclusion? I do not proceed here to a full consideration of alternative modelling approaches, but I suggest that crucial determinants of the response to delay are (i) the variability of generation length and (ii) the stage of the life-cycle that causes damage. It can readily be shown that, where the generation length of the pest is taken as constant, the step-like nature of population change and hence of response to delay necessarily follow from the fact that its age-structure is initially highly uneven: the field is colonized only by adults. Diffuse and continuous immigration is not sufficient to upset this pattern. Consider the simple case where immigration to an approximately synchronous locality is at a constant daily rate e, not merely during the colonizing generation as assumed hitherto. Assume that the adult of the species 84 disperses for one day of its lifespan, during which it may be trapped. Then given a fixed generation length c, constant net rate of increase Q and taking the time step of the difference equation as one day, N t - (/Vt-C <5 )+ e (2.28) and for t < c = e (2.29) Then in general, (2.30) where w is the integer part of the quotient t/c. The growth of the population is clearly periodic and approximately exponential at a rate Q, the approximation improving with time and the extent to which Q exceeds 1. Variability in c however would entail that Nj. in eq. 2.28 was a function not merely of N^_c, but of numbers on many other days. The result would be a more even age structure that would respond in a more continuous fashion to the additional time that delay makes available. It must be recognized that even if variability in c is small, implying that the population of adults will not respond to delay until it exceeds approximately (l-f')c, the numbers of earlier stages clearly will increase when less than this amount of time becomes available. As larvae or nymphs are often responsible for damage to the crop, delay within a locality may be insufficient to be detected in 85 terms of light trap catches, but not as regards economic loss. Finally, it is important to consider the impact of delay on annual as well as seasonal numbers. Within a locality, asynchrony both increases the amount of time available for population increase and reduces the length of the fallow that pests need endure. (If the number of rice crops per year remains constant however, the mean length of the fallow within fields remains unchanged.) It is expected then that asynchrony will increase 0, the carryover coefficient between seasons, as well as the overall increase through the crop, TT Qg. I will not rigorously develop this point here, as in Chapter V my concern will be with evidence of asynchrony's impact primarily within one crop season. 2.10 Changing Maturity of Varieties In many parts of Asia, the traditional rice varieties planted by farmers were photoperiod-sensitive, maturing at a specific time of year, generally after the cessation of the rains. In Central Luzon, Philippines, prior . to the development of reservoir-fed irrigation and the dissemination of the new varieties, the usual time of planting was in May-June and of harvesting in November- December (McLennan 1980) . The IR-series cultivars that are • now dominant are insensitive to day length and mature 3-4 months after transplanting, regardless of time of year. What will be the impact on pest populations of this reduced duration of host plant availability? It is important first to note that the reduction in the length of the crop has not been equally spread over all growth stages; rather it has occurred almost entirely during the vegetative phase, as photoperiod-sensitivity delays the formation of the flower-bearing panicle (Yoshida 1978). A 86 pest such as the rice bug, Leptococorisa acuta, that attacks the plant during ripening is likely to be unaffected by this change. However, all the major pests with which we are here concerned invade the crop during the vegetative phase and thus will confront a reduced useful duration in areas where the modern varieties have been adopted. The argument proceeds much as in the case of asynchrony Consider an approximately synchronous area in which varieties are planted that for a given pest provide a duration D that it is able to exploit. If the generation length of the pest is c, then a maximum of D/c generations can be produced in a season. However, considering complete generations to the adult stage, only the integer portion of the quotient will develop. If the maturity of the varieties planted is reduced by c(l+f'), then one fewer generation can be produced; a further reduction of c will remove another generation. As argued above, if the Qg are similar, the decline in numbers in the final generation of the crop as D is reduced will be approximately exponential, and that of the seasonal density only slightly less than exponential. If D is expressed in days rather than in generations* the reduction in the logarithm of the population should be proportional to r, the intrinsic rate of increase. As in the case of asynchrony, these relationships may be partially obscured by the threshold requirement: if f' is close to 1, reductions in maturity of almost a generation will have no effect on the number of generations that can develop in the crop. 2.11 Conclusions The concern of this chapter has been to develop a mathematical representation of pest populations within agricultural landscapes, one capable of predicting the impact of agrarian change on their dynamics. The questions 87 that have been posed are not specific to rice cultivation and the approach I have sketched may be of some use in other contexts. The dynamics of herbivore populations exploiting resources that are both patchy and transient is not a subject that has attracted a great deal of attention from theoretical ecologists, but is one that may repay closer consideration in terms of practical benefits. The chief conclusions from the model developed here are the following: (1) The response of pest populations to spatial intensification is likely to be no more than proportional to the increase in area devoted to cultivation. Given that in Asia the scope for bringing new land under the plough has been limited, this factor is not likely to have contributed greatly to increased pest densities (2.7.1). (2) With the introduction of dry season, or in general multiple cropping, the carryover of pest populations is likely to increase, raising the levels that colonize new plantings. As carryover is for several species governed largely by physical factors of weather, space and time, it is unlikely to be density dependent. In contrast, numerous inter- and intra-specific forces act on population growth during the crop period and density dependence, though not necessarily fully compensating, is more probable. Multiple cropping will thus make its impact on pest populations apparent through increased carryover, with growth rates within season no higher than under single crop conditions In this it is distinguished from the other elements of the Green Revolution which, with the exception of increased asynchrony are likely to affect rates of increase (2.8). 88 (3) Under fairly unrestrictive conditions, non-sedentary pests will be favoured by asynchrony of planting. The response of pest populations to asynchrony will be proportional to the realized rate of increase (2.9). Within a locality, pest densities are expected to be increased in fields delayed in planting by more than a generation and to rise at a faster rate than in those planted earlier. As farmers who are delayed are often found to be socially and economically disadvantaged, asynchrony will tend to exacerbate inequalities (2.9.1). Variation in planting date with a range of less than a generation is unlikely to increase pest densities in the locality. However, this threshold may be obscured by variability in generation length within the population (2.9.2). (4) Finally, the changing maturity of varieties is similarly expected to alter the time available for population increase. The response of pest populations will again be determined principally by generation length,and the rate of increase (2.10). 89 CHAPTER THREE Population Dynamics under Intensification 3.1 Introduction The clearest test of hypotheses regarding the impact of agricultural change on pest populations is likely to be found by examining the pattern of response at sites that have undergone intensification. In the following sections, I make use of time series of light trap catches at two locations, the IRRI experimental farm at Los Banos, in Laguna province, Philippines and Titi Serong in North Krian State, Malaysia. Records spanning the period of marked agricultural change are available for only a few pest species however. For many, particularly BPH, that became of concern and was censused after intensification was well underway, only between-site comparisons are possible. The second half of this chapter is concerned with such analyses, drawing on light trap records at a number of sites within the Philippines. To both types of data, I apply analytical methods designed to estimate both the degree of density dependence in population processes and the impact of different elements of the intensification syndrome. 3.2 Intensification at One Site 3.2.1 Laguna : the site and the data The International Rice Research Institute farm occupies some 500 ha in Los Banos, Laguna, flanked on the north and east by an extensive area of small hold rice cultivation on the lacustrine plain surrounding Laguna de Bay. Surveys conducted by the University of the Philippines and IRRI from 1966 to 1981 (summarised in Kikuchi et al. 1982), indicate a rapid adoption by farmers of modern 90 varieties in the late 1960's and an increasing use of purchased inputs, notably agrochemicals through the 1960's and 70's. Figure 3.1 illustrates the trend of fertilizer and pesticide use, two of the factors that have been implicated in the increased abundance of the major insect pests (Table 1.1). Accurate data on spatio-temperal intensity and asynchrony over the period are harder to obtain. No long term records of these factors are available for the IRRI farm itself, though a survey in 1982 (Loevinsohn et al. 1982), disclosed a cropping index of 1.43 and a wet season standard deviation of 37 days within 1 km of one of the light traps installed there. Somewhat better information has been obtained through the courtesy of the provincial superintendent's office of the National Irrigation Administration (NIA) for the surrounding areas served by gravity irrigation. Figure 3.2 illustrates the area benefitted in the dry season, expressed as a proportion of that during the wet season, in the adjacent Mabacan and Sta. Cruz River Irrigation Systems. As few farmers are able to plant a 3rd time, the cropping index, Cl, is approximately equal to this proportion plus 1. Dependable data are unfortunately not available prior to 1969/70, nor at a finer spatial scale, from which indices of asynchrony and intensity as a function of distance from the light traps might be calculated. Figure 3.2 however makes clear that an increase in cropping intensity has been part of the recent process of agricultural change in this area of Laguna. This appears to have been made possible by rehabilitation of the canal infrastructure and improvements in operational procedures to extend the area planted during the dry season (Angeles 1973), rather than by new construction. Indeed several of the irrigation system in the province data from the Spanish era, but as most are of the river-diversion type, seasonal water shortage has been a major problem in the past (Stansby and Stisen 1973). Three fluorescent light traps of the Pennsylvania design have been in operation at IRRI since 1963, though nightly catches throughout 91 ) a INPUT INPUT USE (KG PADDY EQUIVALENT / H 1966 1970 1975 1978 1981 Figure 3.1 Changes in farming practices in Laguna province, Philippines: □ per cent of farmers planting modern varieties, a fertilizer use, a insecticide use. Both the latter variables are expressed as rough rice equivalent per hectare. Data from Kikuchi et al. ( 1981). 92 DRY / WET SEASON AREA (%) 100 40 60 80 20 o aalbe ro t 16. aa orey f the of courtesy Data 1969. to prior available not riain ytms Lgn. eedbe eod are records Superintendent. Dependable Irrigation Provincial Laguna. s, system Irrigation e saos n h Mbcn n Sna rz River Cruz Santa and Mabacan the in seasons wet - iue . Rto f ra patd n h dy and dry the in planted areas of Ratio 3.2 Figure - - 1969 RIAIN YEAR IRRIGATION 70 T ------ 71 1 ------ 72 1 ------ 73 1 ------93 74 1 ------ 76 1 ------ 77 1“ 1 78 the year have been recorded only since 1965. Attention in the early years focused on YSB and Chilo suprcssalis and it was not until 1967 that monitoring of BPH and GLH began, and 1975 for CW and leaffolder. The trends of annual catches, summed over the 3 traps, of YSB, BPH and GLH were presented in Fig. 1.2. In what follows, I examine in greater detail the data for YSB, the only one of the principal pests on which this thesis focuses to have been monitored prior to its period of increase. In this and subsequent chapters I will refer to identifiable features of the seasonal trends of pest population as measured in light traps. These are illustrated in Fig. 3.3 which graphs the 4-weekly catches of YSB in the IRRI light traps through a year. I define the "trough" as the minimum 4-weekly catch prior to the increase during a given season, and the "peak" as the largest such catch. Four weekly totals are used in order to avoid the effects of lunar phase. Finally, the seasonal total is calculated as the summed catch from the 4-weekly period following the trough up to and including the subsequent trough. My concern for the most part in what follows is with the wet season total. As the area planted in the wet season appears to have changed relatively little, differences in the number caught at light are likely to more closely reflect changes in pest density in the field and levels of damage than are dry season catches. In the vicinity of IRRI, the dry season crop is planted in October-December and the wet season crop in April-June; thus what I have indicated in Fig. 3.3 as the dry season population peak actually occurs at the beginning of the wet season crop, and vice versa. The identification however is clear, for numbers begin to build up during the latter stages of a crop, as would be expected given YSB's relatively long generation length of around 45 days and the fact that it infests rice from the vegetative through to the ripening stage. The pattern is different for the pests that will be considered in the second half of this chapter and again in Chapter V : caseworm for example, which infests vegetative stage rice primarily and which has a generation length roughly half as long as YSB, builds up rapidly and reaches a peak soon after the crop is planted. 94 catch Per 4 weeks vr ya i te RI ih tas Te filled The traps. light IRRI the in year a over ybl idct te esnl ek, h open the text. the in peaks, defined as seasonal troughs, the the symbols indicate symbols ouain fu-eky ace o ylo stemborer yellow of catches four-weekly population: iue . Faue o a esnly fluctuating seasonally a of Features 3.3 Figure r sao ^ we season et w < ^ season dry < 95 Three further quantities can be computed. The first is the seasonal rate of increase, defined as: rg = In (peak catch/ catch in preceding trough). Similarly, the rate of increase per month is: ■ month = rs/n (3.1) where n is the number of months elapsed between the trough and the peak. Finally, the wet season-to-wet-season carryover coefficient is calculated as: ©ww = ln (trough preceding the wet season in year x / peak in the wet season of year x - 1) Carryover between any two other seasons can be similarly defined. As suggested in the last chapter, carryover is expected to be increased in years when irrigation permits large areas to be planted during the dry season or when there is substantial asynchrony in either season. In what follows I will calculate regressions of the form: ln N2 = a + b ln Nj Then In N2 - In NJ[ = a +(b-l) ln (3.2) 96 When Nj represents the trough and the subsequent peak numbers, the left-hand side of eq'n 3.2 is r When is peak numbers in year x-1 and trough numbers in year x, the left-hand side of 3.2 is ®ww’x- A value o£ b = 0 implies that rg or 0WW x is inversely proportional to Nj and compensates exactly for variation in N^. If b = 1, rs or -Q is equal to a constant and is independent of density. Values of b between 0 and 1 indicate undercompensating density dependence, while b -1 suggests inverse density dependence, that is, r or -©ww increases at higher initial numbers. The O goodness of fit of the regression, measured by R , indicates the amount of variation in N 2 that is systematic, that is, related to Nj, whereas the value of the slope is indicative of the extent to which systematic variation is compensatory. This interpretation is essentially that of Morris (1963), Varley et al. (1973) and Southwood (1967, 1978). It avoids the statistical problems of regressing rg or ©ww themselves against (the same as those in regressing k-factors against (Southwood 1978)), yet provides an equally sensitive index of density dependence. Elsewhere in the literature and ^ are generally measured at successive stages within a generation (e.g. the studies analysed by Stubbs 1977), but there appears to be no reason why the analysis cannot be applied between generations, as long as the population does not undergo extreme fluctuations in the interim (Southwood, 1967). Indeed Kuno and Hokyo (1970) have done so, using field counts, for BPH and GLH in Japan. 3.2.2 . Results Figs. 3.4 a and b illustrate the trends of the logarithm of wet season total and peak catches respectively in the IRRI light traps between 1965 and 1979. The significant increase in the wet season total suggests a real increase in stemborer density over the period. The increase in the peak catch is to be expected as it of course forms a large part of the seasonal total. In both regressions note the apparent levelling off in recent years, particularly in Fig. 3.4 a. In this and following sections I attempt to account for this pattern. 9S Figure 3.4 Trend of wet season catches of yellow stemborer in the IRRI light traps. A. Seasonal total - In Y = -.769 + .129 X (t = 4.32, P < .001) B. Peak catch In Y = -.395 + .107 X (t = 3.42, P <.01 ) In Figure 3.5, I plot the logarithm of the wet season peak catch against the catch in the preceeding trough. The slope is found to differ significantly from 0 (t = 2.31, P < .05). However there is one extreme outlier, the wet season of 1966, lying 2.75 standard deviations below its expected value. Though no obvious explanation can be suggested at this time, 18 years later, the pattern of population development, or recording, appears to have been distinctly anomalous that season. The regression calculated when this point is excluded is illustrated in the figure. The slope is found to differ significantly from both 0 and 1, suggesting undercompensating density dependance in the seasonal growth rate, r . However, as in a regression of this sort both x- and y-variates are subject to error, a Type II regression (Sokal and Rohlf, 1969, Snedecor and Cochran, 1967) is more appropriate than the usual Type I (least squares) method. Using Bartlett's three group technique (Sokal and Rohlf, op. cit.), a slope of A 5 9 is obtained, close to the least squares value, and with 9 5 % confidence limits of .658 and .231. The interpretation is thus reinforced that population growth during the wet season is undercompensating. Inspection of eq'n 3.1 suggests that an equilibrium is reached only if a > 0 and 0 < b < 1 when N = N? = _a_ (3.3) 1 1-b Using the least squares parameter values, an equilibrium would thus be expected at Nj = ^ = 8700, more than 20 times the highest initial numbers yet recorded. In Fig. 3.6, I illustrate the regression of trough numbers prior to the wet season on peak numbers in the dry season immediately preceding 99 LOO WET SEASON PEAK 6 -- - . lp dfes infcnl bt fo 0 t 40, P< 4.05, = (t 0 from both significantly differs slope hn h otyn pit s xldd n 1. The 14. = n excluded. is point outlying the when is: ruh Te ersin siae b least-squares by preceding estimated the in regression numbers The to relation in trough. stemborer iue . Pa wt esn ac o yellow of catch season wet Peak 3.5 Figure 0) n fo 1 t 39, 01). 1 .0 < P 3.98, = (t 1 from and .01) 4 ------O e WET LOGe n N In 4. 1 ------100 2 54 n - + 4.96 + N-j In .504 - K i — ES N TROUGH SEASON . 6. 5. ------1 ------1 ------0 h LOG WET SEASON TROUGH .6 P 01, u nt rm ( = 0, NS). = .03, (t = 0 (t from 1 from not significantly but differs .001), ^ slope P The 4.66, 15. = n es-qae is: least-squares esn n eain o ubr a te ek f the of peak the at numbers to relation in season iue . Cth n h tog pir o h wet the to prior trough the in Catch 3.6 Figure rcdn dy esn Te ersin siae by estimated regression The season. dry preceding n = 94 n - .5 . 1.95 - N In .994 = N In 1 2 101 it. Here, in contrast to the previous case, b is found to differ significantly from 0 but is almost precisely equal to 1, suggesting that carryover through the fallow between these seasons is essentially density independent. Bartlett's technique yields a slightly higher value, as is generally the case (Sokal and Rohlf, op. cit.) whose lower confidence limit just includes 1.0 (b = 1.09, 1.00, 1.19). If trough numbers prior to the wet season are regressed on peak numbers in the wet season of the year previous (Fig. 3.7), a slope is obtained that again does not differ significantly from 1 by either ordinary least squares, as illustrated in the graph, or Bartlett's technique (b = .937; CL^^:. 800 and 1.08). This suggests that over this longer period carryover is still largely independent of density. The filled circles in Fig 3.7 represent the numbers in the years 1971 to 1979. Six of these 9 cases are observed to lie above the computed regression line, suggestive of greater than average carryover. This is to be expected for these were years when double cropping was most extensively practised. The contention finds support in the more detailed picture presented in Table 3.1, which lists 0 and r by year, along with their sum, r , which I refer to as the annual net rate of increase. During the period prior to 1971, r a was negative in 3 years and positive in 2 (2 and 1* years respectively if one excludes the anomalous peak catch in 1966). In the 9 years subsequent, r a was positive 6 times and, in particular, during 4 of the 3 years 1971 - 5 when wet season numbers were increasing markedly (Fig. 3 A a and b). This much is almost implicit in those graphs, for when numbers are increasing, r must be positive. What is not self evident is the pattern of change in its components,K 7 © ww and r . s Between 1971 and 1979, 0 7 ww averaged ° -2.118 _+ .209 while in the period prior to 1971 it was -2.836 .257, (when the 1966 value is excluded), and the difference is found to be significant by the appropriate one-tailed test (t^ = 1.99, .025 < P< .05). These figures indicate that, expressed in terms of proportions, carryover doubled from 5.9% to 12.0% over the period. 102 LOG- WET SEASON TROUGH (t+1) pn ice rpeet h yas rm 96 o 1970 to 1966 from years the represent circles open n te ild ice fo 17 t 1979. to 1971 from circles filled the and .3 P^. ) bt o fo 1 t .9 N) The NS). .49, = (t = 1 (t 0 from not from but significantly l), .O ^ differs P slope The 3.33, 14. = n is: least-squares rcdn wt esn Te ersin siae by estimated the wet of regression The the peak the to at prior numbers season. trough to wet the relation in Catch preceding in season 3.7 Figure O e E SAO PA (t) PEAK SEASON WET LOGe n 2 .7 I N - .9 . 1.39 - N1 In .871 = N2 In 103 Table 3.1 Carryover between wet seasons (0 ww ) and wet season (r ) s and annual (r ) a rates of increase of yellow stemborer, calculated from light traps at the IRRI farm. Ail values expresses in natural logarithms. Data. courtesy of V.A. Dyck. Year 0 r w w •Ls — a 1965 - 3.680 - 1966 -2.232 (.535) (-1.697) 1967 (-2.118) 2.823 (.705) 1968 -2.801 3.256 .455 1969 -2.820 2.404 -.416 1970 -3.489 3.300 -.189 1971 -1.343 1.942 .599 1972 -2.683 2.786 .103 1973 -1.248 2.016 .768 1974 -2.493 1.618 -.875 1975 -1.837 3.027 1.190 1976 -2.996 2.344 -.652 1977 -2.210 2.300 .090 1978 -1.639 2.301 .662 1979 -2.615 2.075 -.540 104 In contrast, seasonal growth rates have declined over this same time. Excluding once again the 1966 value, rs averaged 3.093 _f .219 prior to 1971, but only 2.268 _+ .K3 from 1971 to 1979. The difference is again significant (t^ - 3.28, P< .01). A similar reduction is apparent in the net rate of increase expressed on a monthly basis, rmont|1 : it declines from .759 _+ .125 to .670 +_ .111, though in this case the difference is not statistically significant (t^ = .45, NS). I will return to discuss these results in more depth after examining the Malaysian data. I wish first however to summarise the principal findings to this point. The marked rise in the wet season peak population of Y5B from the early 1970's has been associated with an increased rate of carryover between wet seasons, rather than with increased growth rates within a season; indeed growth rates have been shown to have declined in density dependent fashion. Furthermore, and as expected, the improved carryover occurred at a time when rehabilitation of irrigation systems in the vicinity of IRRI permitted an expansion in the farm area planted in the dry season. 3.2.3 Intensification in Malaysia The process of agricultural change in North Krian appears to have included the same key elements as in Laguna. Traditional photoperiod-sensitive varieties were replaced by fertilizer-responsive short duration varieties, for the most part bred in national institutions (Lim and Heong, 1977). The use of agrochemicals, particularly insecticides, increased markedly, though apparently not to the levels reached in Laguna (K.L. Heong, pers. comm.). Roughly contemporaneous with these changes, gravity irrigation was introduced and dry season cropping over wide areas became possible. 1 Here a two-tailed test would seem most appropriate as there had been no expectation from the analysis of the previous chapter of density dependence or of declining growth rate with time . 105 A light trap of unspecified design was operated at Titi Serong in N, Krian from 1959 to 1975 and records have been made available through the courtesy of Dr. G.S. Lim. Data is available for 3 species : the dark headed stemborer Chilo polychrysus, YSB and GLH, though for differing numbers of years. The trend of annual totals of the first 2 species was presented in Lim and Heong (op. cit.), from which information is also available on the spread of double cropping in the vicinity. 3.2.3.1 Results It will be recalled from section 2.2 that, through the period of intensification, catches of YSB in the IRRI light traps increased proportionately relative to those of Chilo suppressalis. A similar pattern is apparent in Figure 3.8, redrawn from Lim and Heong (op. cit), which illustrates the trend of the annual total of YSB and C. polychrysus catches at Titi Serong. Again there is evidence that YSB has increased relative to the other principal stemborer species occurring with it. In Chapter II, I argued that intensification of rice cultivation in space and time will favour mono-or oligophagous pest habits to the extent that specialization is associated with greater efficiency in locating and exploiting the crop. While YSB is entirely monophagous, both Chilo species have alternative hosts (Grist and Lever 1969). Though at IRRI, C^ suppressalis was found merely to have increased at a slower rate than YSB, at Titi Serong C^ polychrysus can be seen to have declined in absolute terms. An explanation for this difference must be sought in the detailed biology of the 2 Chilo species. A more striking similarity in the pattern of response to agricultural change is revealed in the detailed dynamics of YSB. In Fig. 3.9, I plot the peak population in the wet season against that in the trough immediately preceding. The regression is significant and the slope is within 12% of the value calculated at IRRI and not significantly different from it, suggesting again that population increase through the crop is density dependent but undercompensating. * 106 CO TOTAL X o 15 6 9 - - ra, aasa Rdan rm i ad en (1977). Heong and Lim from Redrawn Malaysia. Krian, 1964 hl plcrss n ih tas t ii eog N. and Serong, stemborer Titi at yellow traps of light total in Annual polychrysus Chilo 3.8 Figure 66 107 68 70 / \ 72 74 LOG. PEAK CATCH 8.5 = 2 Te lp dfes infcnl bt fo 0 from both significantly differs slope The 12. = n aa orey f S Lim. .S. G of courtesy Data s: is t 40) P .1 ad rm ( = .7 P .001). P< 5.17, = (t 1 from and .01) P< 4.09), = (t ruh t ii eog Te es-qae equation least-squares The Serong. Titi at trough tmoe i rlto t nmes n h preceding the in numbers to relation in stemborer iue . Pa wt esn ac o yellow of catch season wet Peak 3.9 Figure n 2 .4 I N, 56 . 5.63 + N., In .442 = N2 In 108 o o When numbers in the trough prior to the wet season are regressed against peak numbers in the preceding wet season, a slope of near 1 is obtained (In = -3.48 + 1.03 In ; R^ = .28, n = ll) but there is considerable scatter and the slope is not significantly different from 0 (t^ = 1.85). A great deal of the variation however appears to be associated with the expansion, beginning in the mid-1960's, of dry season cultivation. In Figure 3.10, the carryover coefficient from wet season to wet season is plotted against the proportion of farm area in the vicinity of the trap that is double cropped. A clear positive relationship is apparent. Interestingly, the mean carryover rate in the 4 most recent years, when double cropping was practised on more than 80% of the area, was 10.7%, compared to 12.0% at IRRI in the years 1971-79 when double cropping was dominant. A somewhat different picture emerges with respect to GLH. When wet season peak numbers are regressed against numbers in the trough preceding it, a slope is obtained that does not differ significantly from 0 (In = 10.8 + .0187 In N^; R^ = .002, t^ = .18 NS). Similar findings have been reported from Japan by Kuno and Hokyo (1970), who analysed field counts of GLH in experimental plots: slopes near 0 they suggested were indicative of strong natural regulatory mechanisms. However in the present context, under uncontrolled conditions, the impact of human intervention, in the form of insecticide application and the replacement of susceptible by resistant varieties cannot be ruled out. The latter factor may be particularly important in the case of GLH (whereas no effective varietal resistance is available against YSB), and Lim and Heong (op cit.) note an increasing adoption of resistant varieties from the late 1960's. 1 The intercept? of the regressions at the 2 sites are not directly comparable as they are affected by such factors as the differing catching efficiences of the traps employed. 109 o> o ra double area ------1 ------ih ta dt cuts o GS Lm cropping Lim, G.S. (1977). Heong of and Lim from courtesy data data intensity trap Light et f am ra n h vcnt dul cropped. double vicinity the in area farm of cent tmoe a Tt Srn, n eain o h per the to relation in Serong, Titi at stemborer h latsurs qain is: equation least-squares The acltd rm ih ta cths f yellow of catches trap light from calculated iue .0 aroe bten e saos , seasons, wet between Carryover 3.10 Figure = .5 - .3 t 50, 001). 1 0 .0 < P 5.04, = (t 4.63 - X 2.05 = Y 25 1 ------ 1 ------110 50 H - 75 4 - rpe (arcsin) cropped o i — 100 However, of possibly greater significance than the resistance of the new varieties to GUI has been their reduced time to maturity. In Fig. 3.11, I plot th** number of months between trough and peak of GLH catches in the wet season (n in eqn. 3.1). A marked decline is apparent, particularly after 1966 when shorter duration modern varieties were first grown on an appreciable scale in West Malaysia (Linn and Heong op. cit.). There is no comparable such reduction for YSB. GLH is expected to be the more susceptible to this change as it primarily infests rice during the vegetative stage, that which has been truncated by the earlier maturing rices (2.10). YSB usually attacks the crop shortly after transplanting, when it is still vegetative, but continues on till ripening and thus would be less affected by the shift in varieties. When one examines the effect of agricultural change on between-seasons population dynamics, GLH is found to behave much as YSB. In Fig. 3.12, the wet season-to-wet season carryover coefficient is plotted against the proportion of the area double cropped. The regression is positive and significant, though less variation is accounted for than in the case of YSB. 3.2.4 Discussion A number of important points have emerged from this study .of the dynamics of rice pests under intensification. Firstly, population growth through the wet season crop had been shown to be density dependent, though undercompensating for YSB, at both Los Banos and Titi Serong. Thus, realised rates of increase are lower at the higher, initial population levels that have prevailed at both sites in recent years. The changes in the agricultural environment listed in Table 1.1 which are thought to affect pest populations by raising growth rates are unlikely therefore to have had a central role in increasing pest abundance. The fact however that the density dependence for yellow stemborer is undercompensating implies that changes in the environment that increase the initial levels infesting the crop, as would improved carryover due to double cropping, will 8 6 - t/) 1 H Z o 2 Z 2 - ' ~i--- 1--- r- 1959 61 63 65 67 69 71 73 75 Figure 3.11 The number of months of increase, n, of green leafhopper in the wet season at Titi Serong. From 1959 to 1966, n averaged 5.63 + .63, compared with 2.89 +_ .35 between 1967 and 1975. The difference is highly significant (tl5 = 3.93, P< .01). 112 O C& ----- 50 *- 113 r — 75 opd (arcsin) - c ropped r ~ 10 0 result in higher peak levels. Moreover, it can readily be shown that for a population regulated in this fashion, a new equilibrium will be attained within a few years of an increase in carryover. The theoretical development in the last chapter assumed population growth to be density independent; it is however a simple matter to derive expectations for populations under intensification where density dependence is taken to be undercompensating. Let Np (x) be the peak population in the wet season of year x and Ntr (x) be that in the preceding trough. It has been found for YSB that: In N p (x) = a, 1 + b. 1 In N. tr (x) with 0 In N tr (x) : a. z + b z In N p (x-I) where cL 1 (carryover is largely density independent), but ^ is increased (made less negative) by increases in the number of crops grown. Where b-z is taken to be exactly 1, a z 7 - -Q ww . Combining the 2 equations. In Np (x) = aj + (In Np (x-i) + ©ww>bj (3.4). An equilibrium will be attained when the seasonal rate of increase exactly balances the carrover between seasons; at this point the peak wet season population will be given by In N p = —a, 1------w + © w — b, 1 1 - b, and that in the trough by In N tr = a, — 1 ------+ © ww . 1 - b, 114 Using the least squares estimates at Los Banos, where a^ was found to be 4.962, b. .5039 and © was -2.836 under the lower levels of double cropping prevailing from 1965-70, an equilibrium peak population is predicted at antilog 7.122, compared to the observed mean value of 6.909 + .219 (n = 5). In the perod 1971-79, © was found to be -2.118, leading to a predicted peak equilibrium of antilog 7.851. This compares to the mean of 7.954 -h .152 during the last 5 years for which data is available, when catches appear to have stabilised. Neither of the predictions differ significantly from the observed values, even if one ignores the uncertainty in the predictions themselves. Moreover, the model appears to capture well the dynamic behaviour of the population after the rapid increase in double cropping (recall that the proportion of land in the vicinity double cropped increased from 39% to 99% between 1969/70 and 1971/72). In Fig. 3.13, I plot the observed peak populations from 1969 to 1979 and those predicted from equation 3.4 assuming that the increase in carryover occurred all at once during the 1970-71 dry season and that © remained constant thereafter at -2.118. The fit appears to be reasonably close, though it must be recognised that the parameters from which the predictions are derived were estimated from data of which the plotted values form a part. Nevertheless, the ability of the simple model to capture the behaviour of the pest population over this critical period of increase gives one some confidence in its adequacy. To this point, I have assumed that in both Laguna and N. Krian it was primarily the increase in double cropping that has led to greater carryover between wet seasons. However, as suggested in Chapter II, asynchrony as well is expected to favour carryover, in addition to its within-season effects. The data at neither site tire adequate to estimate the trend in asynchrony, though Lim and Heong (op. cit.) contend it reached a maximum in N. Krian during the early years of double cropping. In Chapter V, I return to this question, examining the impact of asynchrony on pest populations where the number of crops grown per year is constant. 115 peak wet season catch i — 1969 uig h 17-1 r season. dry 1970-71 the during aroe fo -.3 t -.1 ocre al t once at all in occurred increase -2.118 an to assuming -2.836 3.4, from equation carryover from predicted tmoe i te RI ih tas n those and traps light IRRI the in stemborer iue .3 ek e sao nmes f yellow of numbers season wet Peak 3.13 Figure ---- 1 ---- 71 h 116 73 ---- 1 ---- 75 1 ---- 1 ---- 77 ---- 1 ---- 79 H It should be recognised that, although population growth through the crop from trough to peak has been shown to be significantly density dependent, this regulation occurs over a span of several generations. Within any one, the degree of density dependence is likely to be much less. This is clear from the following: If pest numbers one generation after the trough are given by In Ntr +1 = a + b In Nt and, if the same parameters a and b obtain in subsequent generations, then in the peak In Np = b In Ntr + a (b + b ... + 1) where m is the number of generations from trough to peak. Thus at IRRI the slope of the regression of In on In is found to be ,5 0 k and as there appear generally to be 3 generations within that span, the slope of the regression between succeeding generations is expected to be on the order of .80. This value is only somewhat less than that calculated for stemborer in Sarawk (2.3). Finally, one might consider the likely impact of further increases in cropping intensity, that is from double to triple cropping. Such a development would be expected to further increase carryover between seasons. Given that peak wet season catches of YSB under double cropping at both Los Banos and Titi Serong are below the maximum values they might take (estimated from In = a + b In Ntr), it is anticipated that higher initial numbers due to improved carryover would result in peak levels greater than are now recorded. 3.3. Comparisons between sites 3.3.1 Data and Methods The second set of data I consider comes from study sites of the IRRI Cropping Systems Programme in various provinces of the Philippines. Concentrating for the most part on areas of rainfed 117 cultivation, the programme has brought together agronomists, economists, entomologists, weed scientists, and plant pathologists in an effort to identify superior cropping patterns and management practices for small farmers. A great deal of data on the natural and economic environment has been collected, of which I will draw on part. 1. At each cropping system site established since 1978 (Fig 3.14), a light trap of simple design has been installed : a kerosene - burning fisherman’s lantern, supported on a bamboo tripod above a plastic pan containing water and detergent (IRRI, 1979a). At several, a higher intensity, liquid propane gas lamp with incandescent mantle was also used, and at one site, Batangas, this was the only design employed. Catches at Batangas have been rendered comparable by applying correction factors estimated at the sites where both types were installed. The traps were operated nightly for at least one year and the catch was sorted and recorded by staff on standard farms. Two full years' data were available for all species in Pangasinan and for GLH only in Batangas. The irrigated double cropped sites included in the analyses are 3 of a sample of 23 sites in Nueva Ecija that form the basis of the extensive study of asynchrony reported in the next 4 chapters. Though not randomly selected from this sample, the 3 sites span a range of pest densities, from among the highest to the lowest encountered. Two farmer-tended kerosene light traps were installed at each site and operated in essentially similar fashion to that described above. These sites and the operating procedures will be described in greater detail in the next chapter. The Nueva Ecija study was supported by IRRI under the rubric of the Cropping Systems Programme, but did not involve the full panoply of multi-disciplinary science. 2. Experiments on yield loss due to insect pests have been conducted at each of the Cropping System sites. Plots of ca. 100 m 2 established within farmers' fields are treated with insecticide or left untreated during the different stages of crop 118 CH IN A SEA rgam i te Philippines. the in Programme iue .4 ie o te RI rpig Systems Cropping IRRI the of Sites 3.14 Figure 119 HLPIE SEA PHILIPPINE growth, following what is known as the "partioned yield loss" method (Litsinger et al. 1982). In what follows, I make use of the overall yield loss through the crop, estimated as the difference in yield between plots sprayed regularly from the seedbed to just prior to harvesting and unsprayed controls. 3. Data on fertilizer and pesticide use were obtained from the IRRI Agricultural Economics department regular Farm Record Keeping surveys of farmers. Data were not always available however for the precise year in which light trapping had been conducted. At the Nueva Ecija sites, the information was asked of farmers at the end of the crop season. 4. Data on the number of crops planted per year, the date of crop establishment and the varieties used were obtained from the Farm Record Keeping surveys at the cropping systems sites. The precise location of the farms of respondents was not specified, so that it is impossible to derive accurate estimates of intensity and asynchrony as a function of distance. Most of the farms are thought however to lie within 2 km of the light traps. In Nueva Ecija, detailed information at the level of the rotational area (ca. 30 ha) was collected by personnel of the National Irrigation Administration (NIA) who operate the gravity irrigation system serving the area. Using this and 1 : 20,000 scale maps kindly provided by NIA, it was possible to calculate intensity and asynchrony parameters within radial increments of 200 m around each site. Details of the data gathering and computational procedures are given in the next chapter. 5. Information on the proportion of land area devoted to rice cultivation was obtained during 1982 by purposive survey conducted by IRRI Entomology department staff under my direction. The method used was as follows: Beginning at the trap, the surveyor walked, counting his paces, along a compass bearing for 5 km. Five transects at 72° intervals were followed, after each of which he returned to the trap. At each pace, the surveyor noted whether his foot fell in 120 rice paddy, an upland crop field, on a dike, or on non-agricultural land and he recorded thc^idata every 100 paces. To obtain an overall index of the proportion of land under rice cultivation, a factor was applied to correct for the more intensive sampling in the vicinity of the trap. The survey required the better part of a day to complete at each site, and, in order to have time for each one, it was necessary to restrict coverage to the first kilometer around the trap. Had sampling continued further, the computed indices would likely have been proportional to, though somewhat higher than those reported here, as the traps were generally sited within a few hundred meters of a tender’s house, and residential areas, for ease of operation. Although it was not possible to obtain data for the specific years in which light trapping was conducted, it is thought unlikely that either the area cropped or the proportion devoted to rice changed appreciably at any of the sites in the interim. Accurate small-scale aerial photographs that might have obviated such a labour intensive procedure were not easily obtainable for "national security" reasons. 3.3.2 Results 3.3.2.1 The elements of intensity Estimates of spatio-temporal intensity at the sites and of wet season chemical input use are listed in Table 3.2. Modern varieties bred at IRRI predominate at all sites save those in Cagayan, where photoperiod-sensitive traditional varieties are planted, and in upland Batangas, where a local drought-tolerant, early-maturing variety is preferred to the introduced alternatives. Chemical fertilizer is used at all sites, except in Cagayan, buy most intenstively in Batangas, where farmers grow a variety of profitable cash crops in addition to rice, and in the irrigated lowlands of Neuva Ecija. In terms both of the number of applications and the average dosage, pesticide use is greatest in Nueva Ecija. 121 PROVINCE VILLAGE YEAR SPATIO -TEMPORAL FACTORS CULTURAL FACTORS FACTORS area in rice c r o p mea n fertilizer insecticide use: rice ping index maturity use applications dosage (%) ( c r o p s / y r ) (days) ( k g N/ha) (no./season ) (kg a.i./ha/ application) Batangas Cale 1980 46 1.0 120 66 0 - Cagayan Iraga 1981 60 1.0 156 0 0.3 0.02 Bangag 1981 69 1 .0 174 Pangasinan Lipit 1979 77 1 .0 134 M 1980 77 1 .0 130 33 0.4 0.10 Caaringayan 1979 90 1 .0 129 II 1980 90 1.17 128 Iloilo Buray 1979 72 1.30 110 43 1.3 0.14 Sta Monica 1979 72 1.67 123 Nueva Ecija Sta Rita 1981 90 1.92 119 Manaol 1981 94 1.90 119 57 5.0 0.28 Batitang 1981 85 1.82 121 Table 3.2 Parameters of rice cropping intensity at 10 sites in the Philippines. Relatively little variation is apparent among the sites in the proportion of land area devoted to rice cultivation, again with the exception of Datangas, whose well-drained volcanic soils are suited to a variety of vegetables, legumes and fruit crops. Rice is virtually the sole wet season crop in the other areas. One planting per year is the rule in Cagayan and in most years in Pangasinan. Cropping Systems Programme experiments suggest that 2 rainfed crops with rapid turnaround between them return increased benefits in most years (IRRI 1979), but farmers appear unwilling to attempt this except where, as in Caaringayan in 1980, the development of supplemental pump irrigation reduces the risk of crop failure. In Iloilo, a more even rainfall distribution has favoured adoption of the practice, particularly when vigorous short duration varieties such as IR36 have been available. Double cropping is the norm in Nueva Ecija; the relatively small areas not planted twice suffer from poor drainage and, in Batitang, dry season water shortage. Though as noted earlier, geographically accurate data on time of planting were not available from the rainfed sites, it is clear that there is a considerable range of asynchrony conditions among them. In a sample of 50 farmers in Batangas, the standard deviation of planting date was 8 days, but 45 days among 14 farmers in Iraga, Cagayan. Edaphic and hydrological factors are thought to be chiefly responsible for such differences. In Iraga, planting in the low lying "kalayakan" fields on the flood plain of the Cagayan River is often several months delayed relative to the slightly higher "parang", owing to excessive standing water (IRRI, 1981). In Batangas, such differences due to landscape position appear to be less significant, as rice is grown under unsaturated (upland) conditions. The geography of asynchrony in the double cropped areas of Nueva Ecija will be examined in detail in the next chapter, but under those conditions as well, hydrological factors appear to be key determinants. Standard deviations of planting date will be seen to vary widely : though no site rivals Iraga in asynchrony, at some, well-served by irrigation, conditions appear comparable to those in Batangas. 123 To summarise briefly, the various indices of intensity and asynchrony are found to vary together in a somewhat more complex fashion than might naively have been assumed. However with the exception of fertilizer consumption in Batangas, input use does appear to increase with the cropping index. Estimates of wet season crop loss in the various provinces are listed in Table 3.3, together with the peak wet season light trap catch of the k pests monitored at all sites, given in rank order, and the mean ranking. Where light trap records are available for more than one year or site within a province, these have been averaged. A rough correlation is evident between the pest rankings and the level of crop loss. The relationship between specific symptoms of crop damage and light trap catches of the causative insect will be taken up in greater detail in Chapter V where more extensive data will be available. For the present, it is sufficient to note that the light trap catches appear to reflect pest pressure in the field. To what extent then is it possible to account for differences in aerial density? 3.3.2.2. Pest density and intensity Annual totals of YSB, BPH, GLH, and CW are plotted against cropping index in Figures 3.13 a - d. In all graphs, the points representing Cale in Batangas are conspicuously low. Table 3.2 suggests that cultivation at that site is distinguished quantitatively on a number of grounds. However, what may be a crucial qualitative difference is the upland conditions under which rice is grown. Caseworm, which as a larva disperses between plants on the water's surface in its characteristic leaf case, is not captured in light traps in Batangas and it is likely that the drier conditions would significantly reduce growth rates of the other pests as well. I exclude this site therefore from the analyses that follow. With the exception of GLH, the slopes of the regressions illustrated in Fig. 3.15 are all significant. Moreover, in these 3 cases, the 124 PROVINCE ENVIRONMENT YIELD LOSS PEAK WET SEASON LIGHT TRAP CATCH (RANK) tons/ha % YSB BPH GLH CW MEAN 5.0 Batangas rainfed dryland .25 7 5 . 5 5 5 Pangasinan rainfed wetland .55 12 3 4 3 3 3.3 Cagayan rainfed wetland .50 14 4 2 2 4 3.0 I io ilo rainfed wetland .70 24 1 3 4 2 2.5 Nueva E cija irrigated wetland 2.37 33 2 1 1 1 1.3 Table 3.3 Estimated yield loss to insects at sites in 5 Philippine provinces and peak catches of major rice pests. Yield loss is measured on modern varieties during the wet season and is expressed both in absolute terms and as a per cent of the fully protected yield. Light trap catches are ranked from 5 (lowest) to 1 (highest). Data courtesy of J.A. Litsinger, except the Nueva Ecija * trap catches. 126 A rice cropping index 1.0 1.2 1.4 1.6 1.8 2 .0 rice c Topping index Figure 3.15 Annual light trap catches of 4 pests in relation to the rice cropping index at 10 sites in the Philippines. The least-squares regressions are: A. BPH: In Y = 3.16 + 2.67 X ( t = 3.51 , P < .01 ) n B. C W : In Y = 2.67 + 2.11 X (1 O P <.00 1 ) o 127 o O) a> annual total . L: n Y In GLH: D. 10 I - 4 8 1.0 t ------ 1 ------ie rpig index cropping rice 1.2 1 ------.1 .3 X .837 + 6.71 1 ------128 1.4 1 ------ 1 ------1.6 1 ------ t 16, P<-20) 1.60, =(t 1 ------.8 1 -? r ----- 1 ------.0 2 D 1 increase in the annual total between l and 2 crops per year is greater than a mere doubling and thus represents an increased density per crop : the slope is significantly greater than log^ 2 (.693) for YSB (tg = 2.45, P < .03), BPH (tg = 2.61, P < .03) and CW (tg = 3.42, P < .01). The theoretical analysis in the previous chapter however leads one to expect a relationship of a somewhat different form. Recall that it was suggested in 2.8 that as the proportion of farms planting a second crop, p, increases from 0 to 1, pest numbers will be governed by an inequality of the form O P (N j - N0) + Nq, where Nn,u N, l and N p are the densities where none, all and a proportion p of farms plant a second crop. This implies that a double logarithmic relationship In = a + b lnp should provide a better fit to the data than the single logarithmic equations of figure 3.13. However, it was also argued in Chapter II that when p is close to 0 or when double cropping is newly introduced, the relationship may not hold. The hypothesis that results is rather weak but is tested nonetheless in Table 3.4a, which lists simple correlation coefficients for the 2 alternative models. The double logarithmic equation is seen to marginally improve the fit for YSB and GLH, but not at all for the other 2 species. The sites differ in other aspects of spatio-temporal intensity expected to influence pest density. Table 3.4b gives the simple correlation coefficents between log annual catches and A, the proportion of land planted to rice in either season, its logarithmic transform, and MAT, the mean time to maturity of the varieties planted. As was the case with the proportion of farms double cropped, a logarithmic transorm of A is expected, on the reasoning of Chapter II, to provide a better fit to the data. Again however the improvement is found to be inconsistent and at best slight. Though for both formulations the correlations are in the expected direction, in no case do they attain statistical significance. In contrast, MAT is seen to correlate negatively, against expectation, with log density, and in the case of YSB and 129 Table 3.4 A In YSB In BPH In CW In GLH ** ** Cl .822 .760 .861 *** .476 In p .846** .680 * .858 *** .508 B A .421 .065 .596 .269 InA .415 .057 .600 .269 ** ** MAT -.804 -.304 -.748 -.346 Simple correlation coefficients between selected parameters of cropping intensity and the logarithm of annual catch, n = 11 sites. Cl - rice cropping index = 1 + proportion rice area double cropped, P - proportion rice area double cropped + .005. A - per cent land area devoted to rice cultivation. MAT - average maturity of varieties. * .01 < P< .05 .001 < P< .01 130 CW significantly so. Closer examination of these 2 relationships however reveals that one site, Iraga, dominates the correlation : when this is removed, r is reduced to insignificant levels. Moreover, MAT is negatively correlated, at a level approaching significance with the proportion of the area double cropped (r = -.58, .05 < P< .1); if MAT is entered in a multiple regression of In numbers after In p, it in no case accounts for significant additonal variation in the dependent variable. The some holds true for A or its logarithmic transform . The number of crops grown per year is thus found to account for the largest amount of variation in the logarithm of aerial density for all k pests, and, in each case, in the direction expected on theoretical grounds. It is possible however that the impact of this variable has not ben the direct one supposed in Chapter II, the consequence of a reduced fallow, but due to other elements of the intensification syndrome that have been associated with the adoption of double cropping. As noted above, variation in the use of agrochemicals roughly parallels increases in the cropping index at these sites; it may be then that pesticide-induced resurgence and/or stimulation of pest population growth by nitrogenous fertilizers are primarily responsible for the correlations illustrated in Fig. 3.15. 2.3.2.3. Density dependence within and between seasons It is possible to distinguish this possibility from the argument advanced in Chapter II. As in the analysis of the Laguna and North Krian light trap series, the crucial question is how increase through the crop, as opposed to carrover through the fallow, varies between the sites. 131 In Fig. 3.16 a - d I plot, for each of the pests, the logarithm of peak wet season catch against the logarithm of numbers in the preceding trough.* The slopes are all found to be positive, and all differ significantly from 1. In only 2 cases, those of BPH and CW, does the slope differ significantly from O. In Table 3.5 I summarise these significance tests and calculate the predicted equilibrium^ N;A from eq'n 3.3. In the 2 cases where the slope differs significantly from 0, this value is seen to be many times higher than the mean trough populations found currently at the double cropped sites. A large part of the scatter about the regression lines is found at low initial numbers; in many of the single-crop sites, no insects are caught in the trough month prior to the wet season. The regression is very sensitive therefore to small variations in catch : had only 3 more YSB moths been caught in 2 of the traps during the trough month, the coefficient of determination would have been increased by 46%. If In peak numbers are instead regressed against the logarithm of numbers in the month after the trough, the correlation is much improved£ A for YSB (In N p = 3.97 + .246 In N. tr + . 1 month’ , ttoi 4 = 2.76 , N = 193), but2 not for GLH (In N P = 6.75 + .045 In N tr + 1 month* t 12 = • The ana^Yses thus suggest that increase through the crop is density dependent, though undercompensating, for all of the pests save GLH. In Fig. 3.17 a - c, I plot the logarithm of trough catch against In peak catch in the preceding wet season. Data are not available at all sites for 2 years, so the number of points graphed is less than in the preceding set of figures, and is insufficient for analysis in the case of GLH. Here I treat each of the 2 light traps installed at the 3 Nueva Ecija sites as a separate datum, whereas in the previous regressions against intensity parameters, which were only measured once at each site, I employed their mean values. 2 The regression parameters are also somewhat sensitive . . to the value of the constant, here 0.5, added to the monthly catches to avoid 0 values when taking logarithms. The major conclusions drawn from the analyses however are unchanged if other reasonable values such as 0.05 or 1 are used. 132 9. A Figure 3.16 Peak wet season catches of 4 pests in relation to numbers in the preceding trough. A - brown planthopper; B - caseworm; C - yellow stemborer; D - green leafhopper. The regression equations are given in Table 3.5. 133 c 6. 4 ■ in o ’ 3. • < lu C l -i------1 "i------r----1------r 0. 2. 4. COo < 111 CO D h- 111 $ 8. • d) O O 7, - 6. ■ 1 r ' 1 i------1------i------1------1------1------1------r-1 1 f i i i 0. 2. 4. 6 . L0Ge WET SEASON TROUGH ♦ 0.5 134 t value o A ii species slope + std. / > = 1 N mean under error 2 crops Yellow „ .*** stem borer .136 + .088 1.54 9.81 - 32 Brown * ** planthopper .426 + .159 2.68 3.61 10200 41 Green *** leafhopper .155 + .092 1.68 9.16 - 80 Caseworm .291 + .092 3.14. ** 7.67*** 258 44 Table 3.5 Slopes of the regression of In peak numbers on In trough numbers and the significance of their difference from 0 and i. For the 2 regressions where the slope is found to differ significantly from 0, the predicted maximum, calculated from eq'n 3.3, is listed along with the mean trough population recorded in l£e 6 traps where Rouble cropping is poetised, n = 14 for all species. .01 < P < .05, .001 < P r 01 P Figure 3.17 Catch in the trough prior to the wet season in relation to numbers at the peak of the preceding wet season. A. brown planthopper, b. caseworm, c. yellow stemborer. n = 11. The slopes of all 3 regressions differ from 0: * a. In N = .901 In N - 3.88 (t = 2.82 ), 2 1 * b. In N = 1.51 In N - 6.37 (t = 2.50 ) and 2 1 c. In N = 2.07 In N - 8.53 (t = 1.84, P<.05 2 1 by one-tailed test), but not from 1. The filled symbols represent double-cropped sites. 136 B □ □ T T 4 5. 6. LOGe WET SEASON PEAK ♦ 0.5 (t) 137 The 3 regressions are found to have slopes near or greater than 1, indicative of density independence or inverse density dependence in carryover between seasons, though there is a great deal of scatter. The plots, particularly that of BPH, are suggestive, not so much of consistent increase, as of 2 separate regimes, with the higher points representing double cropped sites. In Table 3.6 I list the seasonal, mean monthly and maximum monthly rates of increase, and the carryover coefficients between wet seasons in relation to the number of crops grown. It can be seen that, no matter how it is calculated, the rate of increase is higher in all but one instance in the single crop sites. A large excess is evident when one considers the seasonal increase; the difference is less marked and verging on significance in only one case when one turns to the mean monthly rate. This is due to the fact that, for each species, the number of months from trough to peak, n, is on average greater in the single crop sites. In part this may be artefactual : where one crop per year is grown there is more time for population decline between seasons. However it may also reflect the greater asynchrony or longer maturity of the varieties planted in some of the single crop sites. Yet as the table indicates, the maximum rates, calculated from the largest increase in any one month, are also consistently and significantly greater where one crop is grown, suggesting that the difference between the 2 environments is traceable to more than variation in the number of months of increase. In contrast, carryover coefficients are found to be greater in the double cropped sites, though significantly so in only 1 of 3 cases. Table 3.6 Wet season rates of increase and carryover between wet seasons in relation to cropping index. There are 5 sites where 1 crop is the norm and 6 where 2 crops are grown on the large majority of farms. Excluded are the 3 sites with intermediate cropping indices, from 1.17 to 1.67. 1 2 n = 4, n = 3. 138 A : seasonal rate of increase YSB BPH CW GIH 1 crop 4.76 + .34 5.77 + .56 4.16 + .16 6.60 + .55 2 crops 1.47 + .13 3.54 + .72 2.16 + .23 3.05 + .38 * *** t 9.72 2.36 6.97 6.91 B : mean monthly rate of increase YSB BPH CW GLH 1 crop 1.05 + .21 1.24 + .15 1.07 + .20 2.97 + 1.00 2 crops .63 + .10 1.17 + .22 1.10 + .36 1.50 + .21 t 1.97 .23 -.08 1.58 C : maximum monthly rate of increase YSB BPH CW GLH 1 crop 3.01 + .29 3.32 + .32 2.84 + .41 4.43 + .96 2 crops .97 + .22 1.95 + .38 1.47 + .26 2.14 + .15 *** t 5.68 2.70 2.97 2.60 D : log carryover YSB BPHCW GLH 1 crop -4 .7 2 + .571 -5.53 + .651 - 2 crops -2.11 + .23 -3.72 + .75 -3.85 + .56 - t -4.86** -1.68 -.70 139 3.4 Discussion and Conclusions The central conclusions from the above results in large measure reinforce those drawn from studies of pest population trends at one site over time. With the possible exception of GLH, population increase through the crop appears to be density dependent, though undercompensating. The levels at which populations would be expected to settle under the influence of regulatory mechanisms are however above those currently observed. In contrast to the rate of increase, carryover between wet seasons shows no evidence of density dependence and is greatest where an intervening dry season crop is grown. It is striking that, expressed as a percentage, YSB's carryover in double cropped Nueva Ecija is almost identical to that in Laguna after 1971 (12.1% vs 12.0%) and close to the value calculated in North Krian (10.7%) under extensive double cropping. In these analyses, I have made use of existing and uncontrolled variation in the parameters of intensity. To some extent the identification of one of these, cropping index (or the proportion of farms double cropped) as key to the increase in pest density is weakened by the fact that others factors are correlated with it. Yet of all the parameters that have been considered quantitatively, the association of cropping index with pest density is the closest and most consistent. Moreover, the detailed pattern of change, the fact that it appears to have occurred, not through increased growth rates while the crop is in the ground, but due to higher initial immigration, is what one would expect from greater double cropping but not, for example, from the effects of pesticide-induced resurgence. It should be stressed that the data I have used in this chapter pertain to populations under non-outbreak conditions. The balance of forces underlying the epidemic-like increase of BPH in most countries of South and Southeast Asia in the early and mid-1970's (Chapter 1) may well have been different. The difficulty in testing the competing hypotheses in this instance have already been noted : little quantitative information is available on the situation ante for BPH, as it was not then of major concern. It may 140 be possible, should no better source of data be found, to combine within- and betwcen-site comparisons (provided similar traps have been employed or the catches are rendered commensurable) to draw inferences about the origins of the outbreaks. At the least however, the analysis of this chapter suggests that the increase in carryover occasioned by double cropping has been sufficient to lift BPH to substantially higher levels than occurred previously. There is no compelling reason to believe the explanations exclusive: transient increases in population growth rate due, for example, to pesticide-induced resurgence would have exacerbated the effect of increased carryover. Overall, rates of increase have fallen under intensive cultivation, yet they might well be lower still were insecticides used more rationally. The implications of these results for the design and mangement of cropping systems should be clearly drawn. Though extrapolation beyond the range of available data is inherently suspect, the results suggest that expansion of multiple cropping to 3 crops per year would result in further increases in pest density and losses to the crop. Clearly, many other factors will weigh in decisions of how or where to intensify, but this consideration should not be lost sight of. Retrenchment to 1 crop per year would be expected to reduce pest density and diversification of the cropping pattern to include upland crops after rice would augment this effect. Such an eventuality would have complex and substantial impacts at all levels, from the village to the national, and, given the political, social and economic environment in which intensification has occurred, must be judgecf to be unlikely. Synchronisation of the cropping schedule, which would reduce the number of pest generations possible per season rather than remove. an entire crop, would seem fundamentally less problematic; in much of the next k chapters, I will be concerned with the efficacity and feasibility of this control technique. 141 Two further points should be made. (1) The effect of increases in the area devoted to rice cultivation must be judged to be unproven with the data that is here available. That is perhaps not to be wondered at, as it was argued in Chapter 2 that, where population growth is density independent, the increase in pest numbers should be less than proportional to the increase in the area under cultivation and, where it is fully regulated, exactly proportional. A factor that acts on populations in approximately arithmetic fashion might easily be masked by stochastic variation among sites. Assuming for the moment that numbers are in fact governed by an inequality of the form Np )> p (Nj ~ Nq) + Nq (3.3.2.2), a number of implications follow. Perhaps most importantly, for a given rice hectarage, mean pest density will be least if double cropping is concentrated in as few localities as possible. For instance, the number of insects produced in 2 localities of equal area, both with p = 0.5, is expected to be greater than if p were 0 in one and 1 in the other. Firmer empirical support is required however before more is made of the theoretical expectation. (2) From both the between-site comparisons in the Philippines and the analysis of light trap records in N. Krian, there are indications of near to perfect density dependence in the seasonal rate of increase of GLH. In both cases, it was suggested that reduction in the length of the season may have played a part, though as earlier noted, Kuno and Hokyo (1970) in Japan have produced evidence for regulation of this insect under more controlled conditions. A further and possibly more sensitive test of the reality of population regulation will be presented in Chapter V where the response of pests to incremental increases in the time available for increase, due to asynchrony, will be presented. As concerns BPH, the results reported in this chapter confirm the conclusions of Kenmore (1980) with respect to the density dependence of population growth in the tropics. In Japan, where the 142 pest does not overwinter and builds up each year from long distance migrants, Kuno and Hokyo (op. cit.) in contrast found no indication of density dependence. Further support for these findings might be sought from other sources of data in the tropics, such as the light trap network established by the All-India Co-ordinated Rice Improvement Project. 143 CHAPTER FOUR The Ecology of Asynchrony in Nueva Ecija In this chapter I examine asynchrony of cultivation as a phenomenon of intensive agricultural production. I begin by describing the origins of rice farming in Nueva Ecija, one of the mainstays of the Philippine rice economy. The history of production in the province is surprisingly brief and one that must be understood if one wishes to make sense of the friction and tension so evident in the day-to-day functioning of the system. I then describe a 20,000 ha study area where asynchrony was studied in detail. Contemporary rice production depends on a large number of inputs, few of which are under the farmer’s direct control. I outline a statistical procedure developed to estimate the contribution of delay in the various requisites of cultivation to variation in the time of planting the crop. Under current conditions, uneven irrigation service is shown to be key to asynchrony. There is however strong evidence that farmers individually compensate for delay and thus minimize asynchrony, in a manner closely analagous to density dependence in population regulation. I then analyse variation in planting date from the perspective of the irrigation system, ascertaining the contribution of delay at the different levels of management. Asynchrony is generally greatest in the wet season, and in one year this was shown to be due to more uneven service at the largest and smallest scales of operation considered (betwen canals and among farms served by a given turnout), while variation was largely unchanged at the intermediate level (among turnouts served by a given canal). Asynchrony varies markedly over the landscape and is greatest in the areas of chronic irrigation difficulties. At these sites, generally in the downstream reaches of the 144 145 system, yields are little more than half of what they are in the well-served areas. Though other factors likely play a part, the trend is such as one would expect if pest densities increased with asynchrony. In the concluding section of the chapter I set the stage for the natural experiment described in Chap. V that tests this hypothesis and show that uncontrolled variation in intensity factors that might obscure the relation is negligible. 4.1 The Origins of Intensive Cultivation The Central Luzon Plain stretches north across the island in a broad sweep, from Manila Bay to the Lingayen Gulf, bounded on the east, north and west by the Sierra Madre, Caraballo Sur and Zambales Mountains. Nueva Ecija province occupies the northeastern third of the plain and surrounding uplands, the area drained by the upper Pampanga River. Though the interior of the plain was largely uninhabited when the Spanish reached it in the late 16th century, the wide distribution of fire-maintained grassland and savanna-parkland suggests that swidden cultivation had long been practised by the indigenous Malay and Negrito peoples, and there is evidence as well of limited wet-rice farming. (McLennan 1980). For nearly two centuries after the conquest of the Philippines, until the late 1700's, the principal justification of the colony in Spanish eyes was as an entrepot on the lucratiye galleon route between China and Mexico and there was little pressure to alter the existing patterns of land use in the hinterland. The declining political and economic position of Spain in Europe and Latin America and growing commercial competition in Asia, particularly from Britain and the United States, forced a reconsideration of the economic basis of the colony and an increasingly mercantilist reliance on the production of export commodities (Fast and Richardson 1979). In Nueva Ecija the impact of this change was first felt in the extreme south, one of the areas where the royal tobacco monopoly was established. Cultivation of the crop was restricted to the sandy, well drained soils on the levees of the Pampanga and Penaranda Rivers and rice was raised for the subsistence of the still sparse local population on the heavier clay soils beyond. Cash cropping expanded northward through the 19th century and came to include sugar cane and to a lesser extent maize, again grown for the most part on the river terraces. In the interior, the principal land use until the mid-19th century was cattle ranching on large Spanish - and mestizo-owned haciendas (estates). Within a span of some 70 years beginning around 1850, the lowlands of Nueva Ecija were transformed from a sparsely occupied and patchily exploited frontier into a virtual rice monoculture, producing the largest marketed surplus of any province in the Philippines (McLennan op. cit., Gleeck 1981). A number of factors influenced this evolution. On one level, one can point to the growing regional specialisation of agricultural production within the colony as a whole and the creation of local rice deficits which Nueva Ecija, with its heavy, poorly drained clay soils, was well placed to fill. The lifting of the tobacco monopoly in 1881 was indicative of the increasingly laissez-faire economic policies that the Spanish administration came to pursue internally, allowing the natural advantages of the regions to be exploited. Both tobacco and sugar cane declined sharply in Nueva Ecija as a result, becoming concentrated in other areas of Luzon and other islands more suited to their cultivation. The Americans, who succeeded as colonial rulers in 1898 after defeating a Philippine army that had all but expelled the Spanish, continued and extended these policies. Nueva Ecija's integration into the colonial and international economy was accelerated by the development of the rail and road network that became a priority of the new administration. More however was involved in the evolution of rice monoculture in Nueva Ecija than agronomy and expanding inter-provincial trade : emerging social institutions and a changing pattern of land tenure played major roles. Much of the labour needed to open the interior of the plain was provided by settlers from the Tagalog-speaking 146 regions to the south and, in larger numbers, from the dry Ilocos coast to the northwest, who migrated to the area after 1850. Many moved on their own or in groups, establishing swidden farms that produced a variety of crops. Others were enticed by landowners who, in exchange for a fixed rent, provided land for clearing, tools, and rations until the first crops were harvested. Gradually however, the independent settlers and leaseholders were reduced to share tenancy through manipulations of the laws on land title and onerous loan arrangements in which the land itself was put up as collateral and often lost. Whereas in 1903 19% of farmers in Nueva Ecija were share tenants, by 1929 this proportion had risen to 66% (Gleeck op. cit.). Some, having lost their land, moved on over the Caraballo Pass to the newly accessible areas of the Cagayan Valley to the north. Those that remained became ever more deeply enmeshed in the debt-peonage system that evolved, particularly on the large haciendas in the central and western parts of the province. Debts at interest of 50 - 200% per 6 months were incurred seasonally as peasants borrowed to tide their families over to the next harvest and after natural disasters such as typhoons devastated crops (McLennan op. cit.). Repayment was often in rice, though cash may have been lent originally (this remains true to this day), thus serving to tie the peasant ever more tightly to the production of rice. Cultivation of the crop was often made a condition of tenancy, and as population density on the* plain increased and the land frontier closed, peasants* bargaining position with landlords eroded (Kerkvliet 1977)*. By 1930, 95% of the farm area of Nueva Ecija was devoted to rice (Gleeck op. cit.). The terms of tenancy deteriorated markedly after about 1920 and landlordism appears to have lost whatever protective function it may previously have had; Kerkvliet (op. cit.) points in explanation to the increasingly favourable rates of return that landlords considered in alternative rural investments. One important development in the early decades of the century was the introduction of mechanical 1 McLennan (op. cit.) points to changing hydrology as another factor that contributed to the evolution of monoculture on the Central Luzon Plain. The deforestation that accompanied its colonisation, and continues to this day in the surrounding uplands due to the demand for firewood, led to increasing aridity and a lowering of the water table making cultivation of dry season non-rice crops difficult, save in the lowest lying areas. 147 threshing by large tractor-powered "tiiladoras", owned by the hacenderos, which replaced the traditional methods of threshing by hand or animal and saddled the tenant with an additional expense (McLennan, op. cit.). The 1920's and 30's saw a growing tide of peasant organisation and increasing conflict, including strikes and seizures of the harvest. The Japanese occupation in World War II enforced a hiatus, but shortly after their surrender, violence erupted between landlord guards and the Hukbalahap guerillas who had fought the occupiers. Full-scale fighting ensued and it was not until the early 1950's that the rebellion was suppressed by the armed forces of the newly independent Philippines, with considerable American assistance (Kerkvliet op. cit.). Land reform on a large scale was introduced in 1972, soon after the declaration of Martial Law. Though there is much dispute about the extent to which ownership has effectively passed to the tillers of the land, or is likely to, in many areas of the province share tenancy has largely been replaced by leasehold (Cordova et. al. 1981), and the landlord no longer directly contributes to production expenses. (A number of landlords however have established Rural Banks that now disburse government-sponsored agricultural loans). Some observers contend that social unrest among the former tenants has abated in recent years with increases in production and at least partial land reform. A more explosive situation is seen to exist among the growing number of landless labourers (Fegan 1980), who in some villages constitute k0% or more of all households. Yet land reform has not reached to all areas of the province and yields on rainfed farms lag behind those where irrigation is available. Violence has erupted in the largely rainfed north-west and in the upland areas in the east of the province, particularly where farmers have been displaced by the construction of the Pantabangan Dam that provides irrigation for the lowlands. The New People's Army, successor to the Huk movement of the 1950's, is now active in these areas. 148 In 1920, Nueva Ecija overtook Pangasinan as the leading rice producing province of the Philippines (McLennan op. cit.). Though the intervening years have seen large increases in production on the southern island of Mindanao, Nueva Ecija retains a leading position in the Philippine rice economy, harvesting more than 16% of the national total, according to figures provided by the Ministry of Agriculture, and supplying more rice than any other province to the Manila market. The provincial mean yield in 1979 was 3.67 t/ha (nearly twice the national average), 4.18 t/ha in the irrigated and 2.61 t/ha in the rainfed areas. Modern varieties bred at IRRI were first grown in the lowlands in 1967 (IRRI 1975), but were not widely adopted in many areas until about 1970. Large increases in production were made possible by the completion in 1975 of the Pantabangan Dam and the canal network of the Upper Pampanga River Integrated Irrigation System (UPRIIS). With an overall service area of nearly 100,000 ha, the system helped stabilize yields in the wet season and made possible dry season cropping for the first time on many farms, though less reliable river diversion systems had previously served some 40,000 ha (Tanfranco 1977). Repeated surveys conducted by IRRI through the Central Luzon region from 1966 to 1979 document a marked increase in the use of chemical inputs and mechanical tillage (Cordova et al 1981), broadly similar to those observed in Laguna (3.2.1). The change in insecticide use over the period is illustrated in Fig. 4.1. The surveys indicate that hired labour was increasingly substituted for family labour in many activities but was displaced from weeding by the adoption of herbicides. Overall, it appears that, despite a 180% increase in the proportion of the area double cropped and a 55% rise in wet season yields, the net annual farm income fell in real terms by nearly 20%. The declining value of rice and the increasing cost of material and labour inputs appear to be primarily responsible. 149 2.5 - Q_ o QL O 2.0 £ Ui Q. * 1.5 I- z ai 2 H < ■ 11 1 .0 K H UJ O < 0.5 a: LU > < 1966 1970 1974 1979 Figure 4.1 Insecticide use among Central Luzon farmers 1966 - 79. Data from Cordova et al. (1981). 150 Interviews with farmers and officials of the Ministry of Agriculture indicate that there have been at least 2 widespread pest outbreaks since the adoption of modern varieties : tungro virus, vectored by GLH, in 1971 and BPH in 1976. In addition, rats were a major problem from 1970 to 1975 in many areas. k .2 The Study Area From 1978 to 1983, an action-research project jointly funded by IRRI and the Ministry of Agriculture operated in selected villages of 4 municipalities in Nueva Ecija. The Small Farmer Organisation Project (SFOP) drew on 3 senior IRRI scientists, an anthropologist, a cropping system entomologist and an irrigation engineer, with the aim of determing the scope for group crop management under the ecological, economic and social conditions of Nueva Ecija (Goodell et al. 1978). A key element of the project was the presence of a team of community organisers, under contract from the Manila-based Agency for Community Educational Services (ACES), whose task was to catalyse group action, building on farmers' "felt needs". Over the course of several years, interest-based groups emerged concerned with irrigation, credit and land issues and appropriate technology. Entomological research in the project was aimed at establishing those aspects of Integrated Pest Management (IPM) that required or benefitted from group action. Initially this centred on paedogogical aspects, that is on how best to transfer the technology from researchers to farmers through classes and demonstrations. Basic studies, including experiments on yield loss due to insects, evaluations of the economics of pesticide use, and surveys of farmers' pest control practices and perceptions, were also conducted in order to derive recommendations appropriate to the area. (Litsinger et al. 1981, Goodell et al. 1982). These involved primarily the use of economic thresholds for pesticide use, co-ordinated with the planting of insect-resistant varieties. Cultural controls - synchronous planting and post-harvest ploughing of stubble - were also discussed in the classes. However, when farmers in 1980 in 2 barrios (villages) attempted to act on these recommendations^ it 151 quickly became clear that there was no basic scientific knowledge to guide them on how large an area was required and within what span of time planting would have to be completed in order to have an appreciable effect on pest populations. It was the recognition of this critical gap in understanding that sparked the research here reported and the expectation that results from it might readily be applied by organised farmers that led to it being conducted in the area. The approach adopted was to examine the impact as well as the origins of asynchrony of cultivation under existing conditions, before considering deliberate attempts at synchronisation. The study area selected was centred on the SFOP barrios but extended in every direction to include the 20,000 ha lying to the southwest of Cabanatuan, encompassing Zones 2 and 3 of District III, UPRIIS (Fig. 4.2). Irrigation flows to the area originate from a single diversion canal, DC-2, and are managed down to the turnout * level by NIA, under the direction of a District Chief Engineer. Data routinely collected by the ditchtenders and section heads provided the main source of information on the variability of planting dates, indeed the only feasible source with the degree of resolution desired for such an area, and the co-operation of key NIA personnel was sought early on. The dominant land use pattern in the area is double cropped rice, save for small areas on the levees along the Pampanga River where maize is grown and in some unirrigated sections along the Chico River to the west, where vegetables or watermelon are cultivated in the dry season. Within Zones 2 and 3, roughly 90% of the area included in NIA's programme is cropped twice, figures provided by the agency indicate. The remaining 10% is concentrated in the downstream reaches, where difficulty in draining water from the fields impedes wet season cultivation, though dry season water shortage is also a problem in some parts. In addition, several 1 A control structure regulating water delivery to the fields' via farm ditches. An average of some 30 ha, 10-13 farms, is served by each turnout in Zones 2 and 3, referred to as a "rotational area". 152 Figure 4.2 The study area in Nueva Ecija. 153 hundred hectares of land previously cropped are at present abandoned and not incuded in NIA's programme because of inadequate drainage. Very light rains during the 1982 wet season failed to adequately refill the Pantabangan reservoir and in the 2 succeeding dry seasons the programmed area was drastically reduced by NIA. The dominance of rice farming in terms of land use is reflected in the narrow economic base of communities. In the villages covered by the SFOP in 1978, nearly 85% of the heads of household in work found their primary employment in rice cultivation, either as farmers or hired labourers (Guino 1978). In early 1981, an office and laboratory were established in Zaragoza on the western edge of the study area. Supported financially and materially by IRRI, it was staffed by the author, a post-doctoral fellow, 2 research assistants and as many as 10 permanent and occasional personnel. Research on various aspects of synchrony constituted the major thrust of the team's work, though studies continued on other aspects of IPM. Save where, otherwise indicated, the research reported in the following chapters was conceived and directed by the author, but relied in the execution on, and benefitted from the intellectual stimulation and moral support of many others. As I have expressed in the acknowledgements, this work would have been impossible without them. 4.3 The Sequence of Cultivation t The timing of planting is affected by a complex of factors that act on it directly and on the preceding farm operations, as illustrated in Fig. 4.3. 4.3.1 Irrigation and drainage Discussions with farmers and irrigation personnel, as well as personal observation, disclose a variety of structural constraints • and management problems that contribute to asynchrony. As mentioned earlier, drainage difficulties in the lower-lying downstream areas exclude some formerly cropped fields from cultivation and in others, 154 Figure 4.3 Factors influencing the timing of cultivation in Nueva Ecija. 155 less severely affected, farmers may be forced either to hasten or delay land preparation and crop establishment because of expected flooding. Elsewhere, downstream farmers may be delayed due to the late arrival of irrigation water. Inadequate releases, design faults in the canal or control structures, reduced flow due to siltation or seepage, and profligate use of water by those upstream may exacerbate the inevitable disparities between the head and tail ends of canal service areas. Fig. 4.4 illustrates the trend in mean planting date within rotational areas served by a short sub-lateral. The gradient in this case is unusually steep, nearly 4 weeks over 3 km, but similar trends can be found along most canals in both wet and dry seasons, and are a common feature in irrigation systems elsewhere in Asia (Chambers 1980). Unfinished canals or farm ditches contribute to asynchrony in other areas. Where the hydraulic head is sufficient, water may be distributed from field to field, causing serious delays and substantial losses due to evaporation. Finally, in some instances, neighbouring farms are served by canals to which water has been released at different times. At the boundary of irrigation systems failure to co-ordinate releases may give rise to even more extreme asynchrony. 4.3.2 Credit The majority of farmers in the area rely on seasonal borrowing to meet their ‘production expenses. As mentioned above, prior to the shift to leasehold in the early to mid-1970's, the landlord was the major source of such loans. Masagana-99, a government-sponsored low-interest credit programme was introduced in 1972, followed by the Special Agricultural Rehabilitation Fund (SARF), and between them the 2 initially reached a majority of lowland farmers. High default rates however, due to a variety of causes, led to a retrenchment in the coverage of the programmes. By 1981, only 10% of farmers in Nueva Ecija still benefitted from M-99 and SARF 156 FEBRUARY evd y sot u-aea cnl Al) uig the during le) (A canal sub-lateral short a by served iue . Dly i patn i rttoa areas rotational in planting in Delays 4.4 Figure 91 r season. dry 1981 T ITNE RM EDAE KM HEADGATE FROM DISTANCE 157 2 T 3 t • loans, according to data provided by The Philippine Crop Insurance Commission/ though the proportion was somewhat higher in the study area itself (vide infra). As few farmers are able to meet expenses from savings, most are thus forced to turn to non-institutional credit from merchants, salaried employees and the local elite. Interest rates from this source are highly variable. Those borrowing from agro-chemical dealers often pay 50-100% on an annual basis, though much of the loan may be in the form of fertilizer or pesticides and there may be an obligation to sell produce to the dealer at harvest (Loevinsohn et al. 1982a). Simple cash loans may bear interest of 250% and more, farmers with whom a relationship of trust has been established report. (Legal maximum rates are 2^-30% p.a. depending on the amount). These rates can be compared to the 12% borne by institutional loans. In the context of the timing of farm operations, farmers borrowing from the banks that administer government loans complain of delays in obtaining credit. Discussions with the manager of a local rural bank disbursing SARF funds suggest that a major stumbling block is the stipulation that 80% of the bank's outstanding lending be recuperated before application is made to the Ministry of Agriculture in Manila for release of the next season's loans. Thus even if an individual farmer repays promptly he may not receive his next loan for some time. In 1982, the Land Bank of the Philippines, the largest source of agricultural credit nationwide, was considering extending credit on an annual rather than a seasonal basis, but this scheme had not been implemented by the end of the study, nor was it being taken up by the other banks active in the area. In contrast, non-institutional credit, though often charging onerous interest, is at least simply negotiated and readily available. Lender and borrower generally reside near one another and little if any paperwork is required. 1 Beginning in that year all such loans were subject to compulsory insurance covering partial or total crop loss, with the premium split between the bank and the farmer. 158 4.3.3 Land preparation and crop establishment At the present time, ploughing is generally done by 2-wheel and less often 4-wheel tractor, while harrowing is by 2-wheel tractor or water buffalo. Many farmers own the smaller tractors and others hire them either within or outside the barrio. Transplanting remains the preferred method of crop establishment in the area and in the wet season it is employed by virtually all farmers. In the dry season, increasing numbers, though still a minority, sow pregerminated seeds onto the puddled soil (direct seeding). The chief advantages of this method are rapid turnaround and reduced labur costs; the major drawbacks are the increased losses to weeds and poor plant stand where water depth cannot be adequately controlled. Transplanting is generally done by hired labour ("upahan" in Tagalog), generally landless or near-landless women. Farmers in the area report that in previous years exchange labour (nsuyuann) between farming families was more prevalent, but that the adoption of the modern varieties with their shorter growth duration has led to a concentration of demand for transplanters that could not be met by family members. Historical accounts (McLennan 1980) suggest however that there has been a trend towards increasing use of hired labour since at least 1920, likely related to the rise of the landless population. The existence of economic markets for both labour and machinery suggests that, where supply is limiting, a concentration in time of demand will lead to an increase in hire rates. Farmers generally prepare their own seedbeds, following the wet-bed technique (De Datta 1981). Seedlings are typically 25-30 days old at transplanting, though both younger and, where there is risk of flooding, older seedlings are common. Their availability does not appear to be an important cause of delay in crop establishment. 159 Fig. 4.3 suggests that farmers are ready and willing to begin cultivation when the requisites become available or affordable. However, other considerations may affect farmers* decisions on when to plant, such as their expectations about the continued availability of water, or their fear of typhoons which can cause close to total destruction of the crop. Yet when farmers were asked, in a survey conducted during the wet season of 1982 and described in more detail below, whether, in order to plant at the times they preferred, they would delay their planting if irrigation water arrived early and abundantly, nearly 80% replied they would not. 4.4 The Components of Asynchrony A simple statistical procedure was developed in order to quantify the contribution of the various factors affecting the sequence of cultivation to the asynchrony of transplanting. With minor modification, the procedure can take account of the slightly different sequence of farm activities for crops established by direct seeding. 4.4.1 Methods Every paddy passes through a well-defined sequence of farming operations before rice can be transplanted into it. The arrival of irrigation water begins land soaking, unless the farmer prevents the water entering his field or relies on some other source, such as groundwater or rainfall. After a variable length of time, generally about a week, the land is ploughed and then harrowed several times (in Nueva Ecija usually twice) and finally transplanted. For any one paddy, this can be represented as: T = S + SP + PH + HT (4.1) where T is the date of transplanting, S is the date at which soaking commences, SP the interval between the beginning of soaking and ploughing, PH the interval between the beginning of ploughing' and harrowing, and HT the interval between harrowing and transplanting. 160 As Fig. 4.3 suggests, difficulties in obtaining each of the requisites of cultivation will enter the sequence at specific points : inadequate canal capacity will delay the beginning of land soaking, a scarcity of labour will affect the time of transplanting and so on. For a large number of farms, the mean date of transplanting can be expressed as the sum of the means of all the quantities on the right side of equation 4.1. The variance of transplanting dates will be given by the more complex expression: v (T) = v(5) + v (SP) + 2 Cov (5, SP) + v (PH) + 2 cov (p, PH) + v (HT) + 2 cov (H,HT) (4.2) where all the terms beginning with v represent the variance of the interval or event in parentheses, cov (S,SP) the covariance between 5 and SP, cov (P, PH) the covariance between the date of ploughing and PH, and so on. The overall asynchrony can then be attributed to factors that enter at each stage; together I refer to the terms on the right of eq'n 4.2 as the "components of asynchrony", analagous to the components of variance often calculated in ANOVA. A part of the variation in the length of time that various operations take may be systematic : where a covariance term is negative, this indicates that those farmers most delayed at the first event take less time than average to reach the second. For example, a farmer who finishes harrowing late may spend more money or look further afield for transplanting labour, or possibly cajole his grandmother into helping. Farmers may choose or be compelled to compensate in this way : if all one’s neighbours have already transplanted, it may be difficult to get a tractor into an interior field without damaging crops and relationships. Silva (1977) has pointed to similar factors promoting synchrony in Sri Lanka. Positive covariances are possible as well, as might occur if those delayed in harrowing because of a shortage of cash for tractor hire have for the same reason more difficulty than most in contracting tranplanters. i 161 The key contribution of certain factors to asynchrony disclosed by this method can be confirmed by further analysis. For example if the terms v (SP) and v (PH) are large compared to the other terms, it might be surmised that tractors or water buffalo are in short supply. If this is the case, it would be expected that the mean dates P and H would be earlier and/or the intervals SP and PH shorter for those farmers that own machinery or for those able to hire them locally than for those that are forced to search further afield. The procedure sketched above is in certain respects similar to the Critical Path Method (CPM) used in operations research (Elmagraby 1977). It differs in 2 respects. Firstly CPM appears generally to be used for predictive ends, for example to estimate how long a complex industrial process will take to complete, given estimates of the duration of the component activities, rather than, as here, in a diagnostic sense, to explain variation in the time of completion. Secondly, the concern of CPM is with the expected duration of a process and variation is taken into account, if at all, only in setting confidence limits for the prediction. In contrast, the procedure developed here focuses on variation in the time taken to complete the sequence per se*. The possible non-independence of successive events that is indicated by the covariance terms in eq'n 4.2 appears to be for the most part ignored by CPM. To obtain the data required to compute the components of asynchrony, 124 farmers were interviewed during the wet season of 1982 and again during the 1983 dry season. The interviews were conducted in the farmers' homes by a team of 3 field workers shortly after the crop had been established. A wide range of questions was asked of farmers, including some to which I will return later in this chapter and again in Chapter VII; in all, nearly one hour was required per respondent. The 124 farmers represented 1 In other agricultural contexts, the mean value may be of central concern. In rainfed areas where the possibility of fitting in a second wet season crop is being considered, rapid turnround after the first is vital. 162 an approximately 10% sample of those cultivating land within a block of some 2500 ha situated within the study area where the concept of synchronous planting was promoted beginning in early 1982 (the hatched area in Fig. 4 .2 ). As will become clear in Chapter VII, for a number of reasons the campaign did not have a major impact on the timing of cultivation and the components of asynchrony are unlikely to have been greatly affected by this intervention. Early in the wet season of 1982, Typhoon Emang struck the area, and as few crops were in the ground at the time, there was little damage. In terms of asynchrony however the typhoon had two effects. The first was that the abundant rain that accompanied it soaked fields in the area fairly evenly and allowed many farmers to begin land preparation. The second was that, due to wind and increased run-off, the main canal was breached a few km from its source on the Pampanga River and it took 6 weeks until it was repaired. For this period, the area reverted to rainfed status, and it was therefore impossible to establish a precise date for the arrival of water and the beginning of land soaking for most farms. The variance of transplanting date under these circumstances can then be expressed as: v (T) = v (P) + 2 cov (P,PH) + v(PH) + 2 cov (H, HT) + v (HT) (4 .3 ) In the following dry season, farmers perforce relied on irrigation releases. As mentioned earlier, the reduced water level in the reservoir caused NIA to cut back on the area served with irrigation. Overall in UPRIIS the reduction was some 30%, though it was not evenly distributed and within the area from which the sample was drawn less than 20% were without irrigation. To those served, water was distributed with greater variance than appeared to have been the case in previous years. Thus both seasons studied were to an extent atypical, the dry season more and the wet season less asynchronous than usual. I will return to consider this point more fully below. 163 4.4.2 Results The components of asynchrony for the dry season of 1983 are illustrated in Fig. 4.5 for those farmers who transplanted the crop. Two points should be noted. Firstly, variation in the date of arrival of water beginning land soaking can be seen to overshadow the contribution of the other factors to the variance of transplanting. Secondly, the three covariance components are all negative, suggesting that farmers delayed at each operation are able to compensate by completing the next one faster. Tests of significance of these terms, or rather of the correlation coefficients derived from them, are not strictly valid however, for essentially the same reason that the regression of k-values on log numbers in the study of population dynamics is invalid : one term, here the date of the first event, appears in both x- and y-variate, breaking the assumption of bivariate normality (Sokal and Rohlf 1969). This problem can be avoided by regressing on each other the dates of successive farm operations, measured and independent variables. These regressions are illustrated in Fig. 4.6 a-d. Slopes of less than 1 are indicative of compensation, as in the previous chapter slopes of less than 1 in the regression of log population at a given stage on that at an earlier one suggest regulation.* The slopes and the significance of their differences from 0 and 1 are given in Table 4.1 Much the same pattern can be seen among those farmers who employed direct seeding rather than transplanting. Establishing their crop in this way, farmers avoid the necessity of sowing a seedbed at least 20 days before tranplanting, and thus direct seeding appears to be favoured where the provision of the various requisites is uncertain or delayed. The slopes of the 1 Strictly speaking, a Type II regression should again be used. However, as the x- and y-variates are both dates estimated by farmers' recall at the same sampling occasion, it might be expected that their variances of measurement error are similar. In this case, Mandel (1964) indicates a Type I or Type II regression will yield similar results. 164 Figure 4.5 Components of asynchrony during the 1983 dry season among transplanted fields, n = 76. The following represent variances of dates or intervals: S - date of land soaking, SP - interval between soaking and ploughing, PH - interval between ploughing and harrowing, HT - interval between harrowing and transplanting. The compensation terms represent (negative) covariances between the adjacent dates or intervals. The variance of T - the date of transplanting - is a measure of asynchrony. 165 COVARIANCE or VARIANCE (days) -100 300 100 200 0 - - S ain 1 sation compen- p S sation 2 sation compen- H P compen- sation 3 sation T H T 166 transplanting h a r r o w i n g p l o u g h in g FEB JAN MAR 19 20 21 21 ARWING HARROW PLOUGHING Oo D= .00 TRANSPLANTING equations are given in Table Table in given are equations nrae hog te eune f utvto. d. cultivation. of sequence the through increase amr dlyd n n oeain opee h next the complete operation one in delayed farmers atr hn h aeae Cmesto apas to appears Compensation average. the than faster opnain vr h etr sqec. h full The sequence. entire the over Compensation esn 93 Soe ls ta 1 niae that indicate 1 than less Slopes 1983. season amn oeain i taslne fed, dry- fields, transplanted in operations farming iue . ac Rgeso o dts f successive of dates of Regression a-c. 4.6 Figure 168 4.1. Table 4.1 t value Regression Equation 0 = 0 /5 = 1 a. Ploughing vs arrival Y = .801 x + 8.41 11.02 *** 2.73 ** of w ater b. Harrowing vs ploughing Y = .771 x + 13.8 16.06 4.76 ¥ r ¥ r c. transplanting vs. harrowing Y = .635 x + 26.1 10.17. ** 5.86„ „ *** d. transplanting vs arrival of water Y = .518 x + 37.9 8.46 *** 7.86 *** Regression of dates of successive farming operations among sample farmers who transplanted the crop during DS 1983. n = 76. ** „ *** .001 < P< .01 ; PC .001. 169 regressions (Table 4.2) are again significantly less than 1, though greater than in the tranplanted fields, suggesting that among those employing direct seeding there is less capacity for compensation. The intercepts however are lower than in the corresponding regressions for transplanted fields and as_ a group, farmers employing direct seeding are able to complete each operation faster. Though the fields that were eventually direct seeded were 6.5 days delayed in relation to those transplanted at the arrival of water, by harrowing the gap had been narrowed to 0.5 days (Table 4.3). The components of asynchrony were similar during the preceeding wet season, despite the fact that the precise date of the arrival of water could not be determined. Variation in the commencement of the first operation, here ploughing, contributed most to the variance of transplanting, and once again all covariance terms were negative (Fig. 4.7). As in the dry season, the slope of the regression of transplanting on harrowing date was the least of any 2 successive farm operations, suggesting that compensation was greatest at this final stage. (Table 4.4.). 4.4.2.1 The availability of inputs and delay in operations From the slopes of the regression of successive farm operations, it appears that in both seasons the capacity for compensation is least between the arrival of water and ploughing. There are indications that this may be traceable to a relative shortage of tractors and water buffalo for land preparation. The fullest set of information on inputs was obtained during the wet season survey. Table 4.5 gives the date of the various farm operations in relation to the source of tractor or draught animal. It can be seen that those forced to hire outside the village were delayed in ploughing and harrowing by nearly 11 days relative to those who owned tractors or animals or were able to borrow them. By transplanting however, roughly half this disparity had been made up. 170 Table 4.2 t value Regression Equation £ - 0 /3 - [ -ft"#- * a. ploughing vs arrival Y = .914 x + 6.06 27.08 2.53 of w ater b. Harrowing vs ploughing Y = .882 x + 8.78 23.20 *** 3.10 ** c. sowing vs. harrowing Y = .821 x + 14.0 13.78 3.01 d. sowing vs arrival of water Y = .650 x + 25.7_ 9.72 _ *** 5.24 *#-* Regression of dates of successive farming operations among sample farmers who direct seeded the crop during DS 1983.n = 43. * .01 < P < .03 ; .001< PC.01; PC.001 171 Table 4.3 Mean date + std. error Arrival of Ploughing Harrowing w ater Transplanted fields 8.0 + 2.0 13.0 + 2.1 23.3 + 1.8 Direct seeded fields 14.3 + 3.9 19.3 + 3.6 23.8 + 3.3 Timing of farm operations in transplanted and direct seeded fields, dry season 1983. n = 76, 43 respectively. January 1 = 1 COVARIANCE OR VARIANCE (days) -200 J -100 200 100 0 e sao aog rnpatd ils n 14 for 124 = n fields. transplanted among season wet iue . Cmoet o aycrn drn te 1982 the during asynchrony of Components 4.7 Figure luhn ad arwn, 1 fr transplanting. for 119 harrowing, and ploughing P sation compen 2 H P 173 compen sation^ T H T Table 4.4 t value Regression Equation f t = 0 ft = i a. harrowing vs ploughing Y = .875 x + 23.5 16.05 *** 2.45 * of w ater *** b. transplanting vs Y = .409 x + 74.5 7.51 10.84 harrowing c. transplanting vs *** ploughing Y = .416 x + 79.8 7.12 9.97 Regression of dates of successive farming operations among sample farmers during WS 1982. n = 124 for a, n = 119 for b and c. Significance indicated as in Table 4.2. 174 Table 4.5 Mean date * + std. error source n ploughing harrowing transplanting Owned or borrowed 49 9.0 + 2.0a 23.5 + 2.2a 48.9 + 1.5a from family or friends Hired within village 51 15.5 + 2. lb 29.6 + 2.1b 50.4 + 1.7a Hired outside village 24 20.1 + 3.4b 34.2 + 3.9b 54.4 + 2.7a Timing of farm operations in relation to source of tractor or draught animal among sample farmers, W5 1982. Means in a column followed by the same letter do not differ at 5% by LSD. * July 1 = 1 175 A rather different picture emerges in relation to transplanting labour, the other major purchased input prior to crop establishment. As in the case of tractors, farmers may hire transplanters residing in the barrio or outside, though there is a preference for local labour, as the farmer is liable for transportation and subsistence expenses. Table 4.6 suggests that it is the farmers delayed in earlier operations who rely on labour from further afield, rather than that reliance being a cause of delay in itself : those who hired transplanters outside the municipality were 9.9 days late in harrowing relative to those contracting within the barrio, but only 3.6 days at transplanting. The data suggest that a reliance on outside labour enabled farmers to compensate for delay. That this should be so, and that, as the earlier regressions indicated, the greatest compensation in the sequence of cultivation is evident between harrowing and transplanting are not to be wondered at when 50% of the households in several of the barrios where the survey was conducted are landless or near-landless (R. de la Cruz, barrio captain of Sta Rita, Jaen and F. Carpio, community organiser, pers. comms.). A relative abundance of labour would be expected to facilitate compensation ; by the same token it would be expected that the response of hire rates to a concentration of demand for transplanting labour would be less than for a commodity such as tractors that appears to be relatively scarce. Concentration of demand occurs naturally each season as farmers seek services for the various operations. If land preparation costs, inclusive of fuel, are regressed on the date at which ploughing begins,* a significant slope is obtained of P 2.24/day (t = 2.00, p < .05, n = 76). A farmer beginning ploughing one standard deviation earlier than the mean would thus pay 6.5% less than the average (P530). If, in the same way, total costs incurred in transplanting, inclusive of transportation and subsistence, are regressed on the date at which transplanting begins, a slope of only P' 0.82/day is 176 T able 4.6 M ean date + std. error source n ploughing harrowing transplanting Hired within 67 11.8 + 1.9ab 26.3 + 2.1ab 49.1 + 1.6a barrio Hired in adjacent barrio 25 16.7 + 3.2bc 29.3 + 2.9bc 50.7 + 2. la Hired within municipality 7 16.3 + 4.7bc 29.9 + 5.6bc 51.3 + 3.0a Hired outside municipality 20 19.9 + 2.7C 36.2 + 2.8C 52.7 + 1.7a Timing of farm operations in relation to source of transplanting labour, WS 1982. 5 of the 124 sample farmers employed direct seeding and are not included here. Means in a column followed by the same letter do not differ at 5% by LSD. 1 July 1 = 1 177 obtained (t - 2.08, P < .05, n = 119). A farmer beginning one standard deviation earlier than the mean would in this case pay 4.0% less than the average (P 243). Among the inputs not under village control, credit appears to have had the least impact on the timing of farm operation during the wet season. As Table 4.7 indicates, the dates of the various operations did not differ significantly between farmers borrowing from institutional and non-institutional sources, nor were the intervals between operations significantly different for the 2 groups. On further questioning, 53% (60 of 113 respondents) claimed that delay in the arrival of loans need not lead to delay in transplanting. To compensate farmers said they may skimp on production expenses, dip into savings, or, more often, borrow further from non-institutional sources. As explained earlier, irrigation-related problems were unusually reduced during the 1982 wet season due to the even soaking that Typhoon Emang provided the area and the temporary breakdown of the irrigation system in the storm’s wake. Nevertheless, uneven water distributuion prior to the typhoon and drainage difficulties thereafter contributed to asynchrony. Farmers who when surveyed claimed that they suffered from problems of drainage were found to plough and harrow 6-8 days earlier thean those who said they did not, though again much of the disparity disappeared by transplanting (Table 4.8). In the following dry season, the dominance of irrigation-related factors was clearer. As there is less water to evacuate than in the wet season, drainage difficulties do not contribute greatly to asynchrony, but those farmers who claimed to suffer from long-standing problems of water distribution in the canal upstream or in the farm ditches after the turnout were substantially delayed. Most affected were those suffering from both (Table 4.9). 1 In both regressions, quadratic terms did not account for significant additional variation, though on theoretical grounds (Stigler 1966), prices would be expected to fall once the mode of the demand had been passed. That this was not observed may be due to delay in some adjacent areas that kept demand high. 178 T able 4.7 Mean date * + std. error source n ploughing harrowing transplanting Institutional 31 13.5 + 2.3a 26.9 + 2.5a 51.9 + 2.3a Non-institutional 69 14.1 + 2.0a 27.3 + 2.1a 49.7 + 1.4a Timing of farm operations in relation to source of credit, WS 1982. 12 of the 124 farmers interviewed did not borrow for production expenses, 11 relied on both institutional and non-institutional credit, and no information is available for 1 farmer. 1 July 1 = 1 179 T able 4.8 Mean date * + std. error claims major n ploughing harrowing transplanting drainage problem? No 74 16.4 + 1.9a 31.2 + 1.9a 51.5 + M a Yes 50 10.0 + 2.0b 23.5 + 2.2b ^9.1 + 1.6a Timing of operations among farmers suffering or not from long-standing drainage problems, WS 1982. Means in a column followed by the same letter do not differ at 5% by t-test. 1 July 1 = 1 180 In comparison, tractor ownership or the source of credit were less clearly associated with delay (Table 4.10), though the information collected with respect to these factors was less detailed than in the wet season. 4.4.3 Discussion If one computes the overall variance of transplanting date during the dry season of 1983, adding 25 days, the average age of seedlings, to the date of sowing in direct seeded fields, one arrives at a figure of 319 days , compared to 141 days the previous wet season. The difference is highly significant, (F^, = .001). It is difficult to escape the conclusion that water-related factors have played the key role in this increase : only these are likely to vary appreciably between seasons. Mechanisation and population growth will affect the availability of tractors and of transplanting labour, but these are processes that act on substantially longer time scales. More detailed investigation is needed to determine the ultimate capacity of the village-based resources of tractors and labour in terms of a minimum variance of farm operations, but clearly they are sufficiently abundant to complete land preparation and transplanting within the shorter period that these spanned in the wet season. The density of tractors in the area, 31 per 100 farms or 137 per 1000 ha, is relatively high and comparable to the national average in South Korea, an agriculturally more advanced country, in the late 1970's (Jayasuriya et al. 1982). Roughly 1 working day per ha is required for each of the 3 land preparation operations - 1 ploughing and 2 harrowings - (McMennamy and Zandstra 1978, quoted in Jayasuriya et al. op. cit., suggest 28 hours for the 3 together). Were the scheduling of land preparation within the area perfectly rational, that is, were tractors used first to plough ail farms before beginning, in the same order, each of the 2 harrowings, each operation would be completed within a span of 7.3 days. A uniform distribution with this range would have a standard deviation 181 T able 4.9 Mean date ^ + std. error Problem n A rrival ploughing harrowing transplanting of w ater Neither 29 4.1 + 3.3a 1 1 .1 + 2.7a 21.5 + 2.5a 39.3 + 2.0a Either 27 6.9 + 2.8a U .« + 3.5a 24.2 + 3.1a M .1 + 2 A a C anal- or Turnout - level Both 15 14.3 + 4.8b 19.7 + 4.9b 30.7 + 4.2b 1(6.5 + it.0 b Timing of farm operations in relation to irrigation problems of long-standing claimed by farmers, DS 1983 (transplanted fields only). No information available for 5 of 76 respondents. In a column, means followed by the same letter do not differ at 5% by LSD. 1 July 1 = 1 182 Table 4.10 Mean date + std. error A. source n A rrival ploughing harrowing transplanting of tractor of w ater or draught animal owned 25 9.7 + 3.6a 15.2 + 3.3a 25.8 + 3.2a 41.2 + 2.8a hired or borrowed 47 5.8 + 2 ,9a 13.2 + 2.6a 23.6 + 2.2a 41.8 + 1.8a B. source of credit institut ional 15 6.2 + 4.8a 17.9 + 9.9a 25.1 + 4.3a 39.3 + 9.0a non - inst itutional 50 10.9 + 2.5a 16.5 + 2.5a 26.9 + 2.3a 99.3 + 1.7a Timing of farm operations in relation to source of tractor or draught animal and source of credit, DS 1983 (transplanted fields only). No information available for 4 farmers in (a) and in (b) not included are 6 farmers who did not borrow for production expenses and 5 farmers who relied on both sources. In either part of the table, means followed by the same letter do not differ at 5% by t-test. * duly 1 = 1 183 of 2.1 days, compared to the wet season standard deviation of 15.5 and 16.7 days at the beginning of ploughing and harrowing respectively*. In practice, the theoretical minimum will be exceeded because some tractors will be out of service at any moment and because movement from farm to farm will require some additional time. Perfect scheduling is impossible where the preceding event, the arrival of water, is uncertain and where communication between farmers requiring the service and tractor owners offering it is not instantaneous. A tight schedule, slightly less than perfect, would also be expected to create a sellers' market for tractor services and drive up prices. There may thus be economic pressures to space out operations between farms. The data available from the surveys are insufficient to estimate the absolute number of tranplanters resident in the area and their capacity for completing transplanting. As labour, like machinery is responsive to demand, the local density is not a dependable guide to the speed with which transplanting can be completed. Labour appears to be the more mobile of the 2 resources: members of landless and smallhold families often travel from as far as Laguna to Nueva Ecija and vice versa in order to take advantage of differences in the cropping schedule. The methods described in this section provide basic information on the origins of asynchrony and the capacity for compensation that is essential if synchronisation is being considered as a pest control tactic. Other concerns must also weigh in any judgement of its feasibility and desirability and I return to these in Chapter VII. The questionnaire and statistical techniques employed here however might be further refined into a survey tool capable of providing a rapid overview of the dimensions of the asynchrony problem in a given area. 1 Neither of these distributions, nor that of transplanting date, is found to deviate significantly in skewness or kurtosis from a normal distribution. For transplanting date, g, = .188 (t = NS) and g2 = .423 (t122 = .96, NS). 184 4.5 The Irrigation System and Asynchrony In this section I focus on that factor which from the previous analysis appears to be the key determinant of asynchrony. Within a gravity irrigation system, water flows from the main canal into laterals and sub-laterals, whence it is diverted through turnouts to the farm ditches serving individual fields within the rotational area. Distribution decisions, physical constraints and design defects influence the flow at each level; the variation of transplanting date can then be attributed to components entering at different points in the system hierarchy. 4.5.1 Methods With the assistance of supervisory personnel, forms were distributed to NIA staff serving the 20,000 ha study area. For each field within a rotational area, the date of transplanting or direct seeding was noted, along with the area planted, the variety used, the date of harvesting and the farmer's reported yield, all data that were routinely collected in any case. It is this information that forms the basis for the analyses described in later sections and in the following chapter. Water Management Technicians, administering sections of approximately 500 ha were responsible for completing the forms, relying on the observations of 3 or more ditchtenders. Where direct seeding was employed, an equivalent date of transplanting was obtained by adding 25 days to the date of sowing. Statistical calculations were carried out weighting the date by the area of the field. * A random effect, nested analysis of variance was calculated of transplanting date within Division 2B. The division includes most of the area sampled in the survey reported in the previous section and is served by 13 sublateral canals and 76 turnouts. As the data constitute a complete census of farms within the area, the total degrees of freedom of the ANOVA need not be reduced by 1, nor are tests of significance required. 185 0 . 2 Results and Discussion The components of variance of planting date, are presented in Table ti. 11 for the dry and wet seasons of 1981, that is respectively 2 years and 1 year previous to those analysed in the survey. The table indicates that variance of transplanting in 1981 was greatest in the wet season. Irrigation records made available to me and operations reports (NIA 1980) suggest that, in most areas of UPRII5, this was the case in 1979 and 1980 as well. As I indicated earlier, in this respect the 2 seasons covered by the survey were atypical. The data indicate that the greater variance in the wet season is due to increase at two levels: between canals and between fields served by a given turnout. Differences between rotational areas on a given canal are on average similar in the two seasons. Though water distribution itself may be eased somewhat by rainfall in the wet season, sharp differences remain between the dates at which up - and downstream rotational areas are served with water; this variation may be exacerbated by drainage problems at the downstream end, largely absent during the dry season. The overall impact of drainage difficulties in the area is attested to by the reduction of 12% in planted area in the wet season. These however are concentrated in the landscape, in areas served by particular canals, and this fact may be responsible for the increased between-canal component of variance. At a finer spatial scale, within rotational areas, low-lying fields may be in danger of damage from excess water and farmers may thus opt to plant either earlier or later than their neighbours. Being low-lying, rainfall runoff will collect in these fields and permit earlier land preparation. In terms of pest pressure, the increase during the wet season in variance at the finest spatial scale, between fields, suggests that, for even the least vagile species, there will be a lengthening of the time available for population growth. Further studies in other years and areas will be needed to confirm the generality of the pattern observed here. The impact on pest populations of seasonal differences in asynchrony will be taken up once more and in greater detail in the next chapter. 186 Table 4.11 Dry Season Wet Season Variance component Variance % Variance % added added Among canals 17.8 13.6 53.5 26.3 Among rotational areas within canals 59.4 45.4 59.1 29.0 Among fields within rotational areas 5 3 .7 41.1 90.9 44.7 Total variance of transplanting date (days ; 130.8 203.5 Area planted (ha) 1813 1592 Components of variance of transplanting date within Division 2B, District III, UPRIIS during 1981. 187 In studies of natural plant communities, methods such as the contiguous quadrat method of Grieg-Smith (1964) have been used in order to isolate the principal spatial scales at which population processes operate. In an agricultural context different procedures are likely to yield greater insight. The method that has been used in this chapter has been to first analyse the sequence of cultivation, determining the factors that exert the greatest influence on variability in the timing of farm operations. In the 2 seasons studied in the previous section, the dominance of water-related variables emerged clearly. It then becomes possible to consider the spatial pattern of planting date in relation to this dominant factor, dividing the landscape in a manner relevant to variation in water supply. Had the availability of transplanting labour been shown to be critical in determining asynchrony, it may have made more sense to consider variation of planting date within and between villages which differ in the proportion of landless households. One is free of course to divide the landscape as one chooses and there are an infinite number of possible schemes. However, a division in units relevant to the dominant factor is likely to be most useful in explaining and predicting trends in asynchrony over years, between seasons and between areas. 4.6 Asynchrony as a Function of Distance Though explanation of variation in asynchrony may be facilitated by adopting the spatial units of irrigation, it is unlikely that pest populations, if they do in fact respond to asynchrony, will align themselves in relation to field, rotational area, and canal. For non-sedentary pests, distance itself is likely to be a more useful metric in qualifying variation in planting date. 4.6.1 Methods In order to study the impact of variation in planting date on pest populations, 21 sites were selected within the 20,000 ha study area over a period of 4 months. In addition, 2 sites were selected 188 beyond the western edge of the NIA-irrigated area, one in a newly completed communal irrigation system, the other in a single crop rainfed area. The sites were located along transects that approximately paralleled the direction of irrigation flow and traversed areas where irrigation or drainage problems gave rise to marked and localised asynchrony. I attempted to maintain a distance of roughly 2 km between sites, but reduced this where the transect crossed the problem area in order to be able to consider a range of conditions of asynchrony as a function of distance. At each site, 2 kerosene light traps were installed and farmer co-operators enlisted to operate them. In some instances it was necessary to move sites from their intended position in order that they be near inhabited areas. The pair of light traps were located from 100 m to 450 m apart, generally determined by the proximity of houses. Details of the trap setting and operation are given in the next chapter. Variance of planting date was calculated for each site, taking as reference the midpoint between the 2 traps. Using a motorcycle odometer, pacing, and a compass, the traps were located on a 1 : 20,000 scale irrigation map that indicated rotational areas but not individual holdings. I designed a transparent overlay, produced by the IRRI graphics department, consisting of concentric circles in increments of 1 cm and lines radiating from the centre at intervals of 5°. When placed over the site centre, the overlay marked off circles in radial in c re m e n ts equivalent to 200 m on the ground, while the radii cut segments from the annuli of readily calculable area (Fig 4.8). By counting the number of segments falling within a given rotational area, its contribution to asynchrony could be estimated. The procedure adopted was as follows: (1) Irrigation records were first summarised in each rotational area, i. For each field or holding reported, j, its date of transplanting was converted to a cardinal number (in the case of direct seeding, 25 days were added to the date of 189 Figure 4.8 Transparent overlay and 1:20,000 scale irrigation map used to calculate asynchrony as a function of distance from a site. 190 sowing) and the area rounded to the nearest "luwang" - the traditional unit of land measure in the Tagalog region, equivalent to 500 m 2 . The sum of dates, x.., and the sum of squared dates Xj. 2 , were calculated, weighting ^ by area, in addition to n., the total area in luwangs of the rotational area. (2) Using the overlay and map, the number of segments within a given annulus occupied by rotational area i was counted and the equivalent area n.1 computed. For example, one segment in the annulus between 1.8 and 2.0 km represents 6 6 A iuwangs (( TT 2.0^ - Tf 1.8^)/72 segments x 100 ha/km x 20 luwangs/ha). On the assumption that within a rotational area variation in planting date is distributed uniformly, within the n.’ luwangs, the estimated sum of planting dates will be n.' — i n.i and the estimated sum of squared planting dates £ x .‘ n.i (3) Within a given circle, these values were summed over each of the rotational areas i falling within it to form quantities N =Xn.' I i X. x. and u■ i 2' ? Xi From these were calculated mean arid variance of planting date in the usual manner, save that as the irrigation records provided a complete census, the parametric formula for variance was employed, dividing the sum of squares by N rather than N - 1. 191 (4) In similar fashion, I calculated the mean maturity of the varieties planted as a function of distance from the site centre. A major problem was encountered in terms of the reliability of the information provided by NIA personnel. Records from the southern part of the study area traversed by transect 3 suffered from obvious flaws in both D5 and WS 1981, the 2 seasons so far analysed. Reported planting dates in a number of key areas did not agree with my personal observations and the records were internally inconsistent in that some downstream rotational areas were reported planted more than a month prior to those upstream on the same canal. Although in some instances it was possible to correct what were clearly errors/ in others it was not. Irrigation field staff appeared in some instances to resent the additional work that filling out the forms entailed, for which they received little but our thanks. In the northern section of the study area however the situation was much better. I came to know several of the key personnel on a first name basis and often discussed irrigation metters with them. The purpose and relevance of the research and of the SFOP in general were also better understood; this and the more personal contact were crucial I believe in securing the assistance of the field staff. No serious inconsistencies were noted in the data and my knowledge of this area was better than in the south. For the two sites lying outside the NIA area, information on planting date was available from Ministry of Agriculture production technicians. However, the absence of accurate tenurial maps precluded calculation of asynchrony as a function of distance. In what follows therefore I will restrict my attention largely to the 14 sites on transects 1 and 2 within the NIA area. 1 Status of Farm Activities reports - less detailed internal documents - were made available to me by NIA and these largely confirmed my observations. 192 4.6.2 Results and Discussions Figure 4.9 illustrates the trend of variance of planting date at these sites as a function of distance from the site centre in two seasons. (The dry season curve is based on only 13 observations as one site had not then been established.) As the previous section indicated, variance is in general greater during the wet season and it is seen here to increase with distance at a faster rate than in the dry. A maximum is attained in both seasons near 1 km, though there is a slow increase up to 2 km in the wet season. Were a larger scale considered, abrupt increases in variance might be expected as areas were encountered served by lateral canals and eventuaiy neighbouring systems following different cropping schedules. Both curves pass through the origin as at the finest spatial scales, within a field, planting is generally completed within one day. As Fig. 4.io indicates, there is considerable variation between sites in asynchrony at a given distance. In the dry season of 1981, variances of from 19 to 339 days 2 were found within 1 km and in the wet season from 41 to 381 days .2 The differences are substantial in ecological terms : at the most synchronous site in the dry season, BPH would find time for less than 1 additional generation within _+ 2 standard deviations of the mean date, over and above what it might complete in an area of perfect synchrony, but more than 3 additional generations at the least synchronous. It is this degree of variation between sites that makes possible the "natural experiments" on the impact of asynchrony reported in the next chapter. The fact that some areas achieve minimal variance in planting without evident exertion gives hope that interventions aimed at reducing asynchrony may be feasible, an issue to which I will return in C hapter VII. The importance of water-related factors is once again apparent if one considers the detailed distribution of planting dates at the least synchronous sites. Fig. 4. H illustrates the pattern around Batitang, near the downstream end of the irrigation network and invariably 193 4------H ---- 1------1------— f .4 .8 1.2 1.6 2.0 RADI US (km) Figure 4.9 Variance of planting date as a function of distance from the site centre. Filled symbols - dry season, open symbols - wet season, 1981. Values plotted are means over 13 and 14 sites respectively. 194 NUMBER OF SITES 0 2 4 0 ■ 2 ---- f h sts a dy esn b wt esn 1981. season, wet b. season, dry a. sites, the of iue .0 aine f lnig ae ihn km 1 within date planting of Variance 4.10 Figure ------1 ---- A I N E WITHIN VARIANCE 100 1 1 ------ 1 ------1 ---- 200 1 ------195 f ------m (days2) 1 km 0 0 3 1 ------ 1 ------00 40 b f Figure 4.11 Batitang, Zaragoza: an area of chronic asynchrony. The values given in the map are variances of planting date within rotational areas. 196 among the areas with greatest variance. Two principal constraints traceable to differences in topography, are evident. In the southeast of the barrio, on relatively high terrain, irrigation is inadequate and farmers fall back on supplemental shallow well pumps. To the west of the road, clogged drainage channels give rise to a serious flooding problem even in the dry season, which however affects fields to varying degrees depending on microtopography. The m ap gives planting date variances within rotational areas and these are highest in these 2 sections. The overall variance is affected as well by a trend in mean planting date between rotational areas, with those in the west and south being as much as 20 days delayed relative to those to the north and east. The result is that within the village's fields one can find rice in any stage of growth at almost any time of the year. Finally, in Fig. 4.12 I plot the mean wet season yield reported to NIA by farmers in the 14 rotational areas where light traps were installed against the standard deviation of planting date within 1 km of the site centre. A significant negative relationship is observed, with yields at the most asynchronous sites little more than half those at the least. The correlation coefficient is found to be maximal with standard deviation calculated at 1 km, rising steeply from near 0 at 0.2 km and declining gradually to .610 at 2 km. The slope of the regression is -2.69 cavans per day of standard deviation (a cavan is a local unit, equal to ca. 50 kg of unmiiled rice); at 1981 prices and exchange rates \ this represents a loss of $27./ha per day of standard deviation. As suggested thoughout this chapter, asynchrony is generally associated with irrigation-related problems and it is likely that their direct impact on yield contributes to this decline. Input use, notably of fertilizer, may also be somewhat greater at the upstream sites which are in general relatively synchronous (see below). However, the trend is also such as one would expect from the fact that, as I hope to show in the next chapter, the populations of the principal insect pests increase with asynchrony. 1 1 kg unmilled rice = F 1.50; $1. = F 7.50 197 mean yield (cavans /ha) e sao 18. 0 aas- 1 o. h least The ton. 1 - cavans 20 1981. season Wet eain o h sadr dvain f lnig date. planting of deviation standard the to relation iue .2 en il i rttoa aes in areas rotational in yield Mean 4.12 Figure qae euto is: equation squares tn a d deviationstandard within 1 (days) km = 0 - .9 ( = 32, C .01). PC -3.24, = (t 2.69X - 107 = Y 198 12 4.7 Variation of Intensity Before proceeding to consider that question, it must be pointed out that there is variation between sites not only in asynchrony but in several parameters of intensity that may affect pest populations. Firstly, the area devoted to rice cultivation is found to vary, but within relatively narrow bounds. From the irrigation maps it is possible to estimate the proportion of land falling within NIA's service area - that which is potentially irrigable (this is larger than the programmed area, which is in turn larger than the area actually served). Within 1 km of the sites, the service area ranges from 85% to 98% of land area, and averages 91.8%, with a coefficient of variation of 4.3% (n = 14). There is variation as well in the varieties that farmers plant and these differ in their time to maturity. Again however the range is restricted : the mean maturity of varieties within 1 km of the sites was 120.0 days in the 1981 wet season, with a coefficient of variation of 3.8%. Farmers in the area, particularly in the wet season show a preference for IR42, a medium duration variety maturing ca. 135 days after germination. The grain has good eating qualities and fetches a favourable price, while in the field the variety is reputed to be comparatively resistant to lodging. In several of the more asynchronous areas however farmers plant a larger proportion of their holdings to early-maturing varieties such as IR36 (110 days duration), possibly because of the greater threat from hazards such as flooding and irrigation cut-off. Among the 14 sites .a significant negative correlation was found between the standard deviation of planting date in the wet season and the mean maturity of varieties within 1 km (r = -.70, P < .01). This finding, while of interest in itself, is of importance in interpreting the results of the following chapter. On theoretical grounds (2.10) the mean time to maturity is expected to be positively correlated with pest abundance. If a significant positive relationship is uncovered between asynchrony and pest density, it is unlikely that this is traceable to differences in maturity. 199 Variation between sites is also found in the proportion of land double cropped, as indicated in 3.3.2. Though the parameter has not been measured at all sites, variation is not expected to be substantial. Furthermore, those sites which are most asynchronous are likely to be those with the largest areas out of production in one season or the other. Once again, a positive correlation between asynchrony and pest density is unlikely to be attributable to underlying differences in cropping intensity. Finally, there is variation between the sites in the use of chemical inputs. A survey was conducted after the 1981 dry season (in collaboration with C.G. de la Cruz) of 36 farmers in 6 barrios within the study area. Expenditure on fertilizer was found to average P 358./ha and P 144./ha on insecticide. Variation between barrios was significant in terms of fertiliser use (F^ = 3.10, .01< P < .025), but somewhat less than significant in the case of insecticide (F^ = 2.32, 05 < P < .1). There was however a significant correlation in farmers' use of the 2 inputs (r = .50, P< .01). In both cases it was the upstream, relatively synchronous areas where expenditures appeared to be greatest, though with only 6 barrios, such an identification is uncertain. If that is in fact the case, then the finding of a positive correlation between asynchrony and pest density is unlikely to be due to the stimulative effects of nitrogenous fertilizer on population growth. Insecticide-induced resurgence would produce the opposite relationship, while in terms of its suppressive effect, the information available on the prevailing pattern of use suggests that insecticide is not applied in a density dependent fashion between farmers (Marciano et al. 1981). In the following chapter it will be possible in some instances to account for variation in intensity factors through multiple regression techniques. For those that are not well quantified however the above discussion suggests that what variation there is among sites is likely to no more than obscure the hypothesized positive relationship between asynchrony and pest density; there is no reason to believe that it will contribute to it. 200 CHAPTER FIVE Population Dynamics and Asynchrony 5.1 Introduction Asynchrony is a characteristic of localities, expected to favour the development of local populations. A necessary consequence of this &t- the level of individual fields is that those that are delayed in planting will suffer increased infestation, and it is with a test of this hypothesis that I begin the chapter. I then describe the farmer-operated light trap network that served to test directly hypotheses concerning the impact of asynchrony itself. The density estimates it provides are shown to be reproducible and significantly correlated with measures of damage on an area-wide basis. I proceed to show that variation in light trap catch between sites in two seasons is positively associated with asynchrony for all but one of 9 species considered. The details of the relationship are shown to accord with what is known of the biology of the pests : the response of a species to a given standard deviation of planting date is roughly proportional to its instrinsic rate of increase and the distance over which it responds to asynchrony is related to the estimated dispersal range which I measure under natural conditions. Finally, I consider the implications of these findings for the implementation of synchronous planting schemes and for other aspects of cropping system design. 5.2 Pest Damage in Relation to Delay In section 2.9.1 it was argued that, under fairly unrestrictive conditions, infestation will be greater in late than in early planted 201 fields and increase at a faster rate. "Late" it was suggested must be defined in a manner relevant to the pest : it was argued that what is crucial is whether delay is sufficient to provide time for the development of an additional generation. This hypothesis is open to experimental test. 5.2.1 Methods During the wet season of 1981, plots were leased from 10 farmers among those tending light traps as part of the network described briefly in section 4.6. One subplot of 250 m 2 was transplanted at about the median date of fields within the surrounding rotational area. As one must sow the seedbed about 3 weeks prior to transplanting, this was necessarily an approximation based on the progress of land preparation in neighbouring fields. In most of the sites however the date of transplanting was within a few days of the median calculated later from irrigation records. Two other subplots of 125 m 2 each were established, one 2 weeks and the other 4 weeks after the median. I will refer to these subplots as "+2 weeks" and "+4 weeks" respectively. All three were planted to IR52 at 25 x 25 cm spacing. 60 kg N/ha in the form of urea was applied, split between a basal treatment and one at panicle initiation. No insecticide was used. The subplots were sampled 4 times for stemborer damage at 4, 6, 8 and 13. weeks after transplanting (WAT), the first 3 occasions for deadhearts (severed vegetative tillers), the last for whiteheads (severed panicles). Leaf folder (RLF) was sampled 4 times as well at 4, 6, 8 and 11 WAT, but caseworm (CW) only once, at 4 WAT, corresponding approximately to the peak of infestation. The levels of these latter 2 leaf feeding pests were assessed as the proportion of leaves damaged in 25 hills systematically sampled along 2 diagonal transects. This intensity was found adequate to produce estimates with a standard error of ca. 25%. Stemborer damage was assessed as the proportion of tillers or panicles damaged in 100 hills similarly sampled; this added effort was necessary in order to obtain standard errors of ca. 25% at the low levels of infestation prevailing. 202 Given the distance between sites and the already crowded sampling schedule, it was found impossible to monitor I3PH and GLH damage as well. This would have required the use of a D-Vac or similar suction device which it was found difficult to maintain in the field. 5.2.2 Results In Fig. 5.1 I plot the mean proportion of deadhearts or whiteheads at the ten sites over the 4 sampling occasions. There is evidence of an increase in infestation with time in all plantings, but the rate of increase is greatest in the +4 weeks subplots. This is confirmed by the ANOVA in Table 5.1 which indicates significant effects due to sampling occasion, planting date and to the interaction of these factors. This latter effect is traceable to the variation between planting dates in the slope of infestation with time : that in the +4 week planting is significantly greater than the slope in the +2 week planting (t^ = 2.82; .01 < P < .025)* and in the median planting (t^ = 2.82; .01 < P < .025) but the latter two do not differ among them selves (t^ = -.16; NS). Figure 5.2 illustrates the mean proportion of leaves damaged by caseworm at the one sampling occasion that this pest was monitored. Though there appears to be a consistent increase with delay, only the difference between the median and the +4 weeks plantings attains significance under a one-tailed test (t^g = 1.84; .025 A completely contrary pattern emerges in the case of leaffolder (Fig. 5.3). Here there is no indication of a common trend of infestation with time, as the ANOVA in Table 5.2 confirms. There is however a significant effect of time of planting, yet it is the subplots at the median date that exceed the other 2 plantings, which do not differ significantly among themselves. 1 Under a one-tailed test, appropriate to the hypothesis. 203 DEADHEARTS OR WHITEHEADS 4 k - wks +4 2 k - wks +2 ein - median transformation ra n 2 n 4 ek ltr Wt esn 1981. season Wet later. weeks 4 and 2 and area er h mda dt o te urudn rotational surrounding the of date median the near h latsurs qain, sn a arcsin an using equations, least-squares The iue . Sebrr aae n upos planted subplots in damage Stemborer 5.1 Figure NG IN T N A L P S N A R T R E T F A S K E E W = Y = Y = Y .9 + -.292 f % damage, of 11 + -1.15 .5 + -.254 204 .178X .352X .171X are: 5 3 ^ 1 3 ^ 33 t ( 5.62, = 6.10, = 5.04, = CL, .001), P< 00 ), 01 .0 < P V o o 1—1 Table 5.1 Analysis of variance of stemborer damage, wet season 1981. No in fo rm a tio n is available for 9 of 120 samples. Source of Variation df Mean square F Probability Between sampling 3 24.536 36.9 P<.001 occasions Between planting dates 2 3.355 5.04 ,005 205 damaged leaves - 6 4 -- - 2 ae. Wet season later. ro, = 0 sites. 10 = n error, h sronig oainl ra n 2 n 4 weeks 4 and 2 of and median area the near rotational planted surrounding subplots in the transplanting iue . Cswr dmg 4 ek after weeks 4 damage Caseworm 5.2 Figure e in 2 ek +4weeks +2 weeks median caseworm 91 Te as niae + std. _+ 1 indicate bars The 1981. 206 DAMAGED LEAVES ra n 2 n 4 ek ltr Wt esn 91 n = n 1981. season Wet later. weeks 4 and 2 and area er h mda dt o te urudn rotational surrounding the of date median the near 0 sites. 10 iue . Lafle dmg i sblt planted subplots in damage Leaffolder 5.3 Figure EK ATR TRANSPLANTING AFTER WEEKS 207 Table 5.2 Analysis of variance of leaffolder damage, wet season 1981. Source of Variation df Mean square F Probability Between sampling 3 .697 .1 NS occasions Between planting dates 2 8.654 9.57 P<.001 Interaction 6 .7 39 1. NS Error 108 .904 Means over sampling occasions (% leaves damaged) Median planting date : 1.265 + 2 weeks : 0.440 + 4 weeks : 0.481 Least significant difference (P = .05) = .425 3.2.3 Discussion Of the 3 pests considered, 2, sternborer and caseworrn, demonstrate the anticipated increase in damage with delay in planting. Leaving aside for the moment leaffolder that does not in any respect conform to expectation, I consider first the differences in the detailed response to delay of these 2 species. In particular, Figures 5.1 and 5.2 suggest that a 2 week delay in planting has an effect on caseworm but not on stemborer damage, though as pointed out above the increase in the former case does not attain the conventional threshold of significance. At the risk of overinterpreting an uncertain result, I suggest that the overall pattern illustrated in the 2 figures is what the hypothesis being tested leads one to expect. Fig. 4.11 gave mean planting date variances as a function of distance during the wet season of 1981. Expressed as standard deviations, these varied between almost 11 days within 0.2 km and 14 days within 2 km. In Fig. 5.4, I illustrate the proportion of the rice area which is expected to have been planted more than 1, 2 or 3 generations prior to a given planting date, where the generation length is that of caseworm, roughly 20 days. I have assumed here that planting dates are approximately normally distributed, as was found to be the case in section 4.4. Within 0.2 km of a site, caseworm would on average have been able to complete a generation on only 4% of the area prior to the median planting; within 2 km, variation in planting date being larger, one generation of the pest might have emerged from 8% of the farm area. However, where planting was delayed 14 days, there would have been sufficient time for a generation to be completed on 29-33% of the area, and a second generation on 1-3%. Thus if population growth is postitive during those generations, the input of eggs to the second experimental planting is expected to have been greater than to the first. By 4 weeks after the median, one generation could have emerged from nearly 3/4 of the area, 2 generations from up to 20% and, beyond 209 Figure 5.4 Proportion of the farm area planted a given number of generations prior to specified dates within 0.2 km (std. deviation of planting date = 11 days) and 2 km (std. deviation = 14 days). The generation length of the pest is taken to be 20 days, equivalent to that of caseworm. Figure 5.5 As in Fig. 5.4, but for a pest with a generation length of 45 days, equivalent to that of yellow stemborer. 210 STANDARD DEVIATION: 11__DAYS 14 DAYS j 1. generat ion ^ 1 generat ion 4% 8% MEDIAN PLANTING DATE <1 generation 96% 9 2 \ >2 generations >2 generations 1% 3% TWO WEEKS AFTER MEDIAN 67% > 2 generations , , <1 generation FOUR WEEKS AFTER MEDIAN 21 1 5TAHDAIU) DEVIATION: 11 DAYS 14 DAYS MEDIAN PLANTING DATE >1 gen. 1% 99% >1 generation >1 generation 6% 11% FOUR WEEKS AFTER MEDIAN < 1 generat ion 94% 89% 212 the immediate vicinity of the site, a 3rd generation from about 1%. Again, positive population growth would be expected to lead to higher initial numbers in the 3rd than in the 2 previous plantings. These graphs illustrate the number of generations that might have emerged prior to transplanting at a given date, but a similar pattern would be found were the calculations performed for any specified stage of crop growth. Figure 3.3 illustrates the number of generations of yellow stemborer that could have emerged from fields established prior to the 3 experimental plantings. Any given variation in planting date in the vicinity or delay in planting in the experimental plots makes possible fewer additional generations of yellow stemborer, with its generation length of roughly 43 days*, than of caseworm with one less than half as long. At the median planting date, no fields are likely to have been established more than 1 generation previous; at the planting 2 weeks later, only 1% of fields within 2 km of the site would have been planted more than 1 generation previous and less than 0.2% within 0.2 km. There would thus be no expectation of substantially greater colonisation of the plots planted 2 weeks after the median than of those planted at the median, as Fig. 5.1 appears to confirm. It is only at the +4 weeks planting that there are significant areas established more than a generation previous and infestation in these subplots is in fact found to be greater than in the 2 planted earlier. Differences in the rate of response of the 2 species to delay may also be explicable in terms of their bionomic characteristics. The 4 week span between the 3 experimental plantings makes possible more generations of caseworm than of yellow stemborer. The extent to which pest numbers and hence damage are affected by the delay however depend not only on the number of additional generations but on fertility and survival during the period, as integrated in r, the rate of increase. In section 5.6.1.6, I present estimates of maximal rates of increase, r calculated from light trap catches during the 1981 wet season : for caseworm 1.92/month and for yellow stemborer 1.13/month. 1 The variability in generation length of this species is considered in the following chapter. 213 The increase in damage of the 2 species in the experimental plantings reflects this difference. Caseworm damage at 4 WAT is 98% greater in the +4 weeks than in the median subplots, while that of yellow stemborer is only 41% greater at this same sampling occasion. Though the change is for both species less than one would expect from the maximal rates of increase, the ratio of the change in damage (98% / 41% = 2.39) is close to the ratio of the finite rates of increase calculated from the r max values (6.82/3.10 = 2. 20 ). The results for these 2 species broadly confirm the hypotheses generated in Chapter II that fields delayed in planting by at least a generation will suffer greater infestation, increasing at a faster rate than in those not delayed. Leaffolder does not conform to expectation and reasons for this can only be suggested. Studies at IRRI (Kamal 1981) have indicated very high rates of predation on this species leading to combined egg and larval mortalities of 95-99%, greater than is generally observed for example on BPH (Dyck et al. 1981). If natural enemies build up rapidly after planting or readily switch from alternate prey to attack RLF, then the pest would be prevented from increasing within any one field, and late planted fields might well have lower infestations than those planted earlier, both patterns that emerge from Fig. 5.3 and Table 5.2. If this is in fact the case, then, in contrast to YSB and CW, one would not expect local differences in the aerial density of RLF to be positively correlated with asynchrony. This hypothesis is tested in section 5.6. 5.3 The Kerosene Light Trap Network As described in the previous chapter, 23 sites were selected within the 20,000 ha study area. Two farmers were approached in each rotational area and asked if they would assist the research by tending a kerosene light trap. A regular effort was involved in cleaning and then lighting the lamp each evening and collecting the catch in the morning, in exchange for which several things were offered. We provided advice on pest control and organised a series of day-long seminars for the entire group 214 of co-opcrators. Wc also arranged to have the farmers’ soil analysed for fertilizer recommendations by the Bureau of Soils and made a number of small gifts from time to time, including new seed varieties, sample packets of zinc sulfate (much of the area suffers from zinc deficient soil, readily alleviated by proper drainage or inexpensive soil amendment), and items like raincoats (which we suggested might be of some use, faute de mieux, as protection while spraying insecticide). For the most part farmers appeared to co-operate readily, though in general it turned out to be the children who were given the chore of minding the light. In only 2 cases out of 46 was it apparent that the lamp was not being lit and neighbouring farmers were enlisted after some tactful excuse was found to switch. Personal relationships were established with several of the farmers by various members of the Zaragoza office staff, and valuable information on different aspects of rice farming, its economics and ecology, was obtained in this way. The first light trap was established in early March 1981, at the beginning of the dry season crop, and the network was maintained for 3 seasons, through July 1982. The lamp employed has been briefly described elsewhere (IRRI, 1979a). Originally intended for use by fishermen on Laguna de Bay, it consists of a kerosene burner fashioned from a baby-food jar, set within a glass-walled lantern with base, roof and chimney fashioned from beaten tins. The lamp is simple and rugged (several were in continuous operation for more than 1 year) and sufficiently inexpensive at P 25. (ca. $ 3.00) to permit extensive replication. Minor modifications were made to adapt it for use as a light trap. A short length of wire was attached to the burner as a flame guide, allowing the farmer to raise or lower the wick to produce a flame of constant height. A supplemental metal roof was also attached to prevent rain from entering the chimney. 215 The lamp was suspended from a bamboo tripod firmly anchored in the farmer's field beside a dike for easy access. A plastic basin 35 cm in diameter was set below the lamp and filled to constant height with water to which detergent was added. The basin was approximately lm 20 above the ground, just higher than the varieties commonly grown. The lamps were sited well away from obstructions such as trees or hedges that might affect the catch and at least several hundred meters from the nearest electric light (only a few of the barrios had electricity) (Fig. 5.6). Each morning the farmer or a member of his family strained the catch through a seive and then transferred it to a pre-labelled plastic bottle containing 70% ethanol. Once a week a team on motorcycle from the Zaragoza office came by to collect the filled bottles and leave new ones, replenish the farmer's supply of kerosene, detergent and matches, and inspect the lamp. Requests for advice on pest control were also passed on at this time. The catches were sorted and counted by 2 lab aides at the Zaragoza office. Seven taxa were monitored : yellow stemborer (YSB), caseworm (CW) and its congenor Nymphula fluctosalis, rice leaffolder (RLF), rice green semi-looper Naranga aenescens (RGS), brown planthopper (BPH) and green leafhopper (GLH), though for the latter, records were kept beginning only with the 1981 wet season. N. fluctosalis is a somewhat larger insect than the common caseworm, N. depunctalis, and appears to prefer aquatic weeds, though it has been shown to attack rice (J. Bandong and J.A. Litsinger, pers. communications). iWales and females of YSB and RLF were counted separately as these are readily distinguished with the naked eye. Sub-samples of 25 BPH per site were examined weekly under the microscope, sexed, and the proportion of the superficially similar Nilaparvata bakeri, which is not thought to be a pest of rice (T.J. Perfect, pers. comm.), determined. Finally, the proportions of the 3 common species of GLH, Nephotettix virescens, N. malayanus, and N. nigropictus were estimated, again from a sample of 25 per site. 216 Figure 5.6 The kerosene light trap. 217 5.4 Variation Among Traps Light traps have been employed by numerous workers in studies of insect population dynamics in both natural (Wolda 1977, 1978) and agricultural environments (Hartstack et. al. 1973; Taylor 1979). For the most part however the traps have been of relatively sophisticated design and have been operated by skilled, though as in the case of the Rothamsted Insect Survey, (Taylor, op. cit.) not always paid personnel. Before further analysis is attempted of the farmer-operated, kerosene light trap network, the reliability of this method of population monitoring must be assessed. What is of prime concern is its ability to distinguish local variation in aerial density, rather than, say, fine-grained temporal differences. For this, the replication of traps within sites is an important asset. Neighbouring traps, operated independently, will be found to have catches more closely related to each other than to those at other sites if 2 conditions are met : firstly, that substantially greater variation in aerial density exists at the scale of the separation between sites, roughly 2 km, than between traps within sites, some 200 m, and secondly, that the catch in a trap reflects aerial density, that the effect of, for example, failure to light the lamp or to clean it of soot, or of later misidentification in counting is insufficient to obscure that relation. Further consideration of the light trap data in order to assess the impact of asynchrony is warranted only if there is a significant between-site component in the variation of catches. 5.4.1 Results and Discussion Table 3.3 presents the results of single level analyses of variance of seasonal total catch of the seven taxa monitored during the wet season of 1981. The seasonal total has been calculated as described in section 3.2.1 and the 14 sites included in the analyses are these for which reliable irrigation records are available (4.6.2). 218 Table 5.3 Yellow C ase- Nymphula Leaf- Rice Brown G reen Stem borer worm fluctosalis folder green plant- leaf- semi- hopper hopper looper F - ratio 5.21 2.52 11.82 6.34 2.43 6.69 24.77 Probability *** **** *** + *** #** Mean seasonal catch per trap 603 31 267 45 396 1441 2032 Range 194 - 8 - 65 - 13 - 132 - 91 - 99 - 1513 149 573 107 1070 8351 8430 Between-site F-ratio from analyses of variance of seasonal total catch, wet season 1981. n = 14 sites, 2 traps per site. The F - ratios have 13 degrees of freedom in the numerator and 14 in the denominator. + .05< P < .1 * .02 >< P < .05 *** P < .005 219 The seasonal total is found to vary widely among traps: there is an almost 8-fold range of YSti catches and one of nearly 90-fold in the case of both I3PH and GLH. As the table indicates, a substantial portion of this variation occurs between sites, and this component of the analysis of variance is significant for all but one of the taxa, RGS, where it almost attains significance. Remarkably, site effects are apparent even in the case of CW where the mean catch per trap over the season was only 32 insects and among RLF females (not listed) of which on average only 21 were captured. The results suggest that the various distorting influences mentioned above are insufficient to obscure the differences in aerial density between sites. It should be recognised however that, were there a correlation between farmers within sites in their propensity to light the lamps or clean them of soot, this too would appear in the analysis as a "site effect". I have no reason to suspect such collusion or systematic lack of co-operation, yet the possibility can only be adequately discounted by providing convincing evidence that the origin of the site effects lies elsewhere. This I hope to do in the following sections. Taylor (1979) has shown that correlations between suction trap catches decline as the distance between the traps increases. The significant between-site component in the above analyses of variance suggests that a similar pattern over at least some range would emerge were the data treated in the same way. In the present instance, I contend that uneven host plant availability, due principally to varying asynchrony, contributes substantially to the spatial autocorrelation of pest populations and their light trap catches. 5.5 Trap Catches and Pest Damage Powerful additional evidence of the reliability of the light trap network as a means of monitoring pest density would be provided by the demonstration that trap catches were significantly related to levels of damage measured in the surrounding fields. Obtaining such evidence requires a substantial sampling effort, given the variability in infestation between and within fields. Nevertheless, an effort was made to establish this correlation for two pests. 220 5.5.1 Methods In section 5.2, I described an experiment in which stemborer damage was measured in plots planted at 3 dates in 10 farmers' fields. In nine of these sites a light trap was located within a hundred meters of the experimental plots and it is thus a simple matter to investigate the correlation between stemborer damage and the seasonal total catch of YSB*. Another opportunity presented itself during that same wet season. Rice tungro virus, of which there had not been a serious outbreak since 1972, increased markedly in several of the areas monitored during the early stages of the crop and an effort was made to follow its progress. Tungro incidence was measured in 6 of the rotational areas in which light traps were located along Transect 1 and in one on Transect 3. In each rotational area, 5 fields were selected at random which were planted to either IR36 or IR42, the two commonest varieties with relatively poor resistance to both the disease and its vector, GLH. (Newer varieties, IR50, 52 and 54, with a different source of resistance were planted at that time by a minority of the farmers). Fields were sampled once during the early ripening stage, by which time most infection would have occurred. One hundred and fifty hills were examined per field for the typical symptoms of the disease : reddening of the leaves, stunting, and poor exertion of the panicle (Ling 1972). However, tungro virus is known to be highly variable in its manifestations and in particular it was found difficult to distinguish late infections, which would not affect vegetative growth or- the emergence of the panicle, from the effects of nutritional disorders such as zinc deficiency. Visual observations were therefore supplemented by a chemical test described by IRRI (1966) on leaf samples taken from suspect hills. The leaves were first boiled in 1 As few moths of other stemborer species were caught in the trap, the damage can safely be ascribed to YSB. 221 ethanol to extract the chlorphyll and then immersed in iodine; a positive reaction was signified by the leaves turning black, indicating the presence of starch. Though the test is not specific for tungro virus, none of the common nutritional disorders or bacterial or fungal diseases are thought to cause significant starch build-up (H.Hibino, pers. comm.)^. Other viral diseases may give a positive reaction, however these are not common in the area and can readily be distinguished from tungro on the basis of visual symptoms 2 The disease recurred the following dry season and its incidence was measured in the same manner, but at only the six sites along Transect 1. 5.5,2 Results and Discussion In Fig. 5.7, I graph the mean proportion of stemborer deadhearts at 4, 6 , and 8 weeks after transplanting in the "median" experimental plot against the seasonal total catch of YSB at the site. The linear regression is highly significant. 1 Plants suffering from what were clearly nitrogen and zinc deficiencies, and leaf streak and bacterial leaf blight diseases were found to test negative in trials at Zaragoza. 2 . The test is quick, simple and can be performed using materials available in most rural towns. Extension technicians expressed interest in it and group of farmers employed it, substituting Nescafe jars for test tubes. Some farmers had been advised by bank or Ministry technicians to plough under their seedbeds or newly-transplanted fields and the test was useful in confirming, or in several cases disconfirming, the visual diagnosis. 222 deadhearts 0 1 . 2 -- ______qain osrie t ps truh h oii is: origin the through pass to constrained equation e sao 18. aae s sesd s h mean the as assessed is Damage 1981. season Wet rprin f ilr dmgd t , ad WT in WAT 8 and 6 4, at damaged tillers of proportion h mda sblt. = . h least-squares The 8. = n subplots. median the esnl ac o te net n ery ih traps. light nearby in insect the of catch seasonal iue . Sebrr aae n eain o the to relation in damage Stemborer 5.7 Figure f ------= .4 lfX t 11.4, P<.001). = (t l(f5XY = 6.54 x esnl oa catch total seasonal 600 1 ------ 223 ¥ ------1200 ------1------“I 1------1------“ 0 1800 ----- The proportion of hills infected with tungro virus is plotted in Fig. 5.8 against the seasonal total catch of its vector, GUI. As the multiple regression makes clear, infection increases, as expected, with the seasonal catch and is significantly greater in the dry than the preceding wet season. This latter effect may be due to the increased probability of an insect landing on a diseased plant and becoming a carrier after the disease had built up through the wet season. In both this and the previous analysis of stemborer damage, the regression lines are constrained to pass through the origin, a procedure justified on both theoretical grounds (if no insects are flying and none caught in the traps it is expected that no damage will be found in the surrounding fields) and by the fact that the intercepts, if they are calculated, do not differ significantly from 0. The regressions that have been calculated are not likely to be of much use in a predictive sense, particularly as the seasonal total can only be determined once the season is over. Further studies are needed to establish whether the kerosene light trap can be used to forecast infestation sufficiently in advance and with sufficient accuracy to become a practical tool in pest management. In the present context however the results serve to increase one’s confidence that light trap catches reflect the actual aerial density of the pests. It now remains to explain variation in this density. 5.6 Trap Catches and Asynchrony The central concern of this chapter is to determine the extent to which local variations in pest density are related to the asynchrony of cultivation in the vicinity. In what follows, I employ regression analysis to establish the relation between the logarithm of seasonal total catch in light traps and the standard deviation of planting date within given distances of the sites. With the variates expressed in these units, a linear relationship indicates exponential increase with time. I then determine whether the inclusion of a quadratic term of standard deviation or the mean duration of the varieties planted in 224 TUNGRO INFECTION - 3 12 6 9 0 A A | -A AA efopr te etr f h dsae i nearby in disease, the of vector the leafhopper, qain osrie t ps truh h oii is: origin the through pass to constrained equation ils n eain o h saoa cth f green of catch seasonal the to relation in fields rp. pn ybl - S 91 coe smos DS - symbols closed 1981, WS - symbols Open traps. hr X i saoa cth n X i a dummy a is X2 and catch seasonal is X1 where 92 2 rp pr ie Te least-squares The site. per traps 2 1982. aetee ae -aus Te vrl Frto is F-ratio in overall numbers The The season). t-values. dry are = 1 wet, parentheses = (0 variable 99 P .0 wt 21 dges f freedom). of degrees 2,11 with .005 (P< 19.9 iue . Icdne f ugo iu i farmers' in virus tungro of Incidence 5.8 Figure ESNL OTL AC x103 CATCH TAL TO SEASONAL ▲ ▲ A A A ---- A A A A = .1 1' t 3.60X2 3 + Xt 10'4 x 6.91 = Y 8 4 1 ------41) (2.72) (4.19) f------r 225 2 6 20 16 12 i i ------A 1------1 ------ 1 the vicinity improves the fit of the equation. Throughout, I base the decision of whether to retain additional variables in the equation on whether they increase the coefficient of determination corrected for degrees of freedom (R ), and reduce the standard error of the estimate (s e ) in a{[ the cases here considered, the two criteria are found to be in agreement. 5.6.1 Results and Discussion - Wet Season 1981 5.6.1.1 Yellow stemborer and brown planthopper In Figs. 5.9 and 5.10 I plot the logarithm of seasonal total catch of YSB and BPH against the standard deviation of planting dates within the distance at which the correlation is found to be maximal. The slopes of both simple regressions are positive and significant. In neither case does the inclusion of either a quadratic term or the mean maturity of the varieties planted improve the fit of the regression. However, male YSB are found to be better correlated with asynchrony than the species as a whole (Fig. 5.11), while females are not significantly correlated with the variable at ariy distance, though the slope is positive up to 1 km from the site. Why the sexes should respond differently is not immediately apparent, though Hartstack (1979) states that light trap catches of male Heliothis moths are the most accurate indicator of field populations in maize, sorghum or cotton. I will return to consider this question in the light of the dry season results. These .results are summarised in Table 5.4. Figure 5.12 illustrates how the correlation between seasonal total catch and the standard deviation of planting date varies as this latter is calculated within increasing distances from the site. For YSB males the best fit is obtained with the standard deviation measured within 400 m of the site, that is within an area of 50 ha, 226 standard deviation of planting within 400m Figure 5.9 Seasonal light trap catch of the yellow stemborer in relation to the synchrony of planting. Wet season 1981. The regression equation is given in Table 5.4. standard deviation of planting within 2000m Figure 5.10 Seasonal light trap catch of the brown planthopper in relation to the synchrony of planting. Wet season 1981. The regression equation is given in Table 5.4. seasonal total tnad eito o patn within planting of deviation standard s ie i Tbe 5.4. Table in given is tmoe mls n eain o h snhoy of synchrony the to relation in males stemborer lnig Wt esn 91 Te ersin equation regression The 1981. season Wet planting. iue .1 esnl ih ta cth f yellow of catch trap light Seasonal 5.11 Figure 228 400 m CORRELATION COEFFICIENT (r) 1.01 itne ihn hc te atr s acltd for calculated is latter the which within distance ugsig ht P i te oe vagile. more the is BPH that suggesting rp ac ad snhoy a a ucin f the of function a as asynchrony, and catch trap elw tmoe mls n te rw planthopper. brown the and males stemborer yellow h rsetv mxm ae t . ad km, 2 and 0.4 at are maxima respective The iue .2 h creain ewe saoa light seasonal between correlation The 5.12 Figure 229 P=.05 while catches of BPH are best related to asynchrony within 2 km, the greatest distance within which it was calculated, equivalent to 1257 ha. In section 2.6 an empirical method of this sort was proposed as a means of determining the dimensions of the locality for a given pest, one likely to provide a more realistic estimate than mark-recapture methods. The radius within which the greatest correlation with asynchrony is obtained will, I suggest, be found to be proportional to the mean distance that adults of the taxon move within the additional generations that asynchrony makes possible; I describe an experimental test of this hypothesis in a later section. For the present however it suffices to note that the implication that BPH is more vagile than YSB finds support in what is known or suspected of the 2 species' dispersal patterns: McNaughton (1946, quoted in Bannerjee and Pramanik 1967) claims that YSB can fly as far as 8 km, while the BPH that colonize Japanese rice fields each year are believed to originate from eastern China, a distance of several hundred kilometers (Kisimoto 1976). 5.6.1.2 Nymphula spp. For both species of Nymphula, the best fit was obtained with a second order equation of standard deviation measured within 400 m, the coefficient of the linear term positive and that of the quadratic term negative. These regressions are illustrated in Figs. 5.13 and 5.14 and suggest that populations attain a maximum at intermediate levels of asynchrony. This would be expected, as suggested in Chapter II, if intra- br inter-specific regulatory mechanisms become effective only after several additional generations. For both species, the vertex of the parabola occurs at 13-14 days of standard deviation, implying that in the 95% of fields planted closest to the median, there is time for nearly 3 additional generations prior to regulation, over and above what could be completed within any one field or within a perfectly synchronised locality. (Again, I assume a normal distribution of planting dates). 230 seasonal total . - 5. tna d eito o patn within planting of deviation standard lcoai i rlto t te ycrn of equation synchrony regression The the 5.4. Table to 1981. in season given Wet relation is in planting. fluctosalis n eain o h snhoy f lnig Wt season Wet planting. of synchrony the to relation in 5.4. 91 Te ersin qain s ie i Table in given is equation regression The 1981. iue .4 esnl ih ta cth f Nymphula of catch trap light Seasonal 5.14 Figure iue .3 esnl ih ta cth f caseworm of catch trap light Seasonal 5.13 Figure 400 m taxon distance within equation overall which best fit F-ratio obtained (km) Y ellow stemborer .4 In Y = 6.02 + .0842 SO 6.15 * (2.48) males A In Y = 5.36 + .0093 SD 9.80 ** (3.13) Brown planthopper 2.0 In Y = 4.31 + .223 SD 14.7 **- (3.83) Caseworm A In Y = -1.23 + .870 SD - .0340 SD2 4.68 *-* (2.85) (-2.66) Nymphula fiuctosalis A In Y = 2.92 + .505 SD - .0180 SD2 4.20 * (2.06) (-1.76) Rice green semi-looper A In Y = 1.77 + .636 SD - .0233 SD2+ .0374 MAT 2.62 x (2.19) (-1.95) (1.90) Green leaf- hopper (all species) .6 In Y = 2.31 + .752 SD - .0226 SD2 6.11 * (2.20) (-1.74) Nephotettix virescens .6 In Y = .908 + .790 SD - .0245 SD2+ .0362 MAT 3.76 * (2.43) (-2.02) (1.24) N. malayanus 2.0 In Y = 2.13 + .316 SD 29.1 *** (5,39) nigropictus 1.4 in Y = -1.33 + 1.17 SD -.0443 SD2 3.66 + (2.32) (-2.10) Table 3.4 Best-fitting regresssion equations relating 1981 wet season total catches to parameters of rice availability. SD - standard deviation of planting date within specified radius; MAT mean maturity of varieties planted within that distance minus 100 days. t-values of the regression coefficients are given in parentheses, n = 14 sites. x .1< P< .2 + .05< P< .1 * .01 < P< .03 ** .OOKPx.Ol *** P < .001, all by 2 - tailed tests. Adding the mean maturity of varieties planted as a third term in the multiple regression did not improve the fit. 5.6.1.3 Rice green semi-looper The best fitting model for this pest was one that included both a quadratic term of standard deviation within 400 m and the mean maturity of the varieties planted within that distance, although the regression as a whole did not reach statistical significance. However, the partial regression coefficient of the linear term on its own was very nearly significant (t = 2.19, compared to a critical value of 2.23) and would have been considered so under a one-tailed test. 5.6.1.4 Green leaf hopper All three Nephotettix species were found to be positively correlated with asynchrony measured within some distance. virescens, generally considered the most serious leafhopper pest of rice, in view of its efficiency as a vector of tungro virus (Ling 1975) and its relatively narrow host range, was the most abundant of the 3, representing 54% of the total catch during the 1981 wet season. The best fit to the seasonal catch of the species was obtained at a radius of 600 m with a 3 parameter model including linear and quadratic terms in standard deviation and the mean maturity of the varieties planted (Table 5.4). N. malayanus has, to my knowledge, never been reported from the Philippines, though it is relatively common elsewhere in Southeast Asia, including Indonesia, Viet Nam and Malaysia (Ghaurie, 1971). 33% of the total Nephotettix catch in the light traps was of this species. Little detailed research has been conducted on its biology, but it is thought to be primarily restricted to weeds, principally Leersia hexandra. However in Nueva Ecija, A. Alviola (pers. comm.) was able to recover malayanus from rice fields by sweep netting, though he encountered difficulties during a first attempt at rearing it on rice under controlled conditions. 233 The seasonal total catch of the species was found to be highly correlated with asynchrony : 71% of the variation in log numbers was associated with the standard deviation of planting date within 2 km, and a quadratic term or the mean maturity of the varieties planted contributed no further explanatory power. The closeness of the relationship suggests that malayanus does in fact exploit rice and is affected by its availability, though it is conceivable that the asynchrony of weeds occuring together with rice may contribute to the correlation. The very different patterns of correlation between light trap catch and asynchrony as a function of distance for N. virescens and malayanus, illustrated in Fig. 5.15, suggest that the latter is the more vagile of the two. 12% of the total Nephotettix catch was of nigropictus, considered to be a less efficient vector of tungro virus than N^_ virescens. The numbers of this species were best predicted by a second order equation in standard deviation of planting date within 400 m, though in this case the result only approached the conventional level of significance. 5.6.1.5 Rice Leaffolder Recall that in the experiment reported in section 5.2, it was found that damage due to RLF did not increase over time either within any one subplot, or between subplots planted at different dates, in contrast to what was observed of caseworm and stemborer damage. It was hypothesized then that in this case the aerial density of RLF would not be positively correlated with the degree of variation in planting date. In Fig. 5.16 the simple correlation coefficient between the seasonal catch of this species and the standard deviation of plating date is plotted as a function of the radius within which the latter variable is calculated. At no distance is the correlation positive, let alone significantly so. Introducing a quadratic term or the mean maturity of varieties as additional independent variables does not improve the fit substantially. 234 SIMPLE OR MULTIPLE CORRELATION COEFFICIENT itne ihn hc te atr s acltd for calculated is latter the which within distance rp ac ad snhoy a a ucin f the of function a as asynchrony, and catch trap epcie aia r a 06 n 2 m suggesting vagile. more km, the is 2 and malayanus 0.6 at are maxima that respective ehtti vrses n mlyns The malayanus. and virescens Nephotettix iue .5 h creain ewe saoa light seasonal between correlation The 5.15 Figure RADIUS 235 (km) correlation coefficient 0. o efodr I cnrs t te te pet, t no at ests, p other positive. the to correlation the contrast is In distance leaffolder. rp ac ad snhoy a a ucin f the for of calculated function is a latter as the which within asynchrony, and distance catch trap iue .6 h creain ewe saoa light seasonal between correlation The 5.16 Figure .4 radius .8 (km) ?.36 .2 1 1.6 0 . 2 Thus for 8 of 9 species, seasonal light trap catches were found to increase with the variability of planting date in the vicinity of the site; in 6 of 8 cases the relationship was significant at the 5% level of probability. Only RLF was not correlated at all with asynchrony, but this had been expected on the basis of its within-field population dynamics. For 5 species there was evidence that the impact of asynchrony saturated at the highest levels encountered, suggesting density dependent regulation. In particular, for 2 of 3 Nephotettix species, equations incorporating quadratic terms were found to provide the best fit to the data, as might have been anticipated from the results of chapter III where there was evidence of strong regulation in the response of green leafhopper populations to intensification. For all taxa however, populations appear to increase over several additional generations. It may be objected that the probability of obtaining a significant result is increased by the procedure adopted, in which the best-fitting equation is found among 30 possible regressions (3 statistical models and 10 distance intervals for each), breaking the assumption of the F- or t-test that the comparison has been planned a priori. However, in 2 of the 3 cases where a multiple regression equation was found to give the best fit to the data at .05, the simplest one parameter model was significant as well at some distance, and the addition of a quadratic term or the mean maturity of the varieties planted merely served to account for residual variation. Moreover, the seasonal total was generally correlated with asynchrony at more than one distance : the number of BPH and N. malayanus for example were significantly correlated with asynchrony at 7 and 8 of 10 distances respectively. This should not be surprising, as the standard deviation from 0-1 km, for example, is well correlated with that from 0-2 km (r = .942), both because the former is a part of the latter and because of spatial autocorrelation in the conditions that promote asynchrony. Thus there are effectively far fewer than 10 independent asynchrony variables, and the likelihood of a significant result is much less than ten times greater than had there been only one measure of asynchrony. 237 5.6.1.6 The rate of response to asynchrony Confidence in the reality of the relationship between aerial density and asynchrony is increased by the correspondence of the calculated regression coefficients with measurable characteristics of the species. The slope of the relationship, b, has dimensions of Mlog units per day of standard deviation". For species whose numbers respond positively to asynchrony, the slope is expected to be related to their potential rate of increase. Again, this hypothesis is subject to test with the available data. As in section 3.3.2.3, the maximum monthly rate of increase, r max’ , can be estimated from the maximum difference in natural logarithms of the catch in successive months. It should be noted that, calculated in this way, rmax measures not only the natural rate of increase, the difference between births and deaths, but the balance of immigration and emigration as well. However, b itself is affected by these factors. In Table 3.5 I list the maximum monthly rates of increase at the 14 sites and the coefficient of the linear term relating log seasonal catch to the standard deviation of planting date from the best fitting model. I include here only those species for which a significant correlation at P .05 was obtained. I also provide only one aggregate value for GLH of each parameter, as the estimates of rmax *or ^dividual Nephotettix species, calculated from relatively small subsamples of the GLH catch, are less reliable than for the other pests. A positive correlation is evident between the 2 parameters, significant under a one-tailed test, despite the fact that only 3 degrees of freedom are available. Two of what I suggest may be the principal determinants of r are also listed in the table, generation length and the degree of pest resistance in the varieties commonly planted. Yellow stemborer stands apart with its much longer generation length; among the others, all with generations of about 20 days, rates of increase appear to reflect the effectiveness 238 Table 5.5 Linear coefficient of the best fitting equation relating wet season 1981 total catch to standard deviation of planting date and the maximum rate of increase on a monthly basis, both as calculated at 14 sites. Theory suggests the 2 parameters should be positively related : the observed correlation between them is 0.813 (.01 < P< .05 by one-tailed test). Also listed are 2 factors that may be important determinants of r max, generation length and degree of varietal resistance to the pest. The generation length of N, fluctosalis has not apparently been measured, but might be assumed similar to that of its congenor, caseworm. 239 Species slope + rmay/rno. approx, generation resistance of St. e rro r + St. e rro r length (days) common v a rie tie s yellow steraborer (0.4 km) .084 + .034 1.13 + .095 40 low brown p la n t hopper (2.0 km) .223 + .058 1.46 + .156 20 high green le a f hopper (0.6 km) .752 + .342 1.53 + .188 20 low (IR 42) moderate (IR 36) high (IR 54) caseworn (0.4 km) .870 + .305 1.92 -»• .221 20 low Nymphula 240 flu c to - (0.4 km) .505 + .245 1.78 + .193 20 (?) low s a lis of varietal resistance. At the time of writing, natural selection of BPH had not yet significantly eroded resistance. This is not the case for GLH : varieties such as IR36 and IR42 that have been planted in the area for 5 years or more are judged in recent tests at IRRI to be "susceptible" and "highly susceptible" respectively. (A. Marciano-Romena, pers. comm.). Newly introduced varieties such as IR52 and IR54 derive their resistance from other, still effective major genes and were grown on a substantial minority of farms by the 1981 wet season. However, none of the varieties available to farmers possess any significant resistance to the 2 Nymphula species and.the measured rates of increase for these are the highest. 5.6.2 Results and Discussion - Dry season 1981 Somewhat less information is available from the previous dry season, the first in which the light trap network was in operation : only 13 sites (26 light traps) had been established in the area from which dependable irrigation data were obtained and catches of GLH were not recorded until the very end of the season. The results however broadly confirm the pattern found in the wet season, though with some revealing differences. Figs. 5.17 and 5.18 illustrate representative results for yellow stemborer males and the rice green semi-looper. Though the relationships between seasonal catch and asynchrony are positive over the range of observed values, both appear to be non-linear and convex downwards. The same is true of all the species or groups for which significant correlations are obtained (Table 5.6) : in each case a second-order equation provides the best fit, with the coefficient of the linear term negative and that of the quadratic term positive. It will be recalled that, during the wet season, in the 5 cases where a quadratic term improved the fit to the data, the relationship was convex upwards. I suggest that this difference between the seasons is understandable in terms of the theoretical development of Chapter II. 241 seasonal tnad eito o patn wti 200m within planting of deviation standard eiloe i rlto t te ycrn of equation synchrony regression The the 1981. to season Dry relation in planting. semi-looper s ie i Tbe 5.6. Table in given is s ie i Tbe 5.6. Table in given is tmoe mls n eain o h snhoy of synchrony the to relation in males stemborer lnig Dy esn 91 Te ersin equation regression The 1981. season Dry planting. iue .8 esnl ih ta cth f h green the of catch trap light Seasonal 5.18 Figure iue .7 esnl ih ta cth f yellow of catch trap light Seasonal 5.17 Figure 242 taxon distance within c iquation overall which best fit F-ratio obtained (km) Yellow o stem borer 1 .4 In Y - 7.84 - .460 SD + .0232 SD + .0568 MAT 4.03 * (-1.98) (2.36) (1.60) males 0.8 In Y _ 6.64 - .166 SD + .0109 SD2 4.12 * (-1.04) (1.57) fem ales 1.8 In Y — 4.98 + .131 SD 5.52 * (2.35) Brown 9 planthopper 2.0 In Y = 12.6 - 1.52 SD + .0531 SDZ 4.24 * (-2.23) (2.39) 2 Caseworm 0.2 In Y = 6.25 - .403 SD + .0271 SDZ 3.83 (-2.20) (2.51) Nymphula 2 depunctalis 0.4 In Y = 8.92 - .630 SD + .0345 SDZ 6.22 * (-3.37) (3.50) Rice green 2 semi-looper 0.2 In Y = 6.13 - .138 SD + .0161 SD 5.57 * (-0.70) (1.37) Table 5.6 Best-fitting regression equations relating dry season 1981 total catches to parameters of rice availability. SD - standard deviation of planting date within specified radius; MAT - mean maturity of varieties planted within that distance minus 100 days. t-vaiues of the regression coefficients are given in parentheses. n = 13 sites. For stemborer females, a 3 parameter equation incorporating a quadratic term in standard deviation and the mean maturity of varieties had a lower s^ and higher R 2 than that which is listed, but the F-ratio was not significant at P .05. + .05< P<.1 * .01 243 As pointed out in section 4.5.2, asynchrony is generally found to be greater in the wet than the dry season. Within 800 m, the standard deviation of planting date was less than 9.0 days at 5 of 13 sites (38%) in the dry season, while in the wet season only 1 of 14 sites (7%) had a standard deviation that small. Taking the generation length of yellow stemborer to be 45 days and assuming planting dates to be normally distributed, in a locality with a standard deviation of 9 days one would expect to find only 0.6% of the area to be planted more than half a generation later than the median and the same area half a generation earlier than the median. In other words, under these conditions, Y5B moths emerging from even the earliest planted fields would be hard pressed to find rice that would provide them time to complete an additional generation, and the overall pest density would not be expected to be any greater than had all fields been planted on the same day. Seasonal light trap catches would only be expected to increase once asynchrony was sufficient to ensure the completion of a further generation, and if observations were fitted to a quadratic equation, this would be reflected in a curve that was convex downwards. On this reasoning, the value of the standard deviation at the vertex, the point beyond which seasonal totals begin to rise, would be related to generation length. In Table 5.7 I list these values, calculated from the best-fitting equations. The vertices of 3 of the 4 species with generation lengths of ca. 20 days are found to be less than that of stemborer with a generation roughly twice as long, the exception being BPH. 244 Table 5.7 The standard deviation beyond which seasonal pest totals begin to rise, calculated from the best-fitting quadratic equation in DS 1981, in relation to generation length. Theory suggests the two should be positively correlated ; the agreement, as the text explains, is only partial. std. deviation at approx, generation vertex (days) length (days) Yellow stem borer 9.9 45 caseworm 7 A 20 ct . Nymphula fluctAalis 9.1 20 (?) Rice green semi-looper 4.3 20 Brown plant-hopper 10.8 20 The standard errors of these values are large, as both the linear and quadratic coefficients from which they are calculated are estimated with considerable uncertainty and perfect agreement with the hypothesis should perhaps not be expected. Confidence in the interpretations given to the above results is bolstered by agreement in 2 crucial respects with those of the wet season. The first is the lack of correlation between rice leaffolder catches and asynchrony : as in the wet season, no significant relationship was found at any distance under any statistical model. The second is the rough similarity in the distance at which the maximum correlation was obtained between seasonal total catch and parameters of rice availability. These distances, which I refer to as d', are listed in Table 5.8 for the 5 species which were positively related to asynchrony in both seasons. 245 Table 5.8 d' ( km ) wet season dry season 1981 1981 Yellow stemborer OA 1. males OA 0 . fem ales - 1. caseworm OA 0 . Nymphula fluctosalis OA 0 . rice green semi-looper OA 0 .: brown planthopper 2.0 2.1 The distance at which the maximum correlation between seasonal total and parameters of rice availability was obtained, d1, in 2 seasons. Female stemborers were not significantly correlated with asynchrony during the wet season. With the exception of yellow stemborer, the estimates are in close agreement. It must be remembered that 2 km is the largest radius within which asynchrony was calculated and it is possible that greater correlation with the seasonal total of BPH would be obtained beyond that distance. The dry season value of d' for yellow stemborer is unstable and varies with the statistical model employed. The results that season suggest females are more vagile than males, as is found to be the case for many species of Lepidoptera (Johnson 1969), and in the next section I consider experimental evidence that bears on this question. The parameter d* at best provides a crude indication of dispersal range. As calculated here, asynchrony is the unweighted standard deviation of planting dates within a given radius. If dispersal is a monotonically declining function of distance, rather than a step function, a measure of asynchrony that gave greater weight to variability in planting near the site centre would likely be found to correlate better with seasonal light trap totals. By calculating the standard deviation within circles, I have also tacitly assumed dispersal to be isotropic; one would expect however that an ellipse with the major axis shifting seasonally to parallel the prevailing wind direction would be a more appropriate shape. Such changes would not only provide more realistic measures of d' and of the dimensions of the locality, they would also improve the ability to explain variation in pest density. Taking account of carryover effects between seasons, that I have to this point ignored, would be expected to make a further contribution in this respect. All these matters should be considered in future studies of population dynamics in relation to asynchrony. That local densities of most pest species are still found to be sighificantly correlated with the unsophisticated measure of asynchrony employed here however suggests that the underlying relationship is a strong one. 247 A final point. Many of the most destructive pest outbreaks in Asia have occurred during the wet season. In Indonesia, the area suffering serious loss due to BPH has been greater in the wet than in the dry season since 1974, reaching 347,000 ha during the 1976-7 wet season (IRRI 1984). The same appears to have been the case in the Philippines. Though other factors such as humidity may be involved, I suggest that the greater asynchrony that typically exists during the wet season may be central. In the present study, planting date variance within 2 km was on average 81% higher in the wet than in the dry season and the BPH population 446% greater (n = 13). 5.7 Measuring Dispersal Range Under Natural Conditions Two studies were conducted in an effort to determine whether the dispersal ranges of the principal pest species were indeed as short as suggested by the values of d' given above. Both are essentially natural experiments that make use of the decline in light trap catches with distance from isolated sources. They thus avoid several of the objections to more traditional approaches such as mark-recapture techniques that, for example, density and physiological state are unnaturally altered. On the other hand, the experimental conditions vary in several uncontrolled or unmeasured directions and this calls for a careful interpretation of results. 5.7.1 Dispersal from an old-field area To the north and west of Batitang, the village described in section 4.6.2, * is an area of low-lying abandoned rice fields, referred to locally as "laon". Though reputed to have once been among the most productive of farmland, since the completion of the UPRIIS canal network in 1975 the area has been inundated with water draining from upstream and is now used largely for grazing and fishing. The vegetation is dominated by the grasses Leersia hexandra and Echinochloa spp., the broad-leafed Monochoria vaginalis and by volunteer rice. 248 Late in 1981, NIA cut off water supply to the lower portion of Lateral A that serves the area immediately upstream in order to carry out construction work. As the dry season progressed, the laon gradually dried out and, as it did so, farmers began to rotovate their land in the hope of being able to plant. Large catches of BPH were obtained during February and early March 1982 in the farmer-operated light traps, with the greatest numbers, up to 10,000/night/trap, at the sites closest to the laon. Catches of the other insect pests were not increased above their previous levels. An effort was made to locate other possible sources of the BPH. Some 10 km to the east, across the Rio Chico in Tarlac province (Fig. 4.2), an area served by pump had just been harvested. Enquiries revealed that only BPH - resistant varieties had been planted and that no notable infestation of the pest had been observed. Within the study area itself there were no large areas of rice from which BPH might have originated : planting of the dry season crop was just getting under way by 10 February when the first large flights occurred. In the analysis that follows, I consider catches in the 32 traps (16 sites) on Transects 1 and 2. Large numbers of BPH, declining from west to east, were also found at the sites on Transect 3, but these may have originated from abandoned farmland to the south of the laon that also dried out, but whose location could not be as precisely delineated. 5.7.1.1 Results and discussion In Fig. 5.19 I plot the logarithm of the BPH catch per trap during the 2 weeks of peak flight activity against distance from the edge of the laon. An exponential decay is found to fit the observations well, the regression accounting for more than 80% of the variation in log trap catch. 249 Fig. 5.19 Light trap catches of BPH from February 10-23 1982 in relation to distance from the edge of the old-field area ("laon") thought to be their source. The best-fitting exponential decay is: catch = 12,730 exp (- .6104 x distance) = .829; n = 32 traps. Source of variation df MS F Probability Due to regression 11 47.4 108.6 P< .001 Deviation 14 1.337 1.91 NS Residual 16 .7116 250 10-23 10-23 FEBRUARY CATCH PER TRAP: ro 3 DISTANCE * However, not all the BPH catch can reasonably be attributed to dispersal from the laon: distant and scattered sources must have contributed a part, though likely a small one. I estimate this background level of flight activity from the mean catch during the 2 weeks prior to 10 February in the 2 sites furthest from the laon, 11.4 and 13.3 km to the northeast, amounting to 2.25 insects per trap. (Similar numbers were caught in other traps). When this value is subtracted from each catch, a slightly improved fit is obtained to an exponential decay (Y = 14,163 exp (-.648X); R = .842). On the basis of this equation (eq'n 5.1), a 50% decrease in density is expected every 1.07 km. A thorough interpretation of these results in relation to the dispersal range of the pest requires an assessment of the prevailing weather conditions during the experiment. Unfortunately, equipment was not available to monitor these, among which wind direction and velocity are likely the most important. Records were however obtained from the government meteorological station at Munoz, some 50 km to the north. Between 10 and 23 February the mean wind direction near the ground at 5 PM was 90°, decreasing to 46° at 5 AM, with wind speed decreasing from 4.8 to 2.7 m/sec. Such conditions are to be expected during February when the northeast trade winds dominate weather patterns. All the sites with but one exception lie between north and east of the laon, suggesting that, if conditions were similar to what they were at Munoz, to reach them BPH would have had to fly into a wind blowing considerably faster than what is thought to be its maximum flight speed, roughly 1 m /sec (T.J. Perfect, pers. comm.). As the evidence appears to point unequivocally to the laon as the source of the majority of the BPH caught, it may be either that they were borne on winds at higher altitude blowing in other directions or that they flew during the daylight hours when winds were more variable. This latter seems the less likely explanation as it contradicts what is known of the flight behaviour of BPH, that activity is primarily crepuscular (IRRI 1981). 252 Ideally, in order to measure the impact of wind on dispersal, one would have wanted at least 2 transects of traps radiating from the source in both up- and downwind directions. In the event however, only one site was situated downwind of the laon, that at Valeriana, the rainfed site 2.5 km to the west. Yet catches in the 2 traps there were not much greater than would have been predicted on the basis of the overall regression : 3500 and 4580 or a mean of 4040 insects, 46% greater than the expected value of 2804. Under the assumption that the decline in catch with distance is similar in all directions, an estimate of the dispersal range of BPH can be derived from equation 5.1 by multiplying the density in a given distance interval from the source by the area of that increment, integrating and rotating about the origin. In Fig. 5.20, I plot the result of this calculation. It can be seen that 50% of BPH are estimated to disperse less than 2.5 km and 80% less than 4.6 km. The significance of this result goes beyond the context of the present study : as far as I am aware, this provides the first firm quantitative estimate of the dispersal range of the insect. It is however consistent with the findings of workers from the Tropical Development Research Institute (IRRI 1982) that the timing of BPH population peaks at a site is more closely related to the pattern of cultivation in the surrounding municipality than in the province as a whole. 5.7.2 Dispersal from an isolated field The second natural experiment on dispersal was somewhat more planned. In November 1981, shortly after the general harvest in the area, a farmer near Carmen, Zaragoza established a 1.5 ha field of direct seeded rice with the aid of a shallow well pump. With the exception of a similarly planted field 1 km to the west, this was the only standing crop within several kilometres. In January 1982, as the crop neared maturity, 2 transects of light traps were set up along parallel lines running eastwards and separated by about 200 m. Traps were positioned within the field and at 5, 20, 50, 95, 170, 290 and 485 m into the fallowed area, a geometric progression that gave greatest resolution near the field. 253 DISTANCE (km) Figure 5.20 Cumulative proportion of brown planthoppers estimated to have dispersed given distances, calculated by integration of equation 5.1. 254 The traps were operated by staff from the Zaragoza office for more than one month until the field was harvested in late February. In what follows, I analyse the catches of various pests from 19 January 1982 until they began to rise after the fallow fields had been planted towards the end of the month. I employ different cut-off dates for each pest, depending on the pattern of catches : 8 and 12 February for the rice green semi-looper and Nymphula fiuctosalis respectively, both short-lived vegetative stage pests, and 16 February for the yellow stemborer. Catches of caseworm and green leafhopper were too low to analyse, while those of BPH were dominated by dispersal from the laon, which I have considered above. 5.7.2.1 Results and discussion In Fig. 5.21 a - d I plot the catch in light traps against distance from the edge of the field and illustrate the best-fitting equation. In each case there is evidence of marked decline in numbers over the range of less than 0.5 km. In Table 5.9 I present the analysis of variance of the catches of these 3 species. In order to derive estimates of dispersal range from these regressions, it is again necessary to estimate the background level of catch. For this purpose I make use of the traps at 2 sites 4.5 km to the east and south, situated as far as any from fields planted out of season. (The equations of Table 5.9 suggest that, with the exception of yellow stemborer males, density would have declined to near 0 well before 4.5 km.). During the periods analysed, an average of 47 YSB males, 4.5 females, 2 N^_ fluctosalis and 1 RGS were caught per trap. When these values are subtracted from the catches, the regression equations listed in Table 5.9 are obtained. The fits of these to the data are essentially unchanged. As with BPH, I multiply the estimated density within a given distance interval by the areay integrate, and rotate about the origin. The graphs in Fig. 5.22 a - d illustrate the results, showing the cumulative proportion of insects estimated to have dispersed a given distance from the source. 255 MEAN CATCH PER TRAP: 1S/1 - 16/2 f esn Te ersin qain ae ie in given are equations regression The season. of ucin f itne rm rc fed lne out planted field rice a from distance of function al 59 a Ylo sebrr ae, . females, b. males, stemborer Yellow a. 5.9. Table iue .1 ac.s f ps seis s a as species pest 3 of Catch.es 5.21 Figure ITNE RM IL ( FIELD FROM DISTANCE 256 m ) d is t a n c e fro m f ie l d (m) c. Nymphula fluctosalis, d. green semi-looper. 257 Table 5.9 MEAN SQUARE VALUES Source of df Yellow stem- df Yellow stem- Nymphuia Rice green variation borer females borer males fluctosalis semi-looper Between transects 1 517.6 (NS) 1 770.1 * .00076 (NS) .0213 (NS) Due to regression 2 1209.9 * 1 2424.6 * 1.5254 * 3.1437 * Deviation from regression 5 103.1 (NS) 6 256.7 (NS) .5136 (NS) .6284 (NS) Residual 7 163.8 7 87.6 .1533 .6110 Equation Y = 48.12 + 33.02 Y = 86.98- Y = 10.99 Y = 6.83 exp In (X +.5) - , £.719 5.738 In exp (-.00 (-.00285X) In (X + . 5Y (X + .5) 193X) R2 .59 .53 .27 .39 • C orrected equation Y = 43.62 + 33.02 Y = 39.98- Y = 8.59 Y = 6.35 exp In (X + .5) - 2.719 5.738 in exp (-.00 (-.00632X) (in X + .5)z (X + .5) 260X) R2 .59 .53 .29 .39 Analyses of variance of catches illustrated in Fig. 5.21 a - d. The corrected equation is the best-fitting regression to the data after subtracting the estimated background density. Y - catch; X - distance. 258 CUMULATIVE PROPORTION orce eutos n al 59 a Yellow a. females, 5.9. b. Table in males, equations stemborer corrected itne, acltd y nerto o the of integration by calculated distances, pce etmtd o ae ipre given dispersed have to estimated species iue .2 uuaie rprin f pest 3 of proportion Cumulative 5.22 Figure 259 A CUMULATIVE PROPORTION c mhl fluctosalis, N vmphula D I S (km)T A N C E 2(>0 . re semi-looper. green d. C Table 5.10 (km ) 50% 80% Brown planthopper 2.5 4.6 Yellow stemborer : males .46 .70 females .76 1.1 Nymphula fluctosalis .64 1.1 rice green semi-looper .27 .47 Estimated dispersal distances of 50% and 80% of the population of 4 pest species, calculated as described in the text from the equations of Table 5.9 and equation 5.1 261 In tabic 5.10, I list the estimated dispersal distances of 50% and 80% of individuals. The median dispersal distances are comparable to the values of d’ given in Table 5.8 and in no case differ by more than a factor of 2.5. It should be recognised however that the calculated distances are sensitive to the assumed background density. YSB catches in particular were quite variable at the isolated sites that were used to estimate this; the levels of N^_ fluctosalis and RGS were low and fairly constant and hence more confidence can be placed in the estimates in these cases. As in the BPH dispersal experiment, the light trap transects ran eastwards from the source and thus both sets of results estimate windward dispersal. It should also be borne in mind that the area through which the transects ran were fallow during both experiments and it is likely that had rice at a suitable stage for colonisation been planted, the dispersal estimates would have been reduced. Note that as the results from the dry season (section 5.6.2) suggested, the calculated dispersal distance of stemborer females is greater than that of the males. 5.8 Conclusions The argument presented in this chapter can be simply summarised: The pattern of infestation within individual fields of major pests is such as must occur if asynchrony is to have an impact on their densities : infestation builds up for at least some period after transplanting and is higher in fields ' delayed sufficiently for an additional generation to develop. The farmer-tended kerosene light trap is shown to be an effective means of assessing local pest density : results are consistent between adjacent traps and the catches over a season of 2 pests correlate well with the damage they cause in surrounding fields. 262 Variation between sites in trap catches are related positively to asynchrony for 8 of 9 species considered. The exception, rice leaffolder, had been found in the infestation study not to increase with time within fields or with delay between fields and was thus not expected to respond to asynchrony in the vicinity. The slope of the relationship between the logarithm of pest numbers and asynchrony is found, as hypothesised, to be related to the species' maximum rate of increase. The distance from the site within which asynchrony is best correlated with pest numbers, d', is consistent in 2 seasons and is similar to estimates of the median dispersal distance from experiments employing naturally dispersing populations of insects. The internal consistency of the results and the strength of the independent corroborative evidence provides confidence that the relationship between asynchrony and pest density is real. Beyond anecdote, this is the first empirical demonstration of the effect, either for rice pests, or, as far as I am aware, for those of any crop, though such a relationship has been widely assumed. This work has a number of consequencesfor pest control, both in its direction and its organisation, and for aspects of cropping system design. Firstly, the fact that pest density increases both with the number of crops per year (Chapter III) and the asynchrony with which any one is planted suggests that efforts to reduce either would result in decreased losses to insects. If natural enemy activity is enhanced by increases in the permanence of crop cover, it is to an extent in" sufficient to contain pest population growth. However, as I have suggested in Chapter III, deintensification of rice cultivation, returning to single- from double-cropping or replacing one of the rice crops by a non-rice crop, would entail a complex of changes in the system of rice production and consumption. The net benefits of such a development, and I consider here more than the economic, are 263 difficult to evaluate, indeed cannot be until a clear alternative or set of alternatives to double-cropping are specified. An incremental change like synchronisation of cultivation is a simpler matter, and in Chapter VII I consider some of its likely effects and requirements. The results reported above suggest that synchronisation would have a substantial impact on pest levels : it was found that BPH density in the wet season for example varied nearly 30-fold between sites and that a large proportion of this variation was associated with differences in asynchrony. It is important to note that some areas are planted relatively synchronously in the natural order of things and apparently without conscious effort; particularly in the dry season, planting at some sites was completed within a period insufficient to permit the development of an additional generation. This fact is crucial, for it suggests that the attainment of an ecologically significant degree of synchrony is a realistic prospect. The findings with respect to dispersal distance have an important bearing as well on the organisation of synchronous planting schemes. It is not necessary that large areas be cultivated together, which as several authors have noted, might well have undesirable economic and social consequences (Bernsten 1980, Goodeil in press). The results suggest that for synchronisation to be effective, areas planted more than a generation apart should be separated by a distance greater than most individuals disperse. For this purpose, one might make use of the estimated distances covered by 80% of insects, given in Table 3.10. In Chapter VII I suggest means by which such scheduling might be implemented without widespread dislocation. A final matter. A considerable amount of research has been conducted, largely at IRRI, into a system of rice cultivation that is in effect the contrary of synchronous cultivation - the so-called "rice garden". In this method a farmer divides his holding into a number of plots and plants them sequentially and at regular intervals, typically of one week. After harvesting, a plot is quickly ploughed and replanted the following week; with a variety such as IR36, up to 4 harvests per year can thus be obtained. 264 This highly intensive and totally asynchronous system serves to spread demand for labour and machinery evenly through the year and reduces the threat from typhoons or other disasters, as a smaller area is at a susceptible stage at any given time. The farmer maintains a steady income flow, less affected by seasonal slumps in harvest price (Morooka et al. 1979). The rice garden however demands dependable and continuous irrigation and ready access to a tractor, requirements that severely restrict the scope for adoption by the mass of small farmers in Asia. Concern has also been expressed about the potential for pest build-up, to which the results of the present study lend considerable support. Pantua (1979) however found no consistent difference in pest levels between a 2 ha rice garden and an adjacent double-cropped field. Two crucial points should be noted. Firstly the results of this chapter suggest that the impact of asynchrony and intensity are scale-dependent. The dispersal estimates reported above indicate that the majority of the insects landing in a 2 ha field would originate from outside it and that density would be affected by factors of host plant abundance over a much wider area. The experience of an isolated, early-adopting farmer may thus be very different from one who takes up the practice well after his neighbours. Where the rice garden has been practised over an extensive area, as on the CDCP farm in Bukidnon province, serious losses have been reported due to brown planthopper, green leafhopper and the virus diseases they vector. Secondly, Pantua's conclusions are based on a comparison of damage between a double-cropped field and that plot of the rice garden planted at the same time. The ecological situation of a small and isolated rice garden is analagous to that of the experiment on the impact of delay in planting described in section 5.2. On the basis of those results, one would expect that plots of the rice garden delayed by more than a generation in relation to the surrounding areas would suffer increased damage. A comparison involving only 265 one of the plots docs not give an accurate indication of the average level of infestation over the entire holding. It should be noted however that though the plots significantly delayed would be subject to increased levels of immigration, within an isolated rice garden, too small to greatly influence the asynchrony of the locality, those plots established early, prior to the rise in populations from the surrounding seasonal plantings, would sustain lower than usual immigration. Similar considerations apply to the assessment of natural enemy levels, which Pantua suggests are greater in the rice garden than the adjacent fields. The real impact of the rice garden on pest density and the effects of its adoption over wide areas are matters of considerable concern when the conclusions of IRRI's research quickly find their way, as in the Philippines, into government-sponsored production programmes and extension recommendations. Based on the results of the present study, it is fortunate I believe that the rice garden system has not been widely adopted in that country, as several officials of the Ministry of Agriculture reported at a Technology Transfer Workshop held at IRRI in 1982. A fundamental contradiction exists in IRRI's support for both synchronous planting for pest control and the rice garden, one that must shortly be resolved. 266 CHAPTER SIX Evolution and Agricultural Change 6.1 Introduction To this point, I have been concerned primarily with the impact of agricultural change on the dynamics of pest populations, considering in Chapter III the effect of increases in the number of crops grown per year, and in the preceding chapter that of alteration in the synchrony with which they are planted. I now take up the question of the impact of these changes on the quality, as opposed to the quantity, of pest populations, that is, the evolutionary pressures that are brought to bear on pests. The two sides of population biology, population dynamics and population genetics, are closely interrelated and as Berry (1979) has argued, workers on either side of the divide ignore processes in the other domain at their peril. The demands of application that have loomed over the present work and that will be dealt with at length in the next chapter also require that the durability of the benefits of purposive change in the intensity or synchrony of cultivation be considered. Rice farmers in South East Asia have direct experience of the interaction of evolutionary change and population dynamics of insect pests : the widespread adoption of varieties incorporating monogenic resistance to the brown plant hopper created uniform selection pressures, resulting in the emergence of 'biotypes'^ that in several regions caused catastrophic crop losses during the 1970's (Chapter 1). Alteration in the duration of rice availibility may similarly be expected to evoke an evolutionary response from pests. 267 6.1.1 Hypotheses The impact of an increased synchrony of cultivation can be understood I suggest in the context of r and K selection. Populations exploiting transient habitats are expected to be characterised by short generation length, high fecundity, small offspring size, but relatively poor competitive ability (Pianka 1970, Southwood 1977). Asynchrony extends the duration of the rice habitat, making possible additional generations for pests that are able to move readily between fields. As seen in the previous chapter, pest densities are higher in asynchronous localities and there is therefore likely to be greater intra-specific competition and, possibly more importantly, increased natural enemy activity if they, as well as their prey, are benefitted by asynchrony. Where cultivation is relatively synchronous, there will be a selective advantage in rapidly completing a generation; indeed, small decreases in the time taken to reach sexual maturity may yield large benefits in terms of realised reproductive output (Cole 1954). On this reasoning, the evolutionary response of pests to synchronisation would be expected to be similar to that in the wake of a shift to the planting of short duration varieties. The consequences of increasing the number of crops grown per year will likely be different. If seasons are separated by a fallow that is longer than the egg laying period of the female, the definition I proposed in Chapter III, and longer than the upper limit of phenotypic variation in this trait, then generation length per se will not be subject to natural selection. However, if double cropping results in increased pest densities and hence in increased natural enemy activity and intra-specific competition, as the results of Chapter III suggest, then adaptations to improve survival, such as larger individual size or 1 A number of researchers contend that the biotypes in fact represent part of the variation present in the original BPH population, rather than being the products of mutation (see e.g. Claridge et al. 1983). 368 immunological defences against parasitism, will be favoured. As well, the cues that determine the shift from what I have termed "fallow strategy" to "crop strategy", for example, the ending of quiescence or diapause in stemborer larvae within rice stubble, may well alter with natural selection upon the introduction of an additional crop. 6.2 Methods These hypotheses were tested in two experiments carried out in Nueva Ecija in 1981 and 1982. The approach I adopted was to examine the life history parameters of yellow stemborers taken from areas of contrasting cropping pattern and reared under common conditions. In June 1981, at the beginning of the wet season crop, stemborer moths were collected from house lights at the Zaragoza office and at the home of a co-operating farmer in Mapalad, Sta Rosa some 30 km to the east. The Zaragoza office is situated less than 500 m from the nearest rice fields which are generally double cropped, farms to the east and south served by NIA, and those to the north by the GAPOMACA communal system. Cultivation is highly asynchronous in the vicinity - within a kilometer of the office it is generally possible to find standing crops at any time of year - both because irrigation releases are not co-ordinated between the two systems and due to problems in water distribution within each. As mentioned in Chapter IV, 1981 was the first year of operation of the GAPOMACA system and unfinished canals and inadequate control structures seriously delayed planting. Though without detailed parcellary maps it is impossible to accurately assess asynchrony as a function of distance, it was conservatively estimated, using Ministry of Agriculture records, that at the centre of the system the standard deviation within 1 km was greater than 25 days during the dry season of 1981. This was more than at any of the 13 sites that figure in the 2 previous chapters. The adjacent areas served by NIA are at the downstream end of their lateral canals and suffer both from waterlogging and water shortage on slightly higher ground, giving rise there as well to considerable asynchrony. The house at Mapalad is situated immediately adjacent to rice fields on gently sloping land at the foot of the Sierra Madre Mountains. A single crop of rainfed rice is grown and from 1979 the dominant variety has been the short duration IR36. The soil is well drained and planting is quite synchronous : from personal observation, I estimate that the standard deviation of planting date within 1 km was no more than 1 week in the west season of 1981. The nearest extensive area of irrigated rice culture is some 10 km to the west. Stemborer moths are sluggish and readily captured on walls where they rest near incandescent or fluorescent lights. Plastic bottles were left with the co-operators at Mapalad and the moths collected the following morning by motorcycle. At the Zaragoza office, females from the two sites were introduced into separate cages with vegetative stage rice, on which they were allowed to oviposit. (It had been found in preliminary experiments that the yield of egg masses was no higher if males were introduced as well, suggesting that most females were already mated). Egg masses were laid at night and these were collected the following morning by clipping the leaves to which they were attached. For the first 5 days, they were kept under daylight in petri dishes on filter paper moistened with a solution of M-Tegucep, a fungistatic agent. On the sixth day, the egg masses were transferred to stoppered vials where they were kept until hatching, generally on the 7th or 8th day. The larvae were infested using a fine camel's hair brush on rice plants (IR36) near maximum tillering stage that had been fertilized with 60 kg N/ha. A constant density of 8 larvae/plant was maintained and 2 2k cm diameter pots, each containing 2 plants, were allocated to the progeny of one egg mass. It was not possible however to adhere to a fully balanced experimental design as varying numbers of egg masses were obtained from the females collected at the two sites, and as some egg masses did not yield 32 viable larvae. After several hours, during which the larvae moved towards the leaf axil where they were observed to bore into the stem, the pot was enclosed with a 1 rn high nylon mesh cage firmly anchored in the soil. Pairs of pots were assigned positions at random within a grid on the grounds to the Zaragoza office and were watered daily so as to maintain 2 cm of standing water. The cages were checked each morning for emerging moths which were removed, sexed and recorded. Females were dissected, either immediately or after refrigeration, and the number of oocytes was counted under a low power microscope as a measure of potential fecundity. The count included the smallest distinct oocyte rudiments which Rothschild (1971) suggests can mature within 2-3 days of emergence. Observations continued until senescence of the plants, when the stems were dissected and checked for larvae or pupae. Mapalad and Zaragosa represent two extremes of rice availability : synchronous single cropped and asynchronous double cropped environments. A second experiment on much the same lines as the first was conducted at the end of the 1982 wet season crop, this time however including a third site, Ibabaw bana, Cabanatuan, adjacent to the NIA main canal and representative of double cropped but synchronous conditions. During the wet season of 1981, a standard deviation of 6 A days had been calculated within 1 km of the light traps installed there. In this experiment, the asynchronous double cropped situation was represented by Batitang, the village described in Chapter IV, some 3 km to the south of Zaragoza, where a standard deviation of 19.5 days had been recorded in the 1981 wet season. Rather than transport live moths from the various sites to the Zaragoza office, oviposition cages were installed in Mapalad, Ibabaw bana and Batitang and the egg masses alone collected. A larger sample of egg masses was obtained from each site than in the first experiment and larvae were infested on IR42, a variety with a growth duration 20 days longer than IR36; aside from these 271 modifications the same procedures were followed as in the first trial. flow ever, possibly due to the high humidity and relatively low insolation under the nylon mesh, some rice plants were attacked after infestation by blast disease. This was eventually controlled by Hinosan, a fungicidal spray, but maturation of many plants was delayed. As well, a prolonged power failure caused the deterioration of moths that had been kept under refrigeration prior to dissection and no information is available therefore on fecundity. The S.A.S. statistical package on the University of London Amdahl computer was used to carry out the mixed model, unbalanced analyses of variance that the data required. 6.3 Results Figure 6.1 illustrates the distribution of times from infestation of the newly hatched larvae to adult emergence in the first experiment, and in Table 6.1 these are subjected to analysis of variance. The mean time to emergence of those originating from Mapalad is significantly less than of those from Zaragoza. However, of possibly greater import than this difference in the means is that in the extreme values : 25% (8/32) of moths originating from Mapalad emerged within less than 3 5 days (the minimum being 2k days) compared with none from Zaragoza, whereas 8% (k/kS) of Zaragoza moths emerged after more than 50 days compared with none from Mapalad. In order to calculate genereration times from these figures, one should add roughly 7 days for the duration of the egg stage (range 6 - 8 days) and 1 - 3 days (Bannerjee and Pramanick 1967) for the period between emergence and oviposition. Though on average males at both sites emerged before females, the difference is not statistically significant. Variation of emergence times between egg masses within a site, due both to genetic differences and to environmental heterogeneity of rearing conditions, does not quite attain statistical significance. 272 Mapalad 30 20 . ■ O 10 - CD U) 0 ------j------1------1------1------r------1------. Zaragoza 30 CD n E 3 20 C 1 0 0 T------1------1------1------T------1------1------ 24 28 32 36 40 44 48 52 56 60 days after infestation Figure 6.1 Time between first instar and adult emergence of yellow stemborers from 2 environments: synchronous single-cropped and asynchronous double- cropped. n = 32, 49 respectively. 273 Tabic 6.1 A. Least Squares Estimate (days) N Mean Time Std Error Zaragoza 49 42.31 1.00 Mapalad 32 38.46 1.24 Males 39 40.82 1.03 Sex Females 42 41.19 0.84 Analysis of variance Source of variation Degrees of Mean square F freedom Between sites 1 263.31 3.26 * Between sexes 1 20.48 0.73 (NS) Among egg masses within sites 13 49.33 1.76 (NS) Among emerging moths within egg masses 63 28.08 Time to emergence of yellow stemborers originating from 2 environments: asynchronous double-cropped, and synchronous single-cropped. A. estimates by site and sex, b. analysis of variance. 274 From the numbers of moths emerging, and knowing how many were initially infested, it is possible to calculate the proportion surviving from the first instar to the adult stage. As Table 6.2 indicates, this was significantly less among Mapalad stemborers. The table also gives an analysis of the sex ratios of the emerging moths, decomposing the total chi-square into its pooled and heterorgeneity components (Sokal and Rohlf 1969). This indicates that overall, the ratio did not differ from 1: 1, nor did the two sites differ significantly among themselves. The distribution of total oocyte numbers in dissected females is illustrated in Figure 6.2, and these are subjected to analysis of variance in Table 6.3. (Only 27 of the 42 females that emerged were used in this study as some escaped, some were damaged, and others deteriorated during storage). The mean among females from Mapalad is significantly greater than among those from Zaragoza. No correlation is apparent between the time to emergence and the number of oocytes among either Mapalad (r = .12) or Zaragoza (r = .01) females. As in the case of emergence times, the "among egg masses" mean square gives no indication of a genetic component to variation in oocyte number within sites, although the unbalanced design and small sample size makes this difficult to detect. 6.3.1 Three-way comparison Figure 6.3 illustrates the distribution of emergence times of moths originating from the 3 sites studied in the second experiment, and the analysis of variance of these data is presented in Table 6.4. As in the first experiment, the mean time to emergence of Mapalad stemborers is significantly less than of those from the asynchronous, double cropped site, here Batitang. The mean emergence time of moths from the synchronous double cropped site does not differ from the Mapalad value but does from that of Batitang. Minimum times for any moth to emerge were 24 days at both Mapalad and Ibabaw bana, but 38 days at Batitang. Table 6.2 Site Moths of which failed to total survival emerged males females emerge infested rate (%) Zaragoza 49 27 22 213 264 18.6 M apalad 32 12 20 232 284 11.3 TOTAL 81 39 42 467 548 df n Sex ratio : overall deviation from 1:1 1 .11 NS homogeneity among sites 1 2.40 NS Survival rate : homogeneity among sites 1 5.21 Survival rates between first instar and adult and sex ratios of yellow stemborers from 2 environments. Mapalad 3 O a? 20 . Q Ui I- o 10 LU C/) l/) Zaragoza 20 < s LU U. 10 t------1------1------r T---- 1 160 200 240 280 320 360 400 TOTAL EGGS IN OVARIES Figure 6.2 Total oocytes in newly emerged female stemborers from 2 environments: synchronous single-cropped and asynchronous double-cropped, n = 13, 14 respectively. 277 A. Total O ocytes in ovaries Site N Mean Std. Error Zaragoza U 233.7 9.8 Mapalad 13 269 A 14.6 B. Analysis of variance Source of variation Degrees of Mean square F freedom * Between sites 1 8576.7 4.56 Among egg masses within sites 13 iS S O J 0 .8 6 NS Among females within egg masses 12 2 185.6 Table 6.3 Total oocytes in newly emerged female stemborers from 2 environments. 27 8 Mapalad rj cu O) L- 0 ) E <1> 0) n E 3 C Batitang 2 0 10 24 28 32 36 40 44 48 52 56 60 64 68 72 days after infestation Figure 6.3 Time between first instar and adult emergence of yellow stemborers from 3 environments: synchronous single-cropped, synchronous double- cropped and asynchronous double-cropped. n = 51, 78 and 80 respectively. 279 A. Least squares estimates (days) N Mean Time Std Error Batitang 80 49.09 0.98 (a) Ibabaw Site Bana 78 45.08 0.81 (b) Mapalad 51 44.43 i .10 (b) Males 88 45.31 0.80 Sex Females 121 47.46 0.67 B. Analysis of variance Source of variation Degrees of Mean square F freedom Between sites 2 243.79 3.29 * Between sexes 1 183.93 5.04 * Among egg masses within sites 37 74.04 2.03 ** Among emerging moths within egg masses 168 36.51 Table 6.4 Time to emergence of yellow stemborers from 3 environments: synchronous single-cropped, synchronous double-cropped and asynchronous double- cropped. A. estimates by site and sex, b. analysis of variance. 280 The significant difference between the means at the two double cropped sites is intriguing, as it suggests a genetic divergence in this trait between areas separated by less than 13 km of continuous rice land, and is consistent with the results of the previous chapter indicating relatively restricted dispersal of this species. Variation among egg masses within sites was significant, though it is not possible to separate its genetic and environmental components. The emergence times in this experiment are substantially greater than those revealed in the first. This may in part reflect the impact of the rice blast outbreak that likely affected the insects' nutrition or possibly the effect of having employed a variety with a longer growth duration. In neither trial were any larvae or pupae found when the plants were dissected at maturity, though some might have been expected, particularly in the second experiment conducted at the end of the wet season. However, quiescence, if it is initiated by increasing aridity, may have been disrupted by the experimental methods that maintained standing water in the pots so as to prevent the moths escaping under the cages. It is striking that the mean emergence time of Mapalad stemborers was an almost constant fraction of those from Zaragoza/Batitang : 0.905 in both trials to 3 significant places. Table 6.5 indicates that the sex ratio of emerging moths did not differ between the sites, but unlike the first trial, the overall proportion was significantly less than 1:1. (The difference in sex ratios between the 2 experiments was not significant : ^ = 0.88). Significant heterogeneity was found in the survival rate from 1st instar to adult emergence, with the Mapalad rate less than that at both Ibabaw bana ^ = 5.93 ) and Batitang = 5.17 ). The two double cropped sites did not differ between themselves if? ^ = .04), contrary to what was observed in the case of emergence times. Once again, a remarkable similarity is apparent in the survival rates obtained in the two trials : neither the Mapalad nor the Batitang values differ significantly from those of a year previous (% [ = .05; -jt = .22 respectively). The results obtained in the two trials are summarised in Table 6.6 . 28 1 Table 6.5 Site Moths of which failed to total survival emerged males females em erge infested rate (%) B atitang 80 32 48 386 466 17.2 Ibabaw bana 78 37 41 364 442 17.6 Mapalad 51 19 32 381 432 11.8 Total 209 88 121 1131 1340 df 7 ! Sex ratio : overall deviation from 1:1 1 5.11 * homogeneity among sites 2 1.51 NS Survival rate : homogeneity among sites 2 7.29 * Survival rates between first instar and adult and sex ratios of yellow stemborers from 3 environments. 232 Site Environment Mean time to Mean no. % em ergence oocytes survival + std. error + std. error Experiment I Zaragoza 2 crops asynchronous 42.51 + 1.00 233.7 + 9.8 18.6 Mapalad 1 crop synchronous 38.46 + 1.24 269.4 + 14.6 11.3 Experiment II Batitang 2 crops asynchronous 49.09 + 0.98 - 17.2 Ibabaw bana 2 crops synchronous 45.08 + 0.81 - 17.6 Mapalad 1 crop synchronous 44.43 + 1.10 - 1U Table 6.6 Summary of the results from the 2 experiments. 283 GA Discussion Aside from the consequences of deploying resistant varieties, surprisingly little scientific work had dealt with the evolutionary impact on pests of agricultural change. In a recent review of the genetic consquences of monoculture, Barrett (1983) focuses solely on this one aspect, the evolution of virulence in pests confronted by varietal resistance. Of what remains, some of the best, indeed classic work pertains to the adaptation of weeds to the crops and cropping systems they infest. The aerodynamic properties of Camelina sativa seeds are found to vary with those of the flax (Linum spp.) varieties with which they occur, which Sinskaia and Beztuzheva (1931, cited by Stebbins 1950) contend reflects the selective pressures of winnowing, those weed seeds that fall out at the same distance as flax seeds being replanted the following season. In terms of the timing of reproduction, Yabuno (1961) points to the correlation between the heading date of barnyard grass Echinochloa crusgalli var. oryzicola and that of rice in regions of Japan which differ as to time of planting. Among insect pests, perhaps the most salient instance of evolutionary change comes again from Japanese rice cultivation. Ishikura (1956, cited by Kiritani and Iwao 1967) report that a shift by farmers to earlier planting within the year was followed by an earlier emergence of the first generation of Chilo suppressalis. In tropical Asia, increases in the duration of rice cultivation, rather than alteration in its timing per se have characterised recent agricultural change, and it is the evolutionary impact of this process that has been considered in this chapter. The life history parameters of stemborers from localities of varying cropping intensity and synchrony have been seen to differ, and in a manner that is likely to ensure that their population rates of increase are greatest in the environments from which they originate. 284 6.4.1 Life history traits, cropping system and population growth rates Consider a locality such as Mapalad where the predominant variety planted is IR36 with a duration of roughly 90 days after transplanting. If the standard deviation of planting date is 7 days and if, as is often observed, moths oviposit on rice soon after the crop is established, then the duration of the stemborer's habitat on 95% of the farm area will be roughly 118 days (90 + 4x7). A population with the longevity of the Batitang and Zaragosa stemborers would be hard pressed to complete 2 generations in this span : considering the distribution of emergence times during the first trial (Fig. 6.1) which were less than in the second, 8% of the insects had generation lengths greater than 59 days (i.e. 51 days from hatching to adult emergence, 8 days assumed to be required for the egg stage and the adult's pre-oviposition period). However, it is possible that these late-developing stemborers would remain in the stubble to emerge the next season. Taking their survival to be the same as those that develop during the crop, and using the mean values from the two trials of fecundity, survival and sex ratio, each "asynchronous" female at the beginning of the season would be expected to give rise to 517 offspring by the end of the 2nd generation. A greater proportion of the stemborers originating from Mapalad itself would be able to complete 2 generations in the 118 day span, indeed ail insects in the first trial had an expected generation length of less " than 59 days. However, a number would be able to complete 3 full generations : in the first tria l 13% and in the second 4% had generation lengths of less than 39 days. Taking the average of 1 Calculation of R , the reproductive output is simplified for an insect such as°YSB with a reproductive stage that is very short relative to its pre-reproductive period. The equation R0 - L m reduces to the product of the rate of survival to the adult stage and the per capita fertility. 285 these, and again using the mean values from the two trials for survival, fecundity and sex ratio, one female would be expected to give rise to 953 offspring by the end of the crop, ignoring those that would not yet have completed a third generation. Under more asynchronous conditions, population growth can be estimated from the value of r, the intrinsic rate of increase, using the approximation given by May (1981) r ~ Ro/T^. Again making use of the mean values from the two trials, Mapalad stemborers would have r ^ .0600 per day and those from Zaragoza/Batitang .0581 per day. Though their advantage is reduced from what it was under a short season, insects with Mapalad life history parameters would do better than indigenous ones under asynchronous conditions and would be expected to eventually dominate the population. It should be remembered however that the survival rates that were measured in the experiment were those from hatching to adult emergence and under caged conditions that excluded some natural enemies. Parasites such as Telenomus rowani that is sometimes trapped at lights attached to the anal hairs of female stemborers, are known to take a toil of their eggs, in Sarawak averaging 49% (Rothschild, 1971). Rothschild also estimates that 15% of potentially viable eggs are not laid by females, and finally, insecticides applied to the crop will depress survival. If one includes in the calculation of Rq a factor f to account for these other sources of mortality or reduced fertility, it is found that the Zaragoza/Batitang life history parameters yield a greater intrinsic rate of increase when f is less than 0.30, that is, more than 70% of potential reproduction is lost to these other sources. Together with the measured mortality from hatching to emergence, this would imply a total survival to the adult of 5% of the total eggs produced by a female from Zaragoza/Batitang. This is thought to be quite feasible : with f at 0.30, r expressed on a monthly basis would be 1.07, close to 1.13, the mean value of r max calculated from light trap catches in section 5.6. In contrast, calculations of the rate of population growth of the two sets of life history parameters under conditions of synchrony, where the number of generations is limited, are less sensitive to the introduction of such a factor. 286 Yet is is individuals, not populations, that are subject to natural selection. Why, if rapid completion of the life cycle is at such a selective premium under synchronous conditions, do only some 4-13% of Mapalad stemborers emerge in time to complete a third generation? The mean difference between Zaragoza/Batitang and Mapalad emergence times of some 4 days, though statistically significant, is largely irrelevant to the dynamics of the pest, given the length of the season and of a generation. An adequate response to this must await more detailed population genetic studies, but I suggest that early emergence likely has costs in terms of reduced survival and possibly, though the data from the first trial gave no hint of it, in terms of reduced fecundity. A polymorphism for generation length could be maintained in the population depending on its mode of inheritance and the distribution of costs associated with it. Immigration from double cropped areas may also blur the population's adaptation to the local cropping system, though dispersal over 10 km is not expected to be significant. 6.4 .2 r and K selection in cropping systems The experimental results of this chapter suggest that stemborers in synchronously planted, relatively transient habitats are characterised by a shorter generation length, increased fecundity, but reduced survival compared to those from areas where, due to asynchrony, the seasonal duration of rice is extended. These results agree with expectations from the theory of r and K selection, both as to the manner in which life history parameters covary and the environmental correlates of these suites of characters (Stearns 1977). The analysis of section 6.4.1 also indicates that the advantage of the K-selected suite increases with the level of pre-reproductive losses, again in accordance with the theory's predictions (Horn, 1978). 287 However, the impact of an increasing duration of rice within a year, through multiple cropping, has been seen to differ from that of increasing its duration within a season. Stemborers from Mapalad and Ibabaw bana have similar generation lengths, as expected, for the length of a season is similar in the two areas. The theoretical analysis of Chapter II and experimental results of Chapter III suggest that population levels, competition, and natural enemy activity are greater where two crops are grown, as at Ibabaw bana, and a premium is therefore placed on adaptations to increase survival. This capacity was tested in the experiments : though most parasites and predators were excluded by the nylon mesh, fungal or baterial diseases were not, and competition at the density of larvae infested, roughly 1 per 2 tillers, was likely substantial. Under these conditions, survival through the larval and pupal stages of stemborers from Ibabaw bana was found to be significantly greater than those from Mapalad and similar to those from Batitang, the asychronous double-cropped site (though an intermediate value might have been expected). It is not known how fecundity differs between Mapalad and Ibabaw bana, but if stemborers at the latter site have achieved both a relatively short generation length and high survival, it might be anticipated that they would pay the price in terms of a reduced production of eggs. Changes may also have occurred in the cues that govern emergence from the stubble, unless these are dependably tied to the cycle of cultivation, such as soaking of the field prior to ploughing, rather than to season-specific cues such as temperature or day length. Those life history traits that have been measured suggest that rice habitats must be considered along more than a single axis of "duration" if one is to make sense of the evolutionary response of the pests that exploit them. I propose "transience", affected by the degree of synchrony between fields and the maturity of cultivars planted within them, and "seasonality", influenced by the extent of multiple cropping, as more useful habitat parameters. If changes in these factors that increase the time available for population growth are not uniform in spatial terms, selection pressure may arise for increased vagility; for active fliers like stemborer this would lead to 288 increased adult size, though that might also be a consequence of selection for fecundity. The adequacy of rainfall or irrigation, the levels of fertilizer and insecticide use, and the planting of resistant varieties might be expected to give rise to a third dimension of "stress" within a crop season as Grime (1977) has suggested, calling forth a different set of pest responses. In rice cropping systems, as in natural communities, it is unlikely that variation along all axes will be correlated or that herbivore life history traits can be adequately described along a single r - K continuum. 6.4.3 Life history traits and the design of integrated contra! schemes Conway (1981) has made provocative use of the r - K categorization of pest life histories to introduce a theoretical basis to the selection of pest control techniques. r-pests he suggests require early and widespread application of pesticides based on forecasting and are vulnerable to cultural controls that limit immigration to the crop, while against K-pests, pesticides must be precisely targeted, their use based on monitoring, and cultural controls aimed at reducing the amount of habitat available to them. Though the need for such a guide to the development of integrated control schemes is clear, the utility of the r - K concept as its basis is less certain. Stearns (1977) has reviewed studies of life history traits from a range of plants and animals and judges that in only 18 of 35 cases do the data fit the r - K schema, and for insects in only 1 of 4 papers. Aside from its empirical failings however, there are areas of ambiguity in such a categorization of life histories that make its application problematical. There is a danger of misplaced concreteness in labelling a pest as either r- or K-selected when life history traits are labile and r- and K-ness may be selected by agricultural change or deliberate attempts at control. In terms of interspecific comparisons, it may be argued that some organism may be found that is either more r- or K-selected than any given pest and that the categorization is inherently relative. It is not clear which of the myriad possible interspecific comparisons are relevant in characterizing a pest. 289 I suggest that a more pragmatic approach, which makes no assumptions about the evolutionary forces moulding life histories, is to develop a set of indicators of the likely impact of potential control tactics on a given pest. For synchronous planting, a key question is whether fields delayed in planting by more than a generation suffer increased infestation, the answer to which it was found in the previous chapter accords well with measurements of the actual impact of asynchrony. In general, cultural controls that act by limiting immigration to a crop will be of benefit where the slope of the function relating log final or peak numbers to log initial numbers is positive and the estimated equilibrium level greater than those currently attained (Chapter III). Crucial to the utility of pest forecasting will be the goodness of fit of this function and the extent to which other readily collected and assimilated data such as weather variables can account for additional variation. Key questions for other forms of pest control, such as the deployment of mono- or polygenic varietal resistance and biological and genetic controls might also be derived. A major advantage of such an approach would be a degree of flexibility in accomodating theoretical or empirical advances and a clearer falsifiability derived from operationally less ambiguous definitions. The development of a theoretically sound and practically useful set of guidelines remains as a challenge to applied ecologists. Guidelines of this sort however would provide only an indication of the biological efficacy of a given control technique in a particular instance; fundamentalquestions remain of whether the technique is economically justified (considered at the various levels at which decisions are made, from the farm to the nation), socially acceptable and capable of being implemented in the agricultural system as it exists. (Norton and Mumford 1983, Perkins 1982). Comprehensive guides to these questions are an even more distant prospect than to the narrowly ecological; in the following chapter, I consider some of these issues in relation to a specific case, the synchronous planting of rice. 290 Durability of synchronous planting as a control technique. One final point should be considered. The results of the second trial suggest that stemborers in synchronously planted, double-cropped environments have been selected for rapid completion of their life cycle, and, though differences in fecundity were not measured, it is expected that under synchronous conditions their intrinsic rate of increase would be greater than those from asynchronous areas. Conscious efforts to synchronise cultivation would likely elicit the same evolutionary response, leading to a lesser reduction in infestation than were life history traits not subject to selection. That synchrony is likely to remain of benefit despite pests' adaptation is suggested by the results reported in Chapter V : stemborer catches in light traps were 3.2 fold greater at Batitang than at Ibabaw bana during the wet season of 1981. The drainage and irrigation conditions that appear to be chiefly responsible for the differences in synchrony have been present since at least 1975 when the UPRIIS system was completed, and benefits would thus appear to remain subtantial even after 6 years. Once again, more detailed population genetic studies would be required to establish the limits to reductions in pests' life cycles or increases in their fecundity under conditions of synchrony and at what point an equilibrium is reached in the distribution of these life history parameters. 291 CHAPTER SEVEN Implementing Synchronous Planting 7.1 Introduction In this Chapter I take up some of the issues inherent in attempts to implement synchronisation as a control technique for rice pests. In the previous two chapters, I have been concerned with its biological efficacy and its durability in the face of the evolutionary response of pests. Here I consider its feasibility under conditions of small farmer cultivation. I first examine some of the general features of cultural pest control tactics, in particular their requirements for co-ordination between and among farmers and support institutions. In a complex social environment such as Central Luzon, one with marked and growing class distinctions, seemingly simple interventions may have unanticipated results. I describe an attempt to consider the unanticipated prior to promoting the concept of synchronous planting in a pilot project in Nueva Ecija. Knowledge of its ecological requirements in spatial and temporal terms is I contend crucial to minimizing the potential negative impacts. I narrate the efforts made over a one year period to gain the support of farmers, landless workers, and institutions for a synchronisation scheme, and show that progress has been limited by the conflicting interests of farmers and institutions, while the concerns of landless workers, that may be threatened by such a scheme, have been ineffectively articulated. Finally, I consider how in other social and institutional environments the response to synchronisation has differed and may differ in the future. Yet even where purposive synchronisation may encounter substantial obstacles, the ecological insights of the previous chapters may be of use in several ways, for example in minimising the potential for pest build-up generated by attempts to schedule cultivation for other purposes, particularly those of irrigation authorities. 292 7.2 Synchrony and Scale Much of the academic debate surrounding the economic and social impact of the "Green Revolution" has turned on the extent to which the benefits of the new technology have been shared by the mass of small farmers. A concensus appears to be emerging that the new technology is, in itself, neutral with respect to scale, that modern seeds, irrigation and agrochemicals can be productively employed on even the smallest of holdings. However, access to these inputs, to the credit to purchase them, to sound technical advise, and to an advantageous price at harvest is generally better among large farmers, and concentration of land holding has in many instances followed upon technological change (Palmer 1976, Lipton 1978, Griffin 1979). Synchronous planting is among a class of techniques that possess inherent economies of scale. The results of Chapter V indicate that densities of the various insect pests are best correlated with the standard deviation of planting date within characteristic radii, suggesting that minimum areas are required for synchronisation to be effective. One farmer's effort to plant his 2 ha holding in as short a span as possible is irrelevant if the mass of insects infesting his crop originate beyond, in the surrounding farmland whose synchrony he cannot directly affect. Large corporate farms operating under unified management on several hundred to a few thousand ha may however be in a position to significantly alter the number of insects immigrating to the crop. At a meeting I attended in December 1980, representatives of some of the largest corporate farms in the Philippines expressed interest in the concept of synchronous planting and one offered his company's co-operation in carrying out the research reported in the previous chapters that was eventually conducted in Nueva Ecija. Several practice the precise opposite of synchrony, the so-called "rice garden" approach, (5.8) in order to maintain an even demand 293 for labour and to take advantage of seasonal fluctuations in harvest price. At least one company CDCP in Bukidnon province, reports large losses to insects and virus diseases and has been forced to apply increasing ammounts of insecticides. Synchronisation, like purposive staggering, requires co-ordination, and is likely to be easiest to achieve where management is unified across an ecologically significant area and able to exert a degree of control over the provision of essential inputs. Where the land is tilled by small farmers, management is by definition fragmented, and their economic and political influence on the institutions responsible for irrigation, credit and marketing is limited in most countries of tropical Asia. The problem of co-ordination is endemic to the implementation of many cultural pest controls, in contrast to insecticidal methods, which are practicable by farmers acting independently.* The problem has been confronted often, notably in cotton cultivation where the maintenance of a closed season has been found beneficial. In Texas, 9 zones have been delineated on the basis of climate in which planting cannot commence before and stalks must be destroyed by dates specified by regulation in order to control pink bollworm and boll weevil (Flint and van den Bosch 1981). Similar measures were enforced in Uganda and Tanzania under colonial administration, beginning in the early years of this century. Cotton had to be uprooted and burned after harvest to limit the carryover of insects and diseases, and farmers were pressed to plant soon after the beginning of the rains so as to prevent pests building up on alternative hosts (Hill and Moffett 1955). Though possibly efficacious on the narrowly biological level , the practice had serious repercussions on the subsistence economy onto which cotton cultivation was grafted. The period after the beginning of the rains 1 There is reason to believe however that spraying may be more effective where it is done synchronously; see e.g. Singh and Sutyoso (1973). The pattern of pesticide use among fields will also affect the rate at which resistance develops (Comins 1977). 2 Coaker (1959) however suggests that in areas of Uganda where cultivated and wild hosts are available the year round, Heliothis armigera is effectively controlled by its natural enemies. Breaking the cycle through an enforced fallow may not be advantageous under these conditions. was the optimal time for planting several food crops and one of labour scarcity. Enforcement of the regulation on early planting of cotton resulted in food shortages, according to both official (Hill and Moffett op. cit.) and other sources (Bowles 1980), and in Uganda contributed to unrest in the post-war period (Jorgensen 1981). Co-ordination was achieved, in East Africa as in Texas, through coercion, but whereas in the latter cotton growers are relatively proserous, politically articulate and organised, farmers in East Africa were unable to effectively bring countervailing arguments to bear to balance a narrow administrative concern with one aspect of agricultural production. Cultural controls are attractive to entomologists aware of the limitations of insecticides, and this is all the more true for those working on food crops in developing countries where insecticide use is often only marginally economical (e.g. Herdt and Jayasuriya 1981). It is sometimes assumed that because cultural controls entail merely altering the timing or frequency of farm operations their costs are likely to be small (Flint and van der Bosch op. cit.; OECD 1977). On the contrary, I suggest that the costs are often indirect and sometimes difficult to perceive by those unfamiliar with the agricultural system. In Chapter 1 I pointed out that post-harvest ploughing of rice fields to reduce pest carryover would destroy the ratoon crop, a limited but possibly significant resource for the rural landless in Central Luzon. Synchronisation may as well have negative repercussions on various aspects of the rice production system. Much depends on the sensitivity with which it is implemented and the degree to which the possibly conflicting interests of those involved are effectively represented. However many of the negative side effects of synchronisation depend as well on its scale, on the number of farms involved and the span of time within which planting is to be accomplished. The results of Chapter V are thus crucial in suggesting minimum spatial and temporal requirements for synchronisation. Moreover, they provide a basis for brokering : should a certain standard deviation in planting date be imposed, say, by limited canal capacity, its impact in terms 295 of increased pest density might be estimated and the costs of relieving the constraint weighed against the losses to the crop. Conversations with Southeast Asian researchers and agricultural administrators (I.N. Oka and S. Partoatmodjo (Indonesia); K.L. Heong (Malaysia) and T.S. Eugenio (Philippines)) indicate that, besides a healthy skepticism as to the biological efficacy of synchronisation, implementation has up to now been hampered by the absence of clear guidance as to the minimum effective scale for such a scheme. The tendancy has been to seek co-ordination over "as wide an area as possible" (Oka 1979) and to press for planting to be completed within as short a time as possible. The difficulty in reaching such absolute goals and the certainty of conflict with those providing essential inputs (the limitations of irrigation and drainage capacity and of transplanting and harvesting labour in particular are often cited) have led to the still-birth of synchronous planting : though widely touted in recipes for integrated pest management (FAO 1982, Kiritani 1979), few concerted and sustained efforts to implement it have been mounted.* 1 One of the few reported cases of deliberate synchronisation is that described by Fernando (1979) in Amparai district, Sri Lanka. Details however are scarce regarding the manner in which the scheme was organised, the extent of synchronisation, and its benefits. It does not appear to have been continued. The distinction should be recognised between attempts to synchronise cultivation and to shift the date of planting per se : the difference is essentially that between altering the variance or the mean of planting date. In Kwangtung province, China, early planting before June 26 is practised to limitinfestation by YSB (NAS 1977). Both early (Kiritani, 1979) and late planting (Nagai 1959, quoted in Khan 1967) have been recommended in Japan for stemborer control. In Java, the white stemborer Tryporyza innotata undergoes diapause during the dry season and was effectively controlled by delaying planting till after emergence following the first rains (Van der Goot 1948, quoted in Khan op. cit). Such tactics may not be effective in tropical and equatorial Asia against pests that do not enter diapause and which successfully exploit rice growing at any time of year due to multiple cropping and asynchrony. The organisational requirements for shifting the mean date of planting and its possible repercussions are likely to be different from those for synchronising cultivation and will be only briefly considered below. 296 In what follows, I describe an attempt to promote the concept in Nueva Ecija that drew on the results of previous chapters regarding the origins of asynchrony and the spatial and temporal requirements for synchronisation. The aim of this effort was to examine the organisational and institutional constraints to synchronisation in smallhold rice cultivation, relying as far as possible on farmers' informed and active participation. 7.3 Possible consequences of synchronisation in Nueva Ecija By late 1981, though not all the results reported in the previous chapters were available, their trend was becoming clear and we began to actively consider the possibility of promoting synchronisation in a pilot area. Those involved in the deliberations were the author, 2 research assistants (J. Bandong and A. Alviola) with long experience in extending integrated pest management to small farmers, and a post-doctoral fellow (P. Kenmore), who left the project early in 1982. Occasional input was also offered by an entomologist (3. Litsinger), an irrigation engineer (A. Early) and an agricultural economist (R. Herdt) at IRRI. National and provincial officials of the Ministry of Agriculture, which was a co-sponsor with IRRI of the Small Farmer Organisation Project (SFOP) (Chapter IV), were aware of our intentions but did not take an active role in the planning. In November 1981, the 2 research assistants and I began a series of informal interviews with experienced farmers in the area who had become known to us in the course of our researches, in order to gain a* better understanding of the limits to synchronisation and of its possible negative impacts. Though we brought discussion around to issues that were of concern to us, such as the availability of transplanting labour, we employed an unstructured format so as to ensure that we were not merely told what we expected to hear, a danger with questionnaire-type surveys. 297 7.3.1 Negative impacts From our discussions and deliberations, 3 potential repercussions of synchronous planting emerged that were of greatest concern : (1) Concentration of demand driving up hire rates. Both tractor and transplanting labour rates may be affected to the extent that supply cannot meet demand, and, as was shown in Chapter IV, this can be seen to occur in the normal course of events within a season, as the demand for services intensifies. Conversely, synchronised harvest, an unavoidable consequence of synchronous planting, may create a market glut, depressing the local price farmers receive. (2) Negative effects on landless labourers. In areas where labour would be in tight supply under a synchronised planting schedule, farmers may shift to direct-seeding, accentuating a trend observed in recent years that has reduced the earning potential of the landless (4.3.3). Farmers might also bring in an increased number of labourers from outside the area, threatening the contractual links between landless and farmers whereby those who transplant gain the right to harvest, a system known in Nueva Ecija as "payok", thus increasing the income insecurity of local landless. (3) These potential effects of synchronisation have been pointed out as well by Bernsten (1980) and Goodell (in press). An aspect that has been much less commented upon is the increasing impact of natural calamities. Though synchronisation is likely to reduce the damage of those pests that disperse from field to field and which reach higher levels the longer rice is present in the environment, it may increase the impact, if not the actual yield loss from typhoons, floods and droughts as well as 298 from those bacterial and fungal infections, such as blast (Ou, 1972) whose spread is tied to particular weather patterns. Unless synchronisation pushes cultivation into a period when the expectation of these occurences is greater than at present, the mean level of loss due to them over a number of years will be unchanged. However the temporal variance of yield will be increased: in some years all farms may escape damage, in others all will be in a susceptible stage when the calamity occurs. It may be that asynchrony functions at present as a form of crop insurance, whereby farmers affected obtain loans or other forms of assistance from those whose harvest is near normal. Such transactions are known to be crucial to the community’s ability to survive disasters in areas of semi-arid India (Binswanger et al., 1980), but little information is available on their functioning in wet rice regions such as Nueva Ecija. In order to gain some insight into the nature of these coping mechanisms, the following question was asked of farmers in the formal survey conducted during the 1982 wet season (4.4.1) : "After a typhoon, as for example the last one that severely damaged your crop, where do you obtain the means to support your family?" As Table 7.1 indicates, some 15% replied that they rely principally on help from family or friends, most residing nearby, while 57% obtain informal sector loans, and 20% have alternative sources of income. It is important to note that, of those relying on assistance from family or friends or on loans, that is 72% of the total, 44% said that this was obtained from another farmer. Unless this person was wealthy enough to weather a bad harvest and still extend loans or assistance, this suggests that excessive synchronisation may imperil this method of coping with calamities. The rural landless, who rely heavily on income from harvesting and who generally have less capital than farmers to buffer the impact of reduced incomes, are likely to be all the more affected by the variability of yields and hence by excessive synchronisation. 299 Table 7.1 Source per cent Informal sector loans 57.3 Alternative sources of livelihood or sale of livestock 20.2 Assistance from family or friends 14.5 (of which family or friends residing in the barrio) (11.3) Bank loans 4.0 No response 2.4 Own savings 1.6 TOTAL 100 Principal source of subsistence following typhoon damage to crops cited by respondents, wet season 1982. The survey methodology was described more fully in section 4.4.1. n = 124. 300 The most destructive of calamities in Central Luzon are typhoons, most frequent during the wet season. In the past few years, three have struck the area during the later stages of the wet season crop: Kading in 1978, Aring in 1980 and Anding in 1981. It can be seen from Figs. 7.1a and b, which draw on maps of typhoon tracks over a 30 year period (PAGASA 1978), that planting at present takes place at a time that puts the crop in considerable danger during the most sensitive stage from just before flowering until harvest. Shifting the mean date of planting more than 2 weeks in either direction would greatly lessen this risk; planting six weeks earlier would halve the probability of late season typhoons from 1 year in 2 to 1 year in 4. Discussions with NIA officials (N, Prieto, pers. comm.) indicate that the pattern of typhoon occurence was not among the meteorological factors taken into account in the design of the cropping schedule when the irrigation system began operations in the mid-1970's. 7.3.2 Other advantages of synchronous planting Besides the principal benefit with which this study has been concerned, the pest suppression effect of a reduced duration of rice in the environment, the completion of planting within a shorter period of time would have a number of other desirable results. (1) Firstly, it would lessen the need for farmers to apply insecticides, resulting in lower production costs and a reduced threat of environmental pollution and human poisoning. However, the realisation of this benefit requires that farmers be aware of the lessened need to spray and that they be free not to purchase insecticides, which has not always been the case up to now (Goodell 1983). There is good reason therefore for synchronisation to go hand-in-hand with the extension of integrated pest control methods, in particular the use of economic thresholds for insecticide application. 301 Figure 7.1 a. Frequency of typhoons touching land within within land touching typhoons of Frequency a. 7.1 Figure PROBABILITY OF TYPHOON DURING 6 WEEKS •PRIOR TO HARVEST ra uig h wt esn f 1982. of season wet the during area arrow represents the mean date of planting in the project project the in planting of date mean the represents arrow variety (post-transplanting) at given planting dates. The The dates. planting given at (post-transplanting) variety b. C alculated probabilities of a typhoon striking C entral entral C striking typhoon a PAGASA of by given probabilities 78 alculated - C 1948 b. from tracks typhoon of Luzon during the final final the maps during from Luzon taken is data 52. The = n Ecija. (1978). Nueva abanatuan, C of A FB A ARMY U JL AUG NOV OCT FEB MARSEP JUL DEC APR JUNMAY JAN 302 6 weeks of the crop for a 90-day 90-day a for crop the of weeks 2 ° latitude latitude ° (2) Irrigation conveyance losses would be minimized were water to be flowing in the canals and farm ditches for a shorter period (A. Early, pers. comm.), enabling water to be diverted to those areas that are short, or to be stored in the dam, lessening the impact of reduced rainfall from which Nueva Ecija has suffered the past 2 years. (3) Similarly, the amount of water that must be drained would be reduced and thus the risk of flooding where drainage facilities are inadequate. (4) Finally, synchronous planting provides an additional incentive to resolve agricultural problems that may be limiting yields and reducing profitability in other ways. Dredging clogged canals for example would improve the water supply to downstream fields and increase yields directly, but may not attract sufficient support or attention until the issue is linked with asynchrony and increased pest levels, which affect farmers further afield. The investment required may not be deemed justifiable when considered from the narrow perspective of relieving water stress, but may result in handsome returns when the overall benefits, including those of reduced asynchrony, are evaluated. 7.3.3 Weighing costs and benefits As noted earlier, the deleterious economic effects of synchronous planting resulting from the concentration of demand for inputs and of supply of rice at harvest are likely to be determined by the scale of synchronisation. Although the results of Chapter V were not available at the time that a decision had to be made on whether to proceed with promoting the concept, it was clear that very low pest levels were found in areas where planting was relatively synchronous in the normal course of events and that this co-ordination was achieved without evident effort or ill-effect. It has since become 303 apparent (5.6.2) that at some sites, generally these near the main canal, planting may be so synchronous, particularly in the dry season, as to be effectively simultaneous; that is, pest levels are no higher than one would expect were all fields to be planted on the same day. It was judged therefore that a responsible strategy was to work with farmers and institutions to improve the distribution of the external inputs of water and credit which appear to be at the root of most ecologically significant delay (Chapter IV), while putting relatively little stress on the scheduling of the village-level resources of labour and machinery, interference with which it was feared might result in dislocation. Limiting synchrony to the levels achieved in well-favoured barrios would also not destroy village-level mechanisms for coping with calamities, though from this point of view the optimum degree of synchrony would be the minimum. It appears evident however from the earlier discussion that the threat to local incomes from typhoons can be more effectively countered by shifting the mean date of planting than its variance, reducing the absolute level of risk rather than merely spreading it within the community. The feasibility of changing the time of planting must be explored by those affected, NIA, other agencies and the farmers, and such an effort might well form part of an attempt to increase synchrony. In this initial scheme however, attention focused on synchronisation alone. I would emphasise that if in Nueva Ecija it appeared possible to largely avoid synchronisation's side-effects by limiting its scale, this may not always be possible elsewhere. A detailed consideration of the factors giving rise to delay - for which the "components of asynchrony" technique described in Chapter IV may be of some use - and close and attentive consultation with those likely to be affected is essential before such a scheme is attempted. In areas such as that of the Muda project in Malaysia where the man-land ratio and the incidence of landlessness are less than in Nueva Ecija, labour shortage may emerge as an important constraint to even moderate synchronisation (see e.g. Afifuddin et al. 1974). Yet even there, inadequate irrigation and drainage facilities appear responsible for considerable delay, and extension of tertiary-level irrigation infrastructure is deemed essential if the span of planting time is to be reduced (Kin 1981). 304 7.4 Promoting Synchronous Planting A roughly square area of some 2,500 ha of rice land was selected within the wider study area described in Chapter IV (fig. 4.2). It included the 4 barrios where Agency for Community Educational Services (ACES) community organisers were already assigned (Marawa, Malabon-Kaingin, Imbunia and Hacienda Romero), and 4 adjacent ones (Carmen, Rajal Centro, Santa Rita and Pamacpacan) where organisers were assigned by July 1982. A target of 3 weeks was proposed in which to complete transplanting during the 1982 wet season (though it was fully expected that this would be exceeded initially), corresponding to the generation lengths of the shortest-lived insect pests (BPH, GLH, CW). No one rice variety was recommended, but it was suggested that where drainage conditions permitted, farmers avoid the tail-stature medium-duration IR42 in favour of other varieties such as IR36, 50, 52, and 54 which mature 107-120 days after seeding. A shift to shorter duration varieties would augment the pest suppression effect of synchronous planting by reducing the overall duration of rice in the environment. IR42 is moreover susceptible to tungro virus which, as mentioned in Chapter IV, affected parts of the area the year previous. Our position as researchers associated with IRRI dictated that, for the most part, we could do little more than make suggestions to farmers and institutions. Though national and some provincial level officials at NIA, The Ministry of Agriculture and Central Bank knew of the SFOP's activities, at the operations level we were acting essentially on our own. We presented our view of the causes and effects of asynchrony. We emphasised the need as we perceived it for co-operation between and among farmers and institutions and did not shrink from pointing out specific areas where co-ordination in the provision of support services appeared essential. However the lack of an executive capacity and the transient nature of the project were to limit the effectiveness of our efforts. 305 After drawing up a tentative timetable for the plan, we began the process of promoting it among the institutions and farmers. I was responsible for liaison with NIA, the Ministry of Agriculture, the banks, the National Food Authority (NFA) and community organisers, while the 2 research assistants under my direction handled relations with farmers. 7.4.1 NIA Considering efficient irrigation delivery crucial to the success of synchronous planting (5P), I approached local officials of NIA in early November 1981 to discuss the broad outlines of the plan. The response of Engr. B*, Chief, Operations and Maintenance, District III, with whom we had earlier been directed to co-ordinate by the UPRIIS project manager, was positive. He readily understood the pest suppression effects of SP and pointed as well to its benefits in terms of reducing irrigation conveyance losses. Having just completed a Master's degree in agricultural engineering, he was eager to be involved with an innovative operational research project. Engr. B was frank about the financial constraints within which the irrigation district and UPRIIS had to work. The operating subsidy from the national budget that NIA had previously received was soon to be discontinued and current expenses would henceforth have to be met from fee collection. Even in a good season, barely 50% of the amount due was collected from the farmers, while he estimated 70% was required at a minimum. Fee collection would remain a major concern he cautioned; however, he recognised that measures to increase yields and reduce farmers' costs such as synchronous planting would eventually mean that farmers would be better able to pay their fees, and he was therefore willing to commit some resources to such a plan. The most important commitment he made at that time was to release water to the project area 10 days ahead of the surrounding areas so as to ensure that transplanting labour and tractors would be readily available. I found that, working within this climate of enlightened institutional self-interest, it was relatively easy to find common ground. 1 In the account that follows, I have altered the names of the principals so as to focus attention as far as possible on the issues rather than the personalities involved. 306 7 A .2 ACES I next presented the proposal to ACES. It quickly became clear that it was necessary to clarify the respective roles of IRRI and ACES in promoting SP, as there was initially some apprehension that the organisers were expected to sell the plan to the farmers. I replied that I saw that responsibility as our own, that I did not consider ACES as our agents. In any event, as they had been trained for the most part as social workers, I consiered that we were the better placed to discuss with farmers the intricacies of pest control and the timing of farm operations. Behind the ACES' initial response was the fear that the freedom to work with farmers on the "felt needs" that organisers uncovered, a freedom that had been guaranteed since the beginning of the project was now threatened. Synchronous planting they argued was our concern, not the farmers', and they disputed that farmers were all that worried about pests when their present control measures were still adequate. I agreed in part with this latter point, yet argued that if synchronous planting were not at the moment a felt need in itself, the farmers might yet come to see it as a rallying point from which to attack common and acknowledged problems, notably those of irrigation and credit. I was willing I said to have the organisers consider SP only when it became an issue in the barrio, an issue we would attempt to raise. I asked only that when discussion did turn to SP, that the organisers ensure that it be considered from all angles, that farmers follow through its implications. Though I emphasised that we were committed to working with the farmer groups and the organisers within the "bottom-up" framework (Goodell et al. 1978) it would take several months and much discussion until ACES' initial suspicions were allayed. 307 7 A . 3 Banks and NFA I next approached the banks, and in particular the Q. Rural Bank (QRB) and the Land Bank of the Philippines (LBP) which are the most important sources of formal credit in the area. The former obtains its funds from the Special Agricultural Rehabilitation Fund (SARF), the latter from the Masagana 99 loan program. I posed three questions to the banks after explaining the nature of the scheme. Firstly, could the realationship that had grown up between the banks and autonomous village-based groups such as the Credit Committee in Marawa and similar organisations in Imbunia and Malabon - Kaingin, which had resulted in improved provision of credit to farmers and increased rates of repayment (Modina, n.d.), be extended to other barrios in the project area? Both Mr. J. of QRB and Mr. M. of LBP, San Isidro office, thought the relationship a useful one and favoured similar links with other barrios. Secondly, I asked to what extent the processing of loans and release of funds could be expedited. Both replied that at their level there was no reason for processing to take more than a few days if the application was submitted on time. Mr. J. in particular pointed to delays in Manila beyond his control. In the event, particularly at QRB, release of loans to individuals is constrained by the overall level of repayment from the previous season : 80% must be received before more funds are committed, though it is not clear at which level this criterion is being imposed, whether locally or centrally. Finally, I asked whether the "package" requirement on the purchase of agricultural chemicals, particularly pesticides, could be relaxed. If SP, or for that matter integrated pest management in general, is to have any impact on reducing the costs of production, farmers must be able to purchase appropriate pesticides only when these are required. Both banks replied that for several seasons already, 308 chemical inputs had been allocated to farmers only upon request. The issue however is more complex than it may appear as bank technicians must approve all such requests. Farmers are also limited to products available at the banks' outlets where prices are often substantially higher than at local dealers. I could do no more however than raise these points. Both banks expressed interest in the 5P plan and pledged to co-operate. I also contacted the R. Rural Bank and the C. Rural Bank whose involvement in the area was minimal, though it had been greater in the past. I explained the plan to them and ensured that they were invited to future meetings in the area. At the National Food Authority I spoke with Mr. E. the provincial manager. Farmer groups had not taken up the issue of marketing, nor contacted NFA, and I in no way wished it to seem that we were negotiating for them. I sought to discover what the scope for participation of NFA in the plan might be and what proportion of the harvest they might purchase at the government support price (well above the prevailing purchase price, thus averting any threat of a glut). Mr. E. declared himself willing to co-operate in the SP plan and to speak with the farmers. NFA would be able to purchase all the production, up to the levels of a normal harvest, and would consider sending trucks, "mobile buying stations", to the area. 7 A A Farm ers We adopted a two-stage approach in presenting our idea to each of the 8 barrios. We met first with village leaders in order to outline the plan and gain their reaction and advice, before presenting it to a general meeting (in the large barrios as many as 3 were needed). These were called by the leaders generally under the auspices of the Samahang Nayon, (a government-sanctioned farmers' association). 309 To both audiences, we began with an outline of the concept of synchronous planting and a summary of our research findings, and then moved on to an open discussion of the local problems in implementing the plan. The meetings were held entirely in Tagalog, and we made every effort to express technical ideas in terms meaningful to the farmers. We relied on a series of Manilla paper posters illustrating points we would make again and again. On one for example, we used horizontal coloured bars to represent the length of time that it took to complete planting in different barrios, and vertical bars to represent the abundance of stemborer moths, which had been measured in the light traps. (Fig. 7.2). In this way, we could point to each barrio in turn and stimulate discussion on why planting was or was not prolonged : Carmen for example, as all knew, was delayed because of an unfinished canal, resulting apparently in an increased level of stemborers. Perhaps not surprisingly, no one during the meetings expressed opposition to the proposal; indeed, most seemed to accept readily, almost intuitively, the idea that planting within a shorter time was a good thing. In talking with individual farmers, many cited their own experience that those who planted late suffered worse bird and rat damage in particular. Extending the concept to insect and disease damage did not seem to stretch their credulity. Most discussion in the meetings centred on the problems of implementing the plan, and for the most part, these involved difficulties in the delivery of inputs. Indeed, once started, the discussion hardly touched on synchronous planting or the control of pests per se : farmers were clearly preoccupied with irrigation and credit problems. 7.4.5 Farmers and the Institutions For the most part we merely observed as farmers and the institutions grappled with these problems, as they had in some cases for a long as 8 years. However, an opportunity to take a more active role presented itself when I learned that some funds had become available to UPRII5 in the framework of a NIA/IRRI agreement on research into the drainage problems in 310 311 A Figure 7.2 Posters used in promoting the concept of synchronous planting among farmers. A. The key experimental results of the project (c.f. Fig. 5.9). The legend reads, in Tagalog: "Village", "length of time to complete planting", and "abundance of yellow stemborer" . B. Sketch map of the irrigation system in the pilot area. During meetings, farmers used chalk to indicate on the map problems in the supply and drainage of water, and possible solutions. C. After considering the causes of asynchrony, discussion generally turned to means of alleviating them. The caption reads: "These problems can only be overcome by cooperation". In the centre of the figure are "farmers and labourers" and, clockwise from top right, "banks (credit and insurance)", "IRRI and the Ministry (research and monitoring)", "NFA (marketing)" and "NIA (irrigation)". u n u * * i v «_»rv U , QUOH ANC MGA PROBLEM A'* u* rxvv-r AY MALULUTAS SA PAKIKIPAG ULUNGAN NG NIA EMNKDSFGUAO ' PATUBIG \ / PAUTANG MAG5ASAKA MAGGGAGAV/A N MANUNUR! BENTAHAN J yiAGASALIKSIK our area. NIA was responsible for carrying out repairs to canals and drainage channels, and though it had already prepared a work plan, Engr. B. agreed it would be useful to discuss what was to be done with the barios affected. We therefore called a meeting for 21 May 1982 which was attended by several NIA officials and farmers from Marawa, Santa Rita, Pamacpacan, and Imbunia. The NIA plan was presented by Engr. B. and farmers then asked for additional specific work to be undertaken to resolve the problems in their areas. Though the work took longer to accomplish than originally scheduled, and despite the fact that many long-standing problems were not resolved, a distinct improvement in the drainage situation was observed in the wet season by farmers in several areas, including parts of Imbunia and Pamacpacan. We also suggested to NIA and farmer leaders that it might be advantageous to meet together to discuss the particular problem of water distribution down the longer sub-laterals. One often observed (4.3. 1) that it took several weeks from the time farmers near the beginning of canals were served until water reached farmers at the downstream end a few kilometers away. Not only did these delays result in significant asynchrony and potential for pest build-up, but downstream farmers also suffered water shortage. They had suggested several means whereby water could be more sparingly used and economically distributed, and were in general willing to meet with NIA and farmers upstream, as was Engr. B. The idea however was never carried through, largely due to the deteriorating relations between NIA and the farmers. NIA's concern with fee collection became increasingly imperative as the dry season campaign, from April to July 1982, wore on. When it began to appear that the collection would again fall short of the 70% objective, the chief of District III, Engr. A. undertook a number of initiatives designed to put pressure on the farmers. In discussions with me, he confirmed that he had approached the mayors of Zaragoza and Jaen and had them ask the police to visit delinquent 313 farmers. He also asked the banks to subtract the dry season irrigation fees from the wet season agricultural loans that were about to be released. Neither of these attempts proved productive, but both contributed to worsening already strained relations with the farm ers. The battle for fee collection had two direct effects on the synchronous planting project. The first was that Engr. A. vetoed the request to release water to the area 10 days before the surrounding areas, a request that had been agreed to by Engr. B., his subordinate, the previous November and confirmed in several subsequent meetings. Secondly, our attempts to propose to all parties a workable planting schedule were rendered futile by Engr. A.'s vcaillation : more than once he agreed, after discussion, to release water by a certain date, only to refuse later. Engr. A. also intimated to me that bulldozers and other earthmoving equipment would be made available sooner to tackle the irrigation problems of the area were the collection to improve. As an observer, it became clear to me that a vicious circle had developed in relations between NIA and farmers : poor service led to poor repayment, which in turn led to an unwillingness on the part of NIA to invest in repairs. Some, like Engr. B. saw the need to break the circle and to undertake improvements that would increase yields and farmers' ability to pay. Engr. A. on the other hand, insisted that the farmers' refusal to pay was primary, and looked with deep mistrust on the ACES' efforts to assist farmers in organising to petition for improvements. Indeed, he once confided that he was trying * to break the groups, approaching individual farmers and promising them their demands would be dealt with if they disassociated themselves from the others and the organisers. Given Engr. A.'s superior position, it was his rather than Engr. B.'s view that prevailed at the local, operations level of NIA. One of the most significant events during the promotion of SP was the general meeting of July 14, 1982. The farmers of Marawa took the initiative in organising it and co-ordinated with the 7 other barrios. At the request of the organising committee 314 consisting of Samahang Nayon and group leaders of Marawa, I passed on invitations to the banks, NIA, NFA, and the Ministry of Agriculture. The purpose of the meeting was to discuss among the farmers and institutions involved in the plan the requirements of synchronous planting, and in particular to make known to the institutions the farmers’ requests for improvements in services that would make it possible. Approximately 200 farmers attended the meeting, as well as representatives from the major institutions, incuding Engr. Z. of NIA Central Office, Mr. E. of NFA, Mr. S. of Central Bank, and Mr. J. of QRB. After opening the meeting, the barrio kapitan of Marawa, Mr. N. called on IRRI to provide a brief explanation of synchronous planting, its potential benefits, and technical requirements. The farmers then broke up into smaller groups for approximately 15 minutes to discuss specific problems in implementing SP, such as irrigation, credit, transplanting labour, seeds, and marketing, while the representatives of the institutions met together with myself to discuss aspects of co-ordination. When the meeting reassembled, a farmer from each group presented the results of the deliberations, summarising the concerns expressed by the different barrios. Where specific demands were raised, the agency involved was given a chance to respond. Farmers brought up a wide variety of issues, many not obviously related to delays in cultivation, but on the whole, the problems of the 8 barrios were succinctly presented. The majority of the discussion centred on problems that involved particular institutions and there were many frank exchanges. Mr. E. responded to allegations that, among others, merchants found it easier to market their rice with NFA than farmers, that payment was much delayed, and that the incentive fee payable to the Samahang Nayon for group sales had not been received. He denied the former, explained that the delays in payment were unavoidable given the volume of NFA's purchases, and informed the farmers that the incentive was held in trust for use on approved post-harvest projects. He promised to have a set of guidelines prepared for the 315 farmers on the marketing of rice with NFA, but regretted that it was not possible, as had been requested, to establish a buying station in each barrio. To the banks, the farmers complained that their debts had accumulated due to natural calamities and poor harvests, and that many were consequently unable to avail of bank loans and forced to turn to local money lenders. An interest moratorium was requested so that farmers might repay their arrearages. Those who were regular bank borrowers complained of delayed release of loans and that the cash portion was insufficient to cover their actual expenses. Mr. J. replied that many of the repayment problems were the farmers' fault, that they had suffered "television catastrophes" or "stereo catastrophes". Loans were released late he said because repayments had been delayed, and, further, that farmers should not expect to meet all their expenses from bank loans. However, he agreed to consider the individual problems of a number of farmers that had been raised. A long list of irrigation and drainage problems was presented to NIA by the discussion group leaders : each barrio complained either of unfinished canals or drainage channels. Mr. T., the irrigation technician in charge of the division, responded. When he suggested that the repair work initiated in Imbunia had been suspended because the life of the bulldozer operator had been threatened by residents, the barrio kapitan, Mr. C. and one of the group leaders, Mr. P. responded angrily and a shouting match ensued. Kapitan N. intervened and eventually managed to quiet the disturbance, whereftpon Engr. A. continued on behalf of NIA. He dealt with none of the specific issues raised by the farmers, but instead asked why the fee collection was so low in most barrios. He suggested that this was traceable to the presence of ACES organisers : Sta. Rita, where no organiser had been assigned during the previous season had paid more than 80% of the fees due. Several farmers rose in response, some explaining that fees were not paid because the service was poor, others denying that the ACES had ever suggested that farmers not pay their irrigation fees. 316 Engr. Z. NIA's deputy administrator, was the last to speak. He said that he had come to see in person how irrigation problems were dealt with by farmers, organisers, and local NIA officials. He had been saddened by what he had seen, and asked, in English, that in future there be less confrontation and more co-operation. Kapitan N. then quickly closed the meeting. SP, he said, was clearly beneficial to farmers, however it was unlikely to come about that season. If problems of irrigation and credit could be dealt with, he thought it possible in the following dry season. As the representatives of the agencies prepared to leave, farmers presented them petitions for the solution of specific, barrio-level problems. From the narrow point of view of launching SP, the rather chaotic end to the meeting was unfortunate in that it prevented, as we had wanted, discussion of and agreement on a target range of planting dates, so as to move as far towards synchrony as the larger constraints would allow. More broadly, the meeting failed to reach a consensus on measures to overcome specific problems. The degree of acrimony between farmers and institutions, evidenced in particular by the exchanges on irrigation, made the degree of co-operation demanded by SP seem unlikely to be readily achieved. However, the mere holding of the meeting was significant, with the attendance of approximately 20% of the farmers affected by the scheme and the presentation of their problems in a relatively co-ordinated fashion. It was clear that, to a considerable extent, farmers had accepted SP and saw it as desirable, if for some only as a vehicle to bring about changes they sought for other reasons. A 7 A .6 The West Season Crop Without agreement on a target range of planting dates, synchrony would likely not be much improved over the situation in previous seasons. I was considering taking around a suggested target to each barrio and to the agencies when Nature intervened. As mentioned in Chapter IV, Typhoon Emang struck Nueva Ecija two days after the meeting with a great deal of rain but rather weak 317 winds, and as few crops were in the groumd, there was little damage. As far as synchrony was concerned, Emang had two effects (4 A , 1). Firstly, it soaked farms in the area more or less equally and with sufficient rain for many farmers to begin land preparation. Secondly, the main canal was breached some kilometers upstream of the area, and it was to be more than a month until it was repaired. For this time the area was essentially rainfed, though some farmers had received irrigation for as long as 10 days before the typhoon. I judged it unwise in this situation to suggest a target, as farmers would be dependent solely on rainfall and would surely base their scheduling of farm operations on their expectation of the rainfall pattern. As mentioned in Chapter IV planting in the event was relatively synchronous : the standard deviation of planting dates, estimated from the random survey of farmers discussed earlier, was 11.7 days, suggesting that approximately 95% had completed transplanting within 47 days. A year previous, the standard deviation, estimated from NIA records, was some 20% greater. I suggest that this reduction reflects the more even distribution of water early in the season, which occurred, perversely, due to the temporary breakdown of the irrigation system. Throughout the wet season crop, weekly classes were held in each barrio dealing primarily with integrated pest control and led by the two research assistants. Discussion centred on identification of pest problems and the use of the "pamantayan" or economic threshold approach to insecticide use. An experimental plot was established in each barrio where different insecticide treatments were demonstrated to the farmers. There was also discussion of a wide range of problems related to SP. In several barrios, notably Imbunia and Carmen, the farmers' class developed into well-attended barrio meetings organised and scheduled by the farmers themselves. Representatives from NIA, the banks, NFA, and the Ministry were at times invited to attend the meetings. 318 7.4.7 Farmers attitudes towards SP One of the objectives of the survey reported earlier was to ascertain farmers' opinions regarding SP and to what extent our promotion of the concept had been successful in reaching them and affecting their views. Table 7.2 indicates that, overall, some 64% had either attended one of the meetings or had heard about the plan in some other way. A much larger proportion of the farmers actually residing in the 8 barrios had been reached compared to those cultivating land in the project area but residing in distant barrios or in nearby towns : 73% as against 13%. The latter group is estimated to represent some 16% of the farmers, a greater proportion than anticipated, and as the time they spend in the area and their contacts with field neighbours are relatively limited, considerable effort would have to be expended to make contact with them. When asked, "In your opinion, would it be a good thing if your barrio finished planting in a shorter time?", only one of the 124 respondents replied in the negative (and he had attended an SP meeting!). Asked why, in an open-ended question, farmers provided a range of reasons, which I have summarised in Table 7.3. Among the sample as a whole, 65% considered reduction of pest damage the principal advantage of greater synchrony. A significantly higher proportion (X = 9.2, P K. .01) of those who had attended the meetings cited this reason compared to those who had never heard of the plan. Moreover, the proportion that mentined reduction of insect 'damage specifically was also significantly greater (JL = 7.7, P .01) among those who had attended meetings, where this aspect of pest loss was stressed, than among those who had not been reached. 319 I conclude from these results that, though we were apparently successful in influencing farmers' avowed opinions*, it is clear that there is a substantial understanding of the principle of synchronous planting in the general population, independently of our efforts. Certainly this agrees with my impression from discussions with individual farmers. The wet season crop was harvested without significant damage from storm or pests, although there was a minor outbreak of blast and leaf streak. Farmers reported yields averaging 4.20 +_ .10 t/ha (n = 115), whereas in the preceding wet season, an equivalent sample drawn from NIA records for the area had an average yield of 3.53 _+ .09 t/ha (n = 116; *2 2 9 = P < *001). However, besides greater synchrony, several other factors may have contributed to this improvement, such as increased solar radiation and the absence of late season typhoons, whose relative importance it is impossible to establish. A second general meeting of those involved in the plan was held on January 28, 1983 to evaluate the progress ache.ived during the first season and to make plans for the dry season that was just beginning. Perhaps what was most significant in this was that planning for the meeting was the responsibility of an ad hoc committee of farmer leaders from each of the eight barrios. The group met several times to discuss common problems, and to formulate objectives and proposals. Most of the issues raised at the July meeting concerning marketing, credit, and irrigation re-emerged in January, and the responses of the agencies were similar. There was, however, noticably less heat in the exchanges, and the meeting took place in good order. A dominant concern of farmers was the shortage of water : the depth in the reservoir forced NIA to withdraw service to some 30% of the 1 I cannot estimate the extent to which those farmers who were aware of our promotion of SP provided answers they assumed we wished to hear. 320 farmers in the district. Little apparently could be done to alleviate this problem, yet the crisis forced farmers to consider some novel ideas, in particular the possibility of negotiating with NIA the taking over of water distribution at the sub-lateral level, as had been attempted on a trial basis in a few areas of Central Luzon. The proposal appeared to take the NIA officials present somewhat by surprise, but they undertook to consider the idea. Table 7.2 Awareness of the synchronous planting plan among farmers residing in the project barrios and those living elsewhere. Percentages expressed with respect to row totals. Wet season 1982. Had not attended Had not attended Had attended Total a meeting, and a meeting, but a meeting (n) had not heard had heard of of the plan the plan Residing in Project Barrios 27% 9% 64% 104 Residing outside 85% 10% 5% 20 Total 36% 9% 55% 124 321 Table 7.3 Benefits of synchronous planting cited by survey respondents. Percentages expressed with respect to row totals. "Reduction of (of which Other Total pests" "reduction of reasons (n) insects" specifically) Had not attended a meeting and had not heard of the plan 49% ( 22%) 51% 45 Had not attended a meeting but had heard of the plan 55% 0 6 % ) 45% 11 Had attended a m eeting 76% 0 9 % ) 22% 67 Total 65% (38%) 35% 124 Farm ers also pressed the desirability of a restructuring of past due loans, a proposal that had been mooted for some tim e. The representative from Central Bank assured the farmers that the idea was being closely considered. 322 The uncertainty in the irrigation supply forced many farmers, among those who were able to plant, to rely on drainage or on shallow-well pumps, and gave rise to considerable asynchrony : some 19 days standard deviation (4.4.3). Again, no concessions were made to enable the area to minimise asynchrony, for example, by announcing well in advance of water release which areas would and which would not be provided with irrigation. I was told that to do so might imperil the collection of fees for the wet season : those who knew they would not be harvesting for another year would be unlikely to pay. Setting a target range of planting dates in those circumstances was' futile. 7.5 Implications of the Project It is difficult to assess at this time what the eventual impact of this series of discussions and meetings over several months will be on the various constraints to synchronous planting in the project area, and how long the initiative to implement SP will be sustained, now that with my departure, IRRI's direct promotion of the concept has ended. The emergence of an area-wide grouping such as the ad hoc committee, and the fact that SP is seen to require the resolution of long standing agricultural problems, may ensure that, on the farmers’ side, the issue remains alive. It is not as clear to what extent the government agencies and private institutions concerned, particularly at the local, operations-level, are committed to the concept. It may be that commitment is not required, that institutional interests and self-interest will gradually lead to the resolution of particular impediments to production. Nevertheless, it is important to ask why SP was not more readily embraced by the institutions, and how future attempts to implement it should be conceived and executed to ensure their involvement. Important questions have also been raised concerning farmer participation and the role of community organisers. Finally, consideration must be given to the impact of SP on landless labourers and on means of ensuring their input into the design of such schemes. These issues will be dealt with in turn. 323 7.5.1 Institutional involvement In Nueva Ecija, promotion of synchronous planting was centred on farmers and the operations-level personnel of agricultural support institutions. Though national level officials were aware of the activities of the SFOP, indeed the Ministry of Aggriculture was its co-sponsor with IRRI, there had been a determined effort from the outset to ensure that knowledge of the project did not influence the behaviour of local personnel, that the advantages of improved farmer organisation would have to make themselves felt within an undisturbed administrative environment. I believe that the narrative of the efforts to promote synchronous planting makes clear that agitation from below and my own liaison with local officials were insufficient to bring about the changes in procedure and the modest investments required to permit greater synchrony. Though in every institution I encountered people sympathetic to the concept, it readily became apparent that SP conflicted with a number of institutional objectives, and that a resolution could only occur at a policy level. NIA's response was crucial, for irrigation and drainage-related factors appear to be the chief causes of ecologically significant asynchrony (4.4.3). Several of the engineers I dealt with recognised that synchronous planting has much in common with sound irrigation practice, particularly efficient scheduling to reduce conveyance losses. In other parts of Central Luzon, for instance the nearby Lower Talavera Irrigation System, NIA itself has been behind efforts to reduce the length of time it takes farmers to complete farming activities (Early 1980, Tapay et al. 1980). Many of the improvements which farmers requested that would have made greater synchrony possible have been recommended internally within NIA, such as rehabilitation of drainage channels (Uwagawa and Punzalan 1978) and completion of unfinished canals. Local officials however were under considerable pressure from Central Office to increase fee collection to at least 70%, failing which layoffs were threatened. In response to requests from farmers to remedy 324 specific irrigation defects, the reply was often "pay first". If farmers, claiming depressed production, could not or would not, then only a relatively high level decision could break the impasse by committing additional funds and management effort in the immediate term in the hope of increasing collection over the longer haul. Key local officials appeared primarily concerned with the immediate issues of institutional survival and were unwilling or unable to break out of the pattern of conflict with farmers. Though there is dispute both within NIA itself and among researchers (e.g. Tapay et al. op. cit) about the causes of low fee payment, a plausible case can be made that inability to pay is a key factor. Tagarino and Torres (1978) show that in Nueva Ecija, some years previous to the present study, the returns to farmers were generally insufficient to permit them to meet the fees charged - 300 kg of paddy per ha per year, or its cash equivalent. Within the larger study area, data furnished by NIA (Fig. 7.3) show a significant positive correlation between mean yield and collection rate in irrigation divisions (ca. 2000 ha). If the relationship is assumed to be direct, then measures aimed at increasing yields, such as synchronous planting* will result in higher fee collection. Institutional self-interest alone would then push the irrigation authority toward supporting such a measure. It should however be noted in Fig. 7.3 that, even at the highest mean yields recorded, nearly 5 tons/ha, collection remains below 70%, suggesting that increases in yield alone will not be sufficient to enable the irrigation system to become self-financing. It is instructive to contrast the response of the agencies in Nueva Ecija to the call for synchronous planting with that of their counterparts in South Cotabato province in Mindanao. There, extreme asynchrony and in some area triple cropping have been implicated in severe crop losses, particularly due to BPH and virus diseases. The situation is so acute that some farmers have 1 Reduced costs of production are also anticipated - see above 325 326 0 0 60 70 80 90 100 MEAN yield (CAVANS /HA > Figure 7.3. Irrigation fee collection from farmers as a precent of the total assessed in the 11 divisions of District III, UPRIIS. + dry, 0 wet season 1981. The best fitting linear equation is: Y = .123 + .00491 X 1 - .110 X2 (F2 {9 = 14.18, P < .005) where Y is percent collection (arcsin transformed), Xj is the mean yield (cavans/ha) and X 2 is a dummy variable ( 1 = dry, 2 = wet season). 20 cavans — 1 t. Data courtesy of the National Irrigation Administration, Cabanatuan City. been forced out of rice and into maize cultivation. Along with depressed yields, irrigation fee collection and loan repayments have reached very low levels, and NIA, the Ministry of Agriculture and the Development Bank of the Philippines have been behind a scheme to synchronise planting and rearrange the cropping pattern (Ministry of Agriculture 1982, Loevinsohn 1983). In Nueva Ecija at the present, losses due to insects are in comparison light, on the order of 15 - 30% where no insecticide is used. Among institutions, as well as farmers, there is nothing of the atmosphere of crisis with respect to pests that prevails in 5. Cotabato. If the concept of synchronous planting is to make headway in such a situation, institutions must be convinced that the correspondingly smaller yield increases from synchronisation are worth achieving, either for their own sake or because it is believed that they will eventually translate into higher loan or fee payments. Efforts to promote synchronisation will have to emphasise to institutions the multiple benefits of improvements in services and infrastructure. It is fortunate that, at least in the study area, the key improvements that synchronisation requires, such as drainage rehabilitation, are ones that have been recommended within the agencies, in that instance from the viewpoint of reducing yield loss due to flooding and allowing submerged land to be farmed again. Where multiple benefits are more difficult to demonstrate, and in particular where investments required to remedy constraints may be substantial, the arguments in favour of synchronisation will have to be closely marshalled and expressed in terms that will permit costs to be ,4 weighed against expected benefits. The acceptance of the concept by irrigation officials may also be quite different in countries such as Indonesia where direct water-use fees are not charged (Taylor, 1978) or China where irrigation management appears to be closely aligned with the concerns of agricultural efficiency (Nickum 1982). 327 7.5.2 Farmer participation The evidence presented above indicates that, working through farmer groups in the 8 villages, we were relatively successful in our efforts to diffuse the concept of synchronous planting within less than one year. The results suggest not only that farmers are aware of the pest suppression effects of syncrhony and likely were even before our extension efforts (7.4.7), but that to a considerable extent they act in such a way as to reduce asynchrony (4.4.2) within the limits imposed by external constraints. The organisation by farmer groups of 2 general meetings with attendances of over 200, and, in particular at the second, the presentation of unified statements of position and proposals under the aegis of an ad hoc committee, speak well of their capacity for the concerted action that synchronous planting requires. It may be argued that, given the emphasis of the scheme on improving the distribution of the external inputs of water and credit, farmers had nothing to lose by supporting it. Analysis showed however that it was primarily constraints at that level that were responsible for ecologically significant asynchrony. Moreover, issues such as the efficient distribution of water down the longer canals do involve farmers directly and generate a degree of conflict between those in the up and downstream sections. Rational scheduling of the village-level resources of machinery and labour might be considered within the farmer groups that have emerged once the larger constraints have been relaxed, should they be shown to be responsible for appreciable and avoidable delay. Though a greater measure of synchrony in planting can no doubt be achieved by irrigation authorities and banks acting on their own, I suggest that representative and responsive farmer groups have an important role to play in such schemes. They will help to ensure : (1) that hitherto unrecognised sources of delay in services are brought to the attention of the relevant agency; 328 (2) that farmers are prepared to make efficient use of the barrio-level resources of tractors, seedlings, and transplanting labour when the external inputs arrive; (3) that farmers' efforts and experience are mobilised to facilitate the distribution of inputs, in particular irrigation; and (4) that possible deleterious and unanticipated side effects of synchronous planting are brought to light. In Nueva Ecija, community organisers played a key role in catalysing farmers' response to our proposals. Their relative independence of the institutions and the credibility in the barrios that they enjoyed as a result facilitated farmers' consideration of the plan on the basis of enlightened self-interest, on which we had presented it. Not tied to any one set of concerns, community organisers could work with farmers on all aspects of production likely to be affected: credit, irrigation, pest management, and relations with landless labourers. The utility of community organisers has been recognised by NIA, which has employed them to facilitate the construction and the initial stages of management of communal irrigation systems (Korten 1981). An appropriate instituational relationship must still be found for community organisers in synchronous planting schemes, one that maintains the advantages of independence but that ensures that agencies do not view them as interlopers. 7.5.3 Involvement of the landless The inability to enter into significant discussions regarding the impact of SP with members of the landless labourer class must stand as an important failure in our efforts to promote the concept. In part this was due to the lack of organisation specific to this group and the difficulty of meeting landless on any but an individual basis, in part to perhaps a natural tendency to concentrate on those most involved in decisions regarding cultivation, the farmers. 329 There are reasons however to be concerned about the effects of synchronisation on the earning potential of the landless, already precarious for many. Though synchronisation need not reduce the overall requirement for labour, unless that is farmers are impelled to shift to labour-saving techniques such as direct-seeding, it may lead to increased income insecurity for landless workers by weakening the basis of the contractual links between them and farmers (Ledesma 1978; section 7.3.1). In Java, Soestrino (1982, cited in KEPAS 1984) has pointed as well to the impact on the near-landless of an enforced shortening of the tranplanting period. Within the span allowed, these farmers could not both transplant their own fields and secure employment in those of the larger farmers, on which they relied, forcing them to rent their holdings to their wealthier neighbours. Measures to address these risks can be conceived, but would require greater co-ordination than appears to exist at present between farmers and landless as groups. As explained below, the way in which synchronisation is scheduled over larger areas would also have a marked impact on the earning potential of the landless. If the implementation of synchronous planting is to avoid the dangers of coercion within a complex social environment, the free and informed participation of those affected, institutions, farmers and landless is crucial. There is a need for a local sponsor, not as in Nueva Ecija a research organisation whose involvement was temporary, and a sustained effort in which initial mistakes can be corrected. In the Phillipines, the obvious choice falls on the Ministry of Agriculture. 7.6 Implementation on a Wider Scale In the pilot demonstration described above, the area involved was limited, some 2,500 ha, and a single target was proposed - the completion of planting within 20 days. In any attempt to extend synchronous planting to a larger area, that degree of rigour would be neither desirable nor necessary. 330 Figure 7 Jf- illustrates an approach to the problem that I refer to as "synchronisation by waves". The map presents an indicative plan for transplanting within the 20,000 ha of Zones II and HI of District III, UPRIIS, based on the dispersal range estimates (5.7.11) and generation length of BPH. A twenty day span is suggested within a radius of 5 km, which would permit no more generations to develop within the distance traversed by 80% of adults than were every field to be planted on the same day. Less vagile and longer lived pests such as YSB would be all the better controlled by such a schedule. This is a formula therefore for extreme synchrony, yet within the entire area, planting would take aproximately the same amount of time, 10 weeks, that it did in the wet season of 1979 (NIA 1980). The principal difference here is that staggering is arranged in a more ordered fashion, with extreme differences in planting date separated by as large a distance as possible. The scheme as illustrated is based on the principle of "tail-first rotation" (Wickham and Valera 1978), a measure designed to improve the equitability of water distribution, but the pest suppression effect would be similar were planting to commence in the upstream areas, as at present. The plan has so far been discussed in only general terms with irrigation officials, and more detailed consultation is required to determine its feasibility, to pinpoint for example if and where canal capacity is exceeded. For those who hire out labour and machinery, landless workers and tractor owners, an ordered progression of planting dates may be more readily exploited and permit secure links to be established with farmers, guaranteeing employment. However, many of the arrangements between landless and farmers appear to be based -on personal contact and this may be strained where their places of residence are separated. Modifications to the plan may be forced by constraints in the provision of the essential inputs to cultivation and by the interests of those who live by them. Such compromises are expected, but the results of Chapter V provide a means of assessing their likely impact and criteria by which to attempt to minimise these. 331 Figure 7 A . Indicative schedule for synchronous planting in Zones II and III of District III, UPRIIS, Nueva Ecija. The irrigation service area of the 2 zones is approximately 20,000 ha. The arcs are separated by 2.5 km and 10 days of variation in planting date is envisaged within each segment, corresponding to bionomic parameters of BPH, as explained in the text. 332 Though it may take considerable time and effort to implement a schedule of optimal synchrony, the fact that for many pests the relationship between the standard deviation of planting date and density is exponential (5.6) suggests that efforts should be directed in the first instance to areas where asynchrony is greatest. Substantial returns may accure to the initial increments of co-ordination, which may be achieved at relatively little cost. Irrigation managers routinely stagger releases so as to minimise water requirements where the supply is limiting, and in order not to strain the system's delivery and drainage capacity. These decisions appear to be made for the most part without consideration of the impact on crop pests and diseases (e.g. Pasandaran 1978, Thavaraj 1978). There is a clear opportunity for those responsible for crop protection to introduce the concerns of pest suppression into these management decisions. Figures 7.5 and 7.6 illustrate proposed irrigation schedules drawn up for 2 Philippine river-diversion systems by Angeles (1973). Water shortage is a major concern in such systems without reservoirs, particularly in the dry season. The span of planting times allowed within each section is not specified for either area, but from the overall duration of the crop it would appear to be not longer than 1 - 2 weeks. A one week gap in planting is proposed between consecutively numbered sections in PENRIS and 4 weeks in SCRI5. While the schedules permit the maximum area to be cultivated given the dry season river flows, the geographical arrangement of sections increases the risk of generating damaging infestations. In Fig. 7.5, section* 1 abuts section 12, producing at least a 3 month range of planting dates across their boundary. In Fig. 7.6, the potential for pest build-up would be reduced were the ordering of sections made consistent from northeast to southwest : where section 1 abuts section k there is as much as a 4 month difference in planting dates, essentially obliterating the fallow between crops. Realigning the sections and redrawing their boundaries so as to reduce the short range variance of planting date would not compromise the original purpose of the scheduling. 333 Figure 7.5 Proposed irrigation schedule for the 18,000 ha Penaranda River Irrigation System (PENRIS), Nueva Ecija, redrawn from Angeles (1973). The scale was not given in the original but has been estimated from areal data provided. 334 Figure 7.6 Proposed irrigation schedule for the 3900 ha Santa Cruz River Irrigation System (SCRIS) in Laguna, redrawn from Angeles (1973). The scale was estimated from areal data in the original. 335 Large gaps in planting date across short distances are also encountered at the boundaries of irrigation systems, particularly where these are under separate management. Small communal systems on the coastal littoral of southern Negros Occidental in the Visayan islands draw their water from different streams draining from the interior uplands and follow independent schedules. Driving along the coastal road, one encounters rice in all stages of growth in close proximity. Where patterns of stream flow are similar, there is opportunity for co-ordinating cropping schedules. Such boundary areas, and, within systems, sites of chronic asynchrony may be important reservoirs of insect pests and diseases and should be a focus for those concerned with surveillance. 336 CHAPTER EIGHT Conclusions The concerns of this thesis have ranged from the theoretical through the experimental to the applied aspects of a pressing ecological problem - the causes of increased pest losses in the wake of agrarian change in South and Southeast Asia, and the prospects for using the insights gained for purposes of control. It is a problem in which technical and social strands are finely interwoven, in which man's actions are responsible for altering the resource base that sustains pest populations and in which the scale of ecological processes determines the utility and influences the form of possible responses. An understanding of the sociological and agronomic context of rice farming is essential if one is to isolate the interactions that determine the distribution and abundance of rice pests (c.f. Krebs 1978); all the more is it essential if one seeks to establish the scope for intervention that does not result in dislocation. When ecology turns to consider the problems of greatest human concern, its perspective inevitably must broaden beyond strictly natural processes. The results reported here have suggested that the increased time for population growth made available by multiple cropping has been central to the greater pest densities that have prevailed in many parts of Asia since the late 1960's. Carryover of populations between seasons has increased and, as hypothesized, shows no evidence of density dependence. Within seasons,* population growth for most species appears to be density dependent but under-compensating, and it is regulation during this phase of the crop cycle that is likely to have been responsible for the re-establishment of equilibria at higher levels (Chap. III). Populations have also been shown to respond positively to the asynchrony with which crops are planted : th e r a te of the response is proportional to the species' rate of increase and the distance over which it responds is commensurate with estimates of dispersal range. Imperfect density dependence is again apparent in the behaviour of several species (Chap. V). 337 Vital parameters of the yellow stemborer, a major pest throughout Asia, are shown to vary in localities of differing intensity and asynchrony, and it is likely that efforts to alter either would provoke a similar evolutionary response. The evidence however indicates that, despite the selection of individuals with greater reproductive output, population densities in areas of relative resource scarcity remain below those where, through double cropping and asynchrony, the potential for increase is greater (Chap. VI). These results suggest that efforts to further increase the number of crops grown per year or to promote asynchronous cultivation through, for example, the extension of the rice garden concept, will result in increased losses to insect pests. Conversely, though reduction in either the cropping index or asynchrony is likely to be effective in suppressing pest populations, the difficulties in deintensifying production are bound to be enormous, and the consequences possibly as far-reaching as those experienced in the initial shift to intensive cultivation (Chapter III). Synchronisation of planting appears to be inherently a less problematic strategy. Asynchrony is the product of a complex sequence of cultivation, affected by the availability of a variety of inputs and by decisions taken at many levels. There is evidence that farmers act in such a way as to minimise asynchrony, compensating for delay in the initial stages of cultivation by completing later ones faster. The regulation of irrigation and drainage are however not generally under farmers' control, and in Nueva Ecija these are found to be key in determining the magnitude of variation in planting data and its distribution across the landscape. Particularly in the dry* season, planting may be so synchronous at some of the sites best served by irrigation that by moving between fields pests are able to complete no more generations than if they remained within any one. In contrast, at the least synchronous sites, areas of serious and often chronic irrigation problems, planting extends over several months, making possible a number of additional generations (Chapter IV). I stress that the contribution of other factors to asynchrony may be greater elsewhere, particularly where a lower man-land ratio causes the availability of labour to become constraining. 338 Synchronisation of planting may potentially have a number of negative repercussions on farming communities. These effects however are likely to be dependent on the scale of what is attempted and the results of Chapter V suggest it is not necessary for planting to be completed "in as short a time as possible over as wide an area as possible". Additional benefits from synchronisation are expected and in Nueva Ecija it was judged that the net effects of moderate synchrony are likely positive. In a pilot area of 2500 ha it was proposed that planting be completed within a span that well favoured sites achieve in the normal course of events. Effort was concentrated on improving the delivery of inputs not under local control that appear to be responsible for most ecologically significant delay. Over the course of a year the proposal attracted the active support of farmers and fostered the spread of farmer groupings. Independent community organizers played a key role in catalysing this response. The main constraint to synchronisation was found to lie in the strained relations between farmers and agricultural support institutions, primarily the irrigation authority. The interests of landless labourers, who may be affected by such a scheme, were not effectively articulated. Though limited success after only one year is perhaps to be expected, the experience gained points to the need for a structure to ensure the representation of all likely to be affected by synchronous planting and for a recognised local sponsor. An approach to introducing synchronous planting over wider areas is proposed, referred to as "synchronisation by waves". Even where an optimal schedule may take some time to implement, important benefits in terms of pest suppression may be achieved at little cost by bringing synchrony considerations into the day-to-day operations of, in particular, irrigation authorities (Chapter VII). Many of the conclusions of the present study invite further research. Key areas where corroboration should be sought are the following: 339 1) The impact of intensification on pest population dynamics. Can it be confirmed elsewhere that, among all the elements of the Green Revolution, it is the effect of an increased period of rice availability that has been central? 2) The dispersal range of rice pests. Are these as short as indicated by the results of Chapter V? 3) The evolutionary response of pests to agrarian change. Can similar trends as demonstrated here be found in the life history parameters of other species? What is the impact of these shifts on the population dynamics of pests under changing host plant availability? How rapidly does such evolution proceed? 4) The social impact of synchronisation. Is the balance of benefits and costs, broadly considered, of an ecologically significant synchronisation of planting likely to be positive under other social conditions? How important is the "crop insurance" function of asynchrony in reducing the impact of natural disasters? What are the prospects of implementing synchronous planting under different political and economic regimes in Asia? In taking up pressing agricultural issues, ecologists encounter both important opportunities and significant challenges. Agriculture offers ecology a laboratory, possibly unexcelled, in which to develop tools to consider the dynamics of animals in relation to varying distributions in time and space of their resources. Perhaps nowhere else are the resources likely to be as well mapped and their genetic and physiological characteristics described. If conditions are simplified in agriculture relative to the complexities of more natural ecosystems, that has always been the hallmark of laboratories and their attraction. 340 The challenge ecologists meet arises from the demands of relevance. Particularly if they make use of increasingly scarce research funds, ecologists must accept the responsibility of providing answers that are of use. In entering the arena of application, scientists must be alive to the questions of how the results of research are to be employed and who is to share in the benefits. The answers determined by prevailing forces may well be ones they would not have wished. 341 Acknowledgements Financial support for this thesis was provided by an overseas scholarship from The Natural Sciences and Engineering Research Council of Canada, for which I am grateful. I am indebted to my supervisors at Imperial College; firstly, to Brian Trenbath for his sustained interest in my work and his support despite the thousands of miles that separated us during the time that most of the research was conducted. Since my return from the Philippines, Gordon Conway has made me welcome at the Centre for Environmental Technology and provided encouragement during the rite of passage that is writing-up. Both have made useful comments on the manuscript. Before and after my fieldwork, I have benefitted from the stimulation of many at Imperial College including Roy Anderson, Yoram Ayal, Mike Hassell, Jorge Soberon-Manero, Richard Markham, Geoff Norton, Jeff Waage and Mike Way. At the International Rice Research Institue, Jim Litsinger provided me more support in the way of material and staff than I could reasonably have expected, and with these, an added incentive to derive answers of practical use. The warmth of his and his family's welcome was an important encouragement. I have benefitted greatly from discussions with many in the Entomology Department including Pipes Carino, Arnie Dyck, Short Heinrichs, Osama Mochida, John Perfect, Lita Marciano-Romena, Lou Sunio, and Gerry Wilde. Nonnie Bunyi and Dodie Capili provided invaluable logistic support. From researchers in other areas, A1 Early, Bob Herdt, Karl Kaiser, Glo Soltes and Amanda Te, I have gained a fuller appreciation of the technical and economic aspects of rice farming. Through long discussions and later correspondence with Grace Goodell, my understanding of the social context of peasant agriculture has deepened considerably. 342 In Nucva Ecija, I learned about the ecology of collaboration with Peter Kenmore who caused me to re-examine many of my earlier ideas in both theoretical and applied areas. Jovy Bandong was unstinting in his support and from him I gained a fuller understanding of farming practices in Central Luzon. Both he and Abraham Alviola promoted the concept of synchronous planting and other aspects of IPM in 8 villages with intelligence and sensitivity. The majority of the data on which this thesis is based was collected by others and my debt to them is immense. At the Zaragoza Outreach Office, Lucy Paladan and Roselle Paragna pored over light trap catches for nearly 2 years. Together with Evelyn Almayda, they also conducted the survey interviews reported in Chapters 4 and 7.Information on the intensity of cultivation, necessitating in some instances pacing on a compass bearing through drainage channels, was collected by Maria Alvarez-Austria, Rudy Apostol and Lino Andrion. Rudy Gabriel, Eric Gabriel, Joey Padayao, George Romero, Romy Sernadilla and Lino Andrion drove several thousand kilometers by motorcyle servicing the light trap network and sampling insect damage in the infestation studies described in C hapter 5. Due to lack of space, I am not able to acknowledge individually the 46 farmers, their wives, sons and daughters who faithfully tended light traps. Among them however, I would like to mention Narciso and Lorenzo Baroga, Fely Beltran, Luis de la Cruz, Mr. and Mrs Miguel Mababa, Justino Ramiscal, Eulogio Romero and Mr. and Mrs Guillermo Tumale from whom, through repeated conversations, I learned much about the natural and social ecology of rice cultivation. Again, it is impossible to thank individually the personnel of the National Irrigation Administration who assisted the research by providing essential information on the timing of cultivation. Among them however, Celso Angeles, Cirilo Malang, Napoleon Prieto, Romy Quemada and Bonifacio Sotto provided many useful insights into the functioning of a large gravity irrigation system. In the banking sector, Josalino Alano, Jose Lustre and Manny Mendoza generously answered my questions regarding the provision of formal credit to farmers. 343 Finally, it was my pleasure to work with the staff of The Agency for Community Educational Services, whose energy and commitment to the self-reliant development of rural communities was inspiring. Among them, the contributions of Valerie Agbayani, Freddy Carpio, Romy Excasinas, Dinky Juliano, Charito Lindo, Roland Modina, Joe Perez, Mila Rupac, Felee Santos and Celia Santos were key. To all of the above I can only say, "maraming, maraming salamat", many thanks. 344 References Afifuddin, H.O., Some aspects of Labour Utilization in the Muda W.H. Soon, and Scheme. Muda Agricultrual Development Authority F. Kasryno 1974. monograph no. 26, Alor Setar, Malaysia. Angeles, H.L. 1973 Water scheduling for diversion irrigation systems. In : Water Management in Philippine Irrigation Systems: Research and Operations. IRRI, Los Banos, Phil. Bannerjee, S.N. and The Lepidopterous stalk barers of rice and their L.M. Pramanick 1967 life cycles in the tropics. In : The Major Insect Pests of the Rice Plant, Johns Hopkins Press, Baltim ore. Barker, R. and R.W. Rainfed lowland rice as a research priority - an Herd't 1978 economist's view. In: Rainfed Lowland Rice. IRRI, Los Banos, Philippines. Barducci, T.B. 1969 Ecological consequences of pesticides used for the control of cotton pests in Canete Valley, peru. In: M.T. Farvar and J.P. Milton (eds) The Careless Technology. Stacey, London. Bannerjee, S.N., Plant protection problems in respect of M. C. Diwaker, and high-yielding cereals in India. Sci. Cult. 39: N. C. Joshi 1973 164-8. Barrett, J.A. 1981 The evolutionary consequences of monculture. In: J.A. Bishop and L.M. Cook(eds) Genetic Consequences of Man Made Change. Academic Press, London. Beddington, J.R., Dynamic complexity in predator-prey models C.A. Free, and J.H. framed in difference equations. Nature 225: Lawton 1975. 58-60 Bellman, R. 1960 Introduction to Matrix Analysis. McGraw Hill, New York Berry, R.J. 1979 Genetical factors in animal population dynamics. In: R.M. Anderson et al. (eds) Population Dynamics. Blackwell, Oxford. Bernsten, R. 1981 Evaluating integrated pest management in developing countries: The Indonesian perspective. Paper presented at the 10th Session of the FAO/UNEP Panel of Experts on Integrated Pest Control. Rome. Binswanger, H.P. The nature and significance of risk in the N.S. Jodha and B.C. semi-arid tropics. In: Proceedings of Barah 1980. the International Workshop on Socio-economic constraints to Development of Semi-Arid Tropical Agriculture, Hyderabad, India 19-23 February 1979. ICRISAT, Patancheru. 345 Bowles, R.D. 1980 The political economy of colonial Tanganyika 1939-61. In: M.H.Y. Kanyiki (ed) Tanzania Under Colonial Rule. Longmans, London. Brader, L. 1979 Integrated pest control in the developing world. Ann. Rev. Entomology 24: 225-78. Bruce-Chwatt, L.J. Malaria debated. Nature 29 4: 302 1981 Butler, G.D., A model to describe the uniform build up of T.R. Pfrimmer, and populations of adult bullworms and cabbage J.W. Davis 1974 loopers. Environ. Entomol. 3: 978 - 80 Carino, F.O., Role of natural enemies in population V.A. Dyck and suppression and pest management of green rice P.E. Kenmore leafhoppers. Saturday Seminar, 22 May 1982, IRRI, Los Banos, Philippines. Catillo, M.B., Nematodes in cropping patterns. Ill Composition M.S. Alejar, and and populations of plant parasitic nematodes in J.A. Litsinger 1977 selected cropping patterns in Batangas. Phil. Agr. 60: 285 -292 Cendana, S.M. and Insect pests of rice in the Philippines. In: F.B. Calora 1967 The Major Insect Pests of the Rice Plant. Johns Hopkins Press, Baltimore Chambers, R. 1980 Basic concepts in the organization of irrigation. In: EW Coward (ed) Irrigation and Agricultural Development in Asia. Cornell Univ. Press, Ithaca. Chambers, R. and Perceptions, technology and the future. In: B.H. Farmer 1977 B.H. Farmer (ed) Green Revolution? Macmillan, London. Chapin, G. and Agricultural production and malaria resurgence R. Wasserstrom 1981 in Central America and India. Nature 293 : 181 - 5. Chelliah, S. and Factors affecting insecticide-induced resurgence E.A. Heinrichs 1980 of the brown plantopper, Nilaparvata lugens of rice. Environ. Entomol 9 : 773-7 Cheng, C.H. 1971 Effect of nitrogen application on the susceptibility of rice to brown planthopper attack. jf. Taiwan Agric. Research 20: 21-30. Claridge M.F., Variation within and between populations of the J. Den Hollander brown planthopper Nilaparvata lugens (stal). In: J.E. Morgan 1983 Knight W.J. et al. (eds). Proceedings of the 1st International Workshop on Biotaxonomy, Classification and Biology of Leafhoppers and Planthoppers (Auchenorrhyncha) of Economic Importance. Commonwealth Inst, of Entomology, London. 346 Coaker, T.H. 1959 Investigations on Heliothis armigera (Hb.) in Uganda. Bull. Ent. Res. 50: 487-506. Cole, L.C. 1954 The population consequences of life history phenomena. Q. Rev. Biol. 29 : 103-137 Colombo, V. Reducing malnutrition in developing countries: D.G. Johnson, increasing rice production in South and T. Shishido 1978 Southeast Asia. The Trilateral Commission. Comins, H.N. 1977 The management of pesticide resistance. Journal of Theoretical Biology 65: 399-420 Conway, G.R. 1981 Man versus pests. In: R.M. May (ed.) Theoretical Ecology. 2nd edition. Blackwell, Oxford. Conway, G.R. , Developing marginal lands in the tropics. I. Manwan, D.5. Nature 304: 392 McCauley 1983 Cordova, V., Changes in rice production technology and their R.W. Herdt, impact on rice farm earnings in Central Luzon, F.B. Gascon and Philippines, 1966 to 1979. Agricultural L. Yambao 1981 Economics paper no. 81-19, IRRI, Los Banos Phil. De Datta S.K. 1981 Principles and Practices of Rice Production. Wiley and Sons, New York. Draper N.R. and Applied Regression Analysis. Wiley, New York. H. Smith 1966 Dyck, V.A., Suppression of planthopper and leaffolder N.Q. Kamal, P.E. populations by natural enemies, expecially Kenmore, A.C. Dulay predators. Saturday Seminar, 25 April 1981 and F.V. Palis 1981 IRRI, Los Banos, Philippines. Dyck, V.A., Ecology of the brown planthopper in the tropics B. C. Misra, S. Alam,In: The Brown Planthopper: Threat to Rice C . N. Chen, C.Y. Production in Asia. IRRI, Los Banos, Hsieh, and R.S. Philippines. Rejesus 1979 Early, A.C. 1980 An approach to solving irrigation system management problems. In : Report of A Planning Workshop on Irrigation Water Management, IRRI, Los Banos, Philippines. Elmaghraby, S.E. Activity Networks: Project Planning and Control 1977 by Network Models. Wiley, New York. Food <3c Agriculture Report of the 11th Session of the FAO/UNEP, Organization Panel of Experts on Integrated Pest Control (F.A.O.) Kuala Lumpur March 1982. F.A.O., Rome. Farmer, B.H. (ed) Green Revolution? MacMillan, London 1977 347 Fegan, B. 1982 The social history a Central Luzon barrio. In : A.W. McCoy and E.C. de Jesus (eds) Philippine Social History: Global Trade and Local Transformations. Ateneo de Manila Univ. Press and Allen and Unwin, Sydney. Fernando, H.E. 1975 The brown planthopper problem in Sri Lanka. Rice Entomol. Newsl. 2: 34-6 Fernando, H.E. 1979 Control of insect pests in Sri Lanka. In: Reddy (op. cit.). Flint, M.L. and R. Introduction to Integrated Pest Management. van den Bosch 1981 Plenum, New York. Geertz, C. 1963 Agricultural Involution: The process of ecological change in Indonesia. Univ. of California Press, Berkeley. Ghaurie, M.S.K. 1971 Revision of the genus Nephotettix Matsumara (Homoptera: Cicadell idae) based on type material. Bull. Ent. Res. 60: 481-512 Gleave, M.P. and Population density and agricultural systems H.P. White 1969 in West Africa. In: MF Thomas and GW Whittington (eds) Environment and Land Use in Africa. Methuen, London. Gleeck, L.E. 1981 Nueva Ecija in American Times, Historical Conservation Society, Manila. Goodell, G. 1983 After land reform. Policy Review. Goodell, G. in press Bugs, bunds, banks and bottlenecks: organisational contradications in the new rice technology. Economic Development and Cultural Change. G reathead, D. 1982 The need for biological control input to the FAO Integrated Pest Control in Rice Programme in South and Southeast Asia. FAO working paper AGP : IPC/ll/WP/7, Rome. Greig-Smith, P. 1964 Quantitative Plant Ecology. 2nd edition. Butterworth, London. Griffin, K. 1979 The Political Economy of Agrarian Change. 2nd edition. MacMillan, London. Grime, J.P. 1977 Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory. Am. Nat. Ill: 1169-94 Grist, D.H. and Pests of Rice. Longmans, London. R.J.A. Lever 1969 Guino, R.A. 1978 Baseline survey for the project : Improved Organisation for Small Farmers for Intensified Rice Production. Paper presented at the Thursday Seminar 2nd November, 1978, IRRI, Los Banos, Phil. 348 Gutierrez, A.P., The within-field dynamics of the cereal leaf W.H. Denton, beetle Oulema mebropus (L.) in wheat and oats. R. Shade, H. Maitby J. Anim. Ecoi. 43: 627 640 T. Burger, and G. Moorhead 1974 Hambleton, E.J. Heliothis virescens as a pest of cotton, with notes on host plants in Peru. J. Econ Entomol. 37:660-66. Hargrove, T.R., Genetic interrelationships of improved rice W.R. Coffman, and varieties in Asia. IRRI Research Paper series V.L. Cabanilla 1979 no. 23. Hartstack, A.W. Jr. A population dynamics study of the bollworm and J.P. Hollingsworth, the tobacco budworm with light traps. Environ. R.L. Ridgway and Entomol. 2 : 244-52 J.R. Coppedge 1973 Hassell, MP 1978 The Dynamics of Arthropod Predator - Prey Systems. Princeton Univ. Press, Princeton. Heinrichs, E.A., Development and implementation of insect pest R. C. Saxena, management sytems for rice in tropical Asia. S. Chelliah. 1978 In : Sensible Use of Pesticides, Food and Fertilizer Technology Centre, Tokyo. Herdt, R.W. 1982 Focusing field research on future constraints to rice production. IRRI Research paper series, no. 76 Los Bunos, Phil. Herdt, R.W. and The economics of insect control on rice in the S.K. Jayasuriya Philippines. Agric. Economics Dep't paper no. 1981 81-10. IRRI, Los Bunos, Phil. Herdt, R.W. and Adoption Spread and Productivity Impact of C. Capule 1983 Modern Rice Varieties in Asia. IRRI, Los Banos, Philippines. Hibino, H. 1979 Rice ragged stunt, a new virus disease occuring in tropical Asia. Rev. of Plant Protection Research 12: 98-110. Hidaka, T. 1974 Recent studies on the rice gall midge Orseolia oryzae (Wood-Mason), (Cecidomydae, Diptera). Review of Plant Protection Research 7 : 99-143. Hidaka, T. and Ecology and control of the rice gall midge, with V. Yaklai 1980 with special emphasis on measures to prevent its spread in Thailand. Paper presented at the 12th meeting of the FAO Plant Protection Committee, Chiengmai, Thailand. Hill, J.F.R. and Tanganyika. A Review of its Resources and J.P. Moffett 1955 Their Development. Government of Tanganyika, Dar es Salaam. Hirano C. and Changes in Japanese rice cultivation. In: K. Kiritani 1976 Proceeding of The International Congress on the Human Environment, Kyoto Japan. 349 Ho, D.T. and Stemborers in various rice ecosystems in Kenya. 3.G. Kibuka 1983 Int. Rice Res. Newsl. 8 (5) : 18. Hokyo, N and E. Life table studies on the paddy field population Kuno 1977 of the green rice leafhopper Nephotettix cincticeps Uhler (Hemiptera : cicadellidae) with special reference to the machanism of population regulation. Res. Popul. Ecol. 19 : 107-124. Horn, H.S. 1978 Optimal tactics of reproduction and life history. In 3.R. Krebs and N.B. Davies (eds) Behavioural Ecology : an Evolutionary Approach. Blackwell, Oxford. International Rice Annual Report for 1965. IRRI, Los Banos, Research Institute Philippines. (IRRI) 1965 International Rice The Major Insect Pests of the Rice Plant. Research Institute Proceedings of a symposium held at IRRI, Los (IRRI) 1967 Banos, September 1964. 3ohns Hopkins Press, Baltim ore International Rice Changes in Farming Practices in Rice Research Institute Farming in Selected Areas of Asia. IRRI, Los (IRRI) 1975 Banos, Phil. International Rice Cropping Systems Programme. In : Annual Research Institute Report for 1978. IRRI, Los Banos, Philippines (IRRI) 1979 International Rice An inexpensive kerosene light trap to monitor Research Institute rice insects. Rice Res. Newsl. 4 (2) : 17. (IRRI) 1979a International Rice Insects. In : Annual Report for 1980. Research Institute International Rice Research Institute, Los (IRRI) 1981 Banos, Philippines. International Rice Annual Report for 1981. International Rice Research Institute Research Institute, Los Banos, Philippines. (IRRI) 1982 International Rice A decade of co-operation and collaboration Research Institute between Sukamandi (AARD) and IRRI 1972-1982. (IRRI) 1984 . International Rice Research Institute, Los Banos, Phillipines. Jayasuriya S.K., Mechnisation and cropping intensification : A. Te, R.W. Herdt economic viability of poer tillers in the 1982 Phillipines. IRRI Saturday Seminar, 9 Oct. 1982, IRRI, Los Banos, Philippines. Jeyaratnam, 3., Survey of pesticide poisonings in Sri Lanka. R.5. d'A Seneviratne Bulletin of the World Health Organisation 60: and 3.F. Copplestone 615-619. 3ohnson C.G. 1969 Migration and Dispersal of Insects by Flight. Methuen, London. 350 Johnston, J.S. and Dispersal of Drosophila : The Effect of baiting W.B. Heed 1975 on the behaviour and distribution of natural populations. Am. Natur. 109 : 207-216. Johnston, J.S. and Dispersal of desert-adapted Drosophila : The W.B. Heed 1976 Saguaro-breeding D. nigrospicracula. Am. Natur. 110 : 629-651. Jones, O.T. and Oriented responses of carrot fly larvae T.H. Coaker 1977 Psila rosae, to plant odours, carbon dioxide and root volatiles. Physiol. Entom. 2 : 189-97. Jorgensen, J.J. 1981 Uganda : A Modern History. Croom Helm, London. Kalode M.B. 1974 Recent Changes in relative pest status of rice insects as influenced by cultural, ecological and genetic factors. Paper presented at the International Rice Research Conference, 22-25 April 1974, IRRI, Los Banos, Philippines. Kalode M.B. 1983 Leafhopper and planthopper pests of rice in India. In W.J. Knight et al. (eds) Proceedings of the First International Workshop on Leaf hoppers and Planthoppers of Economic Importance. Commonwealth Institute of Entomology, London. Kamal, N.Q. 1981 Suppression of Whitebacked Planthopper Sogatella furcifera (Horvath) and Rice Leaffolder Cnaphalocrocis medinalis (Guenee) Populations by Natural Enemies. PhD thesis, Araneta University, Manila. Kenmore P.E. 1979 Limits of the brown planthopper problem: implications for integrated pest management. Saturday seminar. IRRI, Los Bunos. Kenmore P.E. 1980 Ecology and Outbreaks of the Brown Planthopper Nilaparvata lugens Stal, an Insect Pest of the Green Revolution. PhD Thesis, University of California, Berkeley. Kenmore, P.E. 1980a Recent widespread outbreaks of brown plant hopper Nilaparveta lugens (Stal) (Horn., Delphacidne) in the tropics : critical comparison of four explanations. Proceedings of the XVI International Congress of Entomology, Kyoto. Kennedy, J.S., Host finding by aphids in the field III. Visual C.O. Booth, and attraction. Ann. Appl. Biol. 49 : 1-21. W.J.S. Kershaw 1961 Kerkvliet, B.J. 1979 The Huk Rebellion : a Study of Peasant Revolt in the Philippines. Univ. of California Press, Berkely. 351 Khan M.Q. 1967 Control of paddy stemborers by cultural practices In : IRRI (1967). Kikuchi M., F.B. Changes in technology and institutions for rice Gascon, L.M. Bambo, farming in Laguna, Philippines, 1966-1981 : a and R.W. Herdt 1982. summary of five Laguna surveys. Agricultural Economics department paper 82-22, IRRI, Los Banos, Philippines. Kin, Ho Nai 1981 An Overview of Water Management Requirements and Agricultural Practices for Rice Cultivation in the Muda Area - with Special Reference to Muda II Irrigation Project. Muda Agricultural Development Authority, Alor Setar, Malaysia. Kiritani, K. 1979 Pest Management in Rice. Ann. Rev. Entomology 24 : 279-312. Kiritani, K. 1979 The biology and life cycle of Chilo Suppressalis and 5. Iwao 1967 (Walker) and Tryporyza (Schoenobius) incertulas (Walker) in temperate-climate areas. In : IRRI (1967). Kisimoto R. 1976 Synoptic weather conditions inducing long distance migration of planthoppers sogatella furcifera Horvath and Nilaparvata lugens Stal. Ecol. Entomol. 1 : 95- 109. Kiyosawa S. 1972 Theoretical comparison between mixture and rotation cultivations of disease-resistant varieties. Ann. Phytopath. Soc. Japan 38 : 5 2 -5 9 . Kiyosawa S. 1976 A comparison by simulation of disease dispersal in pure and mixed stands of susceptible and resistant plants. Japan J. Breeding 26 : 137-45. Kiyosawa S. and A theoreticl evaluation of the effect of mixing M. Shiyomi 1972 resistant variety with susceptible variety for controlling plant diseases. Ann. Phytopath. Soc. Japan 38 : 41-51. Kiyosawa S. and Simulation of the process of breakdown of M. Shiyomi 1976 disease-resistant varieties. Japan J. Breeding 26(4) : 339-352. Korten F.F. 1981 Building national capacity to develop water users' associations : experience from the Philippines. Paper presented at the World Bank Sociological Workshop, July 27, 1981, Washington. Krebs, C.J. 1978 Ecology : The Experimental Analysis of Distribution and Abundance. 2nd Edition. Harper and Row, New York. 352 Kuno, E. and Comparative analysis of the population dynamics N. Hokyo 1970 of rice leafhoppers Nephotettix cincticeps Uhler and Nilapurpata lugens with special reference to natural regulation of their numbers. Res. Pop. Ecol. 12, 154-84. Lagemann, J. Land use, soil fertility and agricultural J.C. Flinn, and productivity as influenced by population density H. Ruthenberg 1976 in Eastern Nigeria, mimeo. International Institute of Tropical Agriculture, Ibaden, Nigeria. Ledesma, A.J. 1978 Rice farmers and landless rural workers : perspectives from the household level. Agricultural Economics Dep't paper no. 78-19, IRRI, Los Banos, Phil. Lehninger, A. 1975 Biochemistry : The Molecular Basis of Cell Structure and Function. 2nd ed'n. Worth, New York Levin S.A. 1974 Dispersion and population interactions Am. Nat. 108 : 207-228. Levins, R. 1966 Strategy of model building in population biology. Am. Sci. 54 : 421-31. Lewontin, R.C. and On population growth in a randomly varying D. Cohen 1969 environment. Proc. Natl. Acad. Sci. U.S. 62 : 1056-1060. Lim, G.S. 1972 Studies on penyakit merah disease of rice III. Factors contributing to an epidemic in North Krian, Malaysia. Malaysia Agr. Journal 48: 278-285. Lim, G.S. and Habitat modification for regulating pest K.L. Heong 1977 populations of rice in Malaysia. Malaysia Agricultural Research and Development Institute report no. 50. Ling, K.C. 1972 Rice Virus Diseases. International Rice Research Institute, Los Banos, Philippines. Ling, K.C. 1975 Experimental epidemiology of rice tungro virus disease. Part I - Effect of some factors of vector (N. virescens) on disease incidence. Phil. Phytophathology 11 : 11-20. Ling, K.C. 1976 Recent studies on the rice tungro disease at IRRI. IRRI Research Paper series no. 1. Ling, K.C. 1977 Transmission of rice grassy stunt by the brown planthopper. In : The Rice Brown Planthopper. Food and Fertilizer Technology Centre for the Asian and Pacific Region, Taipei. Lipton, M. 1978 Inter-farm, inter-regional and farm- non-farm income distribution : the impact of the new cereal varieties. World Development 6 (3) : 319-37. 353 Litsinger, J.A. Pest control in cropping systems - research approach and methodology. Committee report to the Cropping Systems Working Group meeting, Dacca, Bangladesh, 15-18 Feb. 1977. Litsinger, J.A., Cultural control of rice insect pests. Lectures P.E. Kenmore, presented at the Regional Training Seminar on and R.C. Saxena 1978 Integrated Pest Control for Irrigated Rice in South and Southeast Asia, 16 Oct - 18 Nov 1978. IRRI, Los Banos, Philippines. J.A. Litsinger, A methodology of determining insect control M.D. Lumaban, recommendations. IRRI Research Paper series J.F. Bandong et al. no. 46. IRRI, Los Banos, Philippines. 1980. J.A. Litsinger, Rice insect pest management technology transfer G.E. Goodall, P.E. to small-scale farmers : a case study involving Kenmore, C.G. de la surveillance by Filipino farmers. Paper Cruz et al. 1981 presented at the FAO Rice Pest Surveillance Workshop, 7-11 Sept. 1981, Kuala Lumpur, Malaysia. Loevinsohn, M. A basis for predicting rice insect pest J.A. Litsinger, abundance in cropping systems. Paper N. Panda et al. presented at the International Rice Research 1982 Conference, 19-23 April 1982, IRRI, Los Banos, Philippines. Loevinsohn, M. Interest rates in Nueva Ecija. Mineographed J. Bandong, memorandum, IRRI, Los Banos, Philippines. A. Alviola 1982a. Loevinsohn, M. 1983 Report on a visit to the proposed synchronous planting project in South Cotabato, Philippines, mimeo, IRRI (Entomology Dep't), Los Banos. Loevinsohn, M. Evidence of increased death rates in rural Nueva In prep. Ecija, Philippines : a link with pesticide use? MacArthur R.H. and The Theory of Island Biogeography. Princeton E.O. Wilson 1967 University Press, Princeton. MacQuillan, M.J. Influence of crop husbandry on rice 1974 planthoppers (Hemiptera' : Delphacidae) in the Solomon Islands. Agro. Ecosystems 14 : 339-58. Mandel J. 1964 The Statistical Analysis of Experimental Data. J. Wiley and Sons, New York. M arciano V. P. Insect management practices of rice farmers in A.M. Mandac, J.C. Laguna, Philippines. Agricultural Economics Flinn 1981 Department paper no. 81-03, International Rice Research Institute, Los Banos, Philippines. May, R.M. 1981 Models for single populations. In : R.M. May (ed.) Theoretical Ecology. 2nd Edition. Blackwell, Oxford. 354 McLennan M. 1980 The Central Luzon Plain. Alemar-Phoenix, Quezon City, Phil. Ministry of Five Years of Agricultural Research and Agriculture Development. Agency for Agricultural Research (Indonesia) 1980 and Development. Jakarta. Ministry of Inter-agency action program on integrated pest Agriculture management in South Cotabato Province, mimeo. (Philippines) 1982. Ministry of Agriculture, Region XI, Davao, Phil. Misra B.C. 1980 The Leaf and Planthoppers of Rice. Central Rice Research Institute, Cuttack, India. Mochida, O. and Outbreaks of planthoppers (and grassy stunt) in T. Suryana 1975 Indonesia during the wet season 1974-5. Paper presented at the International Rice Research Conference, April 1975, Los Banos, Phil. Mochida, O., Recent outbreaks of the brown planthopper T. Suryana and in Southeast Asia (with special reference to A. Wahyu. 1977 Indonesia). In : The Rice Brown Planthopper. Food and Fertilizer Technology Centre for Asia and the Pacific Region, Taipei. Modina, R.B. n.d. The Marawa credit group : a study in organisational development. mimeo. Agency for Community Educational Services, Manila. Morris R.F. 1963 The development of predictive equations for the spruce budworm based on key-factor analysis. In : R.F. Morris (ed.) The Dynamics of Epidemic Spruce Budworm Populations. Mem. Ent. Soc. Can. 31. Morooka Y. An analysis of the labour intensive continuous R.W. Herdt, rice production system at IRRI. IRRI Research and L.D. Haws 1979 Paper Series 29. Moulton, T.P. 1973 The effects of various insecticides, especially Thiodan and BHC, on fish in the paddy fields of West Malaysia. Malays. Agr. of 49 : 224-53. Murdie G. and M.P. Food distribution, searching success and Hassell 1973 predator-prey models. In" : Bartlett M.S. and R.W. Hiorns (eds) The Mathematical Theory of The Dynamics of Biological Populations. Academic Press, London. Nakasuji F. 1974- Epidemiological study on rice dwarf virus transmitted by the green rice leafhopper Nephotettix cinceteps Jap. Agr. Res. Q. 8 : 84-91. National Academy of Insect Control in the People's Republic of Sciences (NAS) 1977 China - A Trip Report of the American Insect Control Delegation. NAS, Washington. 355 National Irrigation Irrigation Evaluation Report for 1979. UPRIIS, Administration (NIA) Cabanatuan City, Philippines. 1980 Nickum, 3.E. 1982 Irrigation management in China. Staff Working Paper no. 543, the World Bank, Washington. Norton G.A. and Decision making in pest control. Adv. Appl. 3.D. Mumford 1983 Biol. 8 : 87-119. Numerical Algorithms Eigenvalues and Eigenvectors. Volume 4 Group (NAG) 1981 (Mark 8). Oxford. Oka, I.N. 1979 Feeding populations of people versus populations of insects : the example of Indonesia and the rice brown planthopper. Proc. IX Int. Congress of Plant Protection, 5-11 August 1979, Washington. Oka, I.N. 1979a Cultural Control of the brown planthopper. In : Brown Planthopper : Threat to Rice Production in Asia. IRRI, Los Banos, Philippines. Oka, I.N. n.d. Decreasing diversity : genetic uniformity and the case of the brown planthopper mimeo., Ministry of Agriculture, Indonesia. Oka, I.N. and Integrated pest control programmes on some Soenardi 1979 important rice pests in Indonesia. In : Reddy (1978) Organisation for Report of the steering group on pest control Econmic Co-operation under the conditions of small farmer food and Development production in developing countries. OECD, Paris (OECD) 1977 Ou, S.H. 1972 Rice Diseases. Commonwealth Mycological Institute, Kew. Packham, 3.R. and Ecology of Woodland Processes. Edward Arnold, D.3.L. Harding 1982 • London. Palmer, I. 1976 The New Rice in Asia : Conclusions from four Country Studies. UNRISD no 76.6, Geneva. Pantua, P. 1979 Population fluctuations of insect pests and their natural enemies in a continuous weekly-planted rice garden. Paper presented at the 10th national conference of the Pest Control Council of the Philippines. May 2-5, 1979, Manila. Pantua, P. and Life History and plant host range of the rice J.A. Litsinger 1984 green semi-looper. Int. Rice. Research Newsl. 9 : 26 356 Pasandaran, E. 1978 Water management decision-making in the Pekalen Sempean Irrigation Project, East Java, Indonesia. In : Irrigation Policy and Management in Southeast Asia. IRRI, Los Banos, Phil. Pathak, M.D. and Breeding approaches in rice. In : F.G. Maxwell R.C. Saxena 1980 and P.R. Jennings (eds) Breeding Plants Resistant to Insects, Wiley, New York. Pathak, M.D. and Trends and strategies for rice insect problems G.S. Dhaliwal 1981 in tropical Asia. IRRI Research Paper Series no. 64. Pearson, E.O. 1958 The Insect Pests of Cotton in Africa. Commonwealth Institute of Entomology, London. Perkins, J.H. 1982 Insects Experts and the Insecticide Crisis. Plenum Press. New York. Philippine Tropical Cyclone Summaries from 1948 to 1978. Atmospheric PAGASA. Quozon City. Geophysical and Astronomical Services Administration (PAGASA) 1978 Pianka, E.R. 1970 On r and K selection. Am. Nat. 104 : 592-7. Pielou, E.C. 1977 Mathematical Ecology. Wiley, New York. Prabhu, N.U. 1965 Stochastic Processes. MacMillan, New York. Reddy, D.B. (ed) Integrated Pest Control in Rice : papers 1979 presented at the technical consultation on the inter-country programme for integrated pest control in rice in South and Southeast Asia, March 20-24 1978. F.A.O. Bangkok. Roff, D.A. 1974 Spatial heterogeneity and the persistence of populations. Oecologia (Berlin) 15 : 245-258. Rothschild, G.H.L. The biology and ecology of rice stemborers in 1971 Sarawak (Malaysian Borneo). J. Appl. Ecol. 8 : 287-322. Roughgarden J. 1972 The evolution of niche widths. Am. Nat. 106 : 683-718. R uthenberg, H. 1976 Farming Systems in the Tropics. 2nd Edition. Clarenden Press, Oxford. Searle S.R. 1966 Matrix Algebra for the Biological Sciences. Wiley, New York. Seneta, E. 1973 Non-Negative Matrices. Wiley, New York. Sharma, V.P. and Return of malaria. Nature 298 : 210. K.N. Mehrotra 1982 357 Silva W.P.T. 1977 Chena-paddy interrelationships. In : BH Farmer (ed) Green Revolution? MacMillan, London. Singh, S. 1968. Plant protection problems of cropping patterns. In : Proc. of a Symposium on Cropping Patterns in India, Indian Council of Agronomic Research, New Delhi. Singh, S.R. and Effect of phosphomidon ULV aerial application on Y. Sutyoso 1973 rice over a large area in 3ava. 3. Econ. Ent. 66 : 1107-9. Siwi, S.S., and Species composition and distribution of green M. Roechan, 1983 leafhoppers Nephotettix spp. and the spread of the rice tungro disease in Indonesia. In : W.3. Knight et. al. (eds) Proceedings of the 1st International Workshop on Biotaxononomy, Classification and Biology of leafhoppers and Planthoppers of Economic Importance, London 4-7 October 1982. Commonwealth Institute of Entomology, London. Sokal, R.R. and F.3. Biometry. Freeman, San Francisco. Rohlf 1969 Sogawa K. 1979 The brown planthopper in India and Sri Lanka. Nekken Shiryo, 43, Tropical Agricultural Research Centre, Yatabe, 3apan. Sogawa K. 1983 On the brown planthopper problems in North Sumatra, mimeo. Ministry of Agriculture, 3akarta. Southwood, T.R.E. The interpretation of population change. 3. 1967 Anim. Ecol. 36 : 519 : 529. Southwood, T.R.E. Ecological Methods. 2nd edition. Chapman and 1978 Hall, London Southwood, T.R.E. Bionomic strategies and population parameters. 1981 In : R.M. May (ed) Theoretical Ecology. Blackwell, Oxford Southwood, T.R.E. Ecological background to pest management. In : and M.3. Way 1970 R. L. Rabb and F.E. Guthrie (eds) Concepts of Pest Management, North Carolina State University. Stansby I. and S. The development of the Diezmo Irrigation System. Stisen 1973 In : Water Management in Philippine Irrigation Systems : Research and Operations. IRRI, Los Banos, Phil. Stearns S. 1977 The evolution of life history traits : a critique of theory and a review of the data. Ann. Rev. Ecol. Syst. 8 : 145-71. Stebbins G.L. 1980 Variation and Evolution in Plants. Columbia Univ. Press, New York. 358 Stigler, G.J. 1966 The Theory of Price. 3rd edition. MacMillan, New York. Stinner, R.E. 1979 Biological monitoring essentials in studying wide-area moth movement. In : R.L. Rabb et al. (eds.) Movement of Highly Mobile Insects : Concepts and Methodology in Research, North Carolina State University. Stubbs, M. 1977 Density dependence in the life cycles of animals and its importance in K- and r- strategies. J. Anim. Ecol. 46 : 677-688 Tagarino, R.N. and The pricing of irrigation water : a case study R.D. Torres 1978 of the Philippines' Upper Panpanga River Project. In : Irrigation Policy and Management in Southeast Asia, IRRI, Los Banos, Phil. Tanfranco D. 1977 Operations Policy for the Upper Pampanga River Project Reservoir System in the Philippines. PhD dissertation, Univ. of Arizona. Tapay, N.E., M.L. Socioeconomic issues in irrigation management. Sipin, and A.C. Saturday Seminar May 3, 1980. IRRI, Los Bunos Early 1980 Philippines. Taylor, D.C. 1978 Financing irrigation services in the Pekalen Sampean Irrigation Project, East Java, Indonesia. In : Irrigation Policy and Management in Southeast Asia. IRRI, Los Banos, Philippines. Taylor, D.C. 1980 Economic analysis to support irrigation investment and management decisions. In : Irrigation Water Management. IRRI, Los Banos, Philippines. Taylor, L.R. 1979 The Rothamsted insect survey - an approach to the theory and practice of synoptic pest forecasting in agriculture. In : R.L. Rabb and G.G. Kennedy (eds). Movement of Highly Mobile Insects : concepts and Methodology in Research, North Carolina State jJniversity, Raleigh. Taylor, R.A.J. The relationship between density and the 1978 distance of dispersing insects. Ecol. Entomol. 3 : 63-70. Thavaraj S.H. 1978 The importance of integrating non-engineering aspects in irrigation system design. In : Irrigation Policy and Management in Southeast Asia. IRRI, Los Banos, Phil. Tothill J. D. 1958 Some reflections on causes of insect outbreaks. Proc. 10th Intern. Congr. Entom. (Montreal, 1956) 4 : 525-31. Usher, M.B. and A deterministic matrix model for handling the M.H. Williamson birth, death and migration processes of 1970 spatially distributed populations. Biometrics 26 : 1 - 12 . 359 United States Foreign agricultural circular : grains. FG - 20 Department of - 79. December 1979 A griculture (USDA) 1979 Uwagawa, M. and Report on the drainage improvement, UPRIIS. I.V. Punzalan 1978 mimeographed report, National Irrigation Administration, Quezon City. Van der Plank J.E. Principles of Plant Infection. Academic Press, 1975 New York. Van der Schalie H. World Health Organisation Project Egypt 10 : 1969 a case study of a schistosomiasis control project. In : M.T. Farvar and J.P. Milton (eds) The Careless Technology. Natural History Press, New York. Varley, G.C., G.R. Insect Population Ecology: an Analytical Gradwell, and approach. Blackwell, Oxford. M.P. Hassell 1973 Velusamy, R. and Bionomics of the rice leaf roller : T.R. Subramaniam Cnaphaiocrocis medinalis Guen. (Pyraidiae : 19 74 Lepidoptera). Indian J. Ent. 36 : 185-9. Waggoner, P.E. 1962 Weather, space, time and the chance of infection. Phytopathology 52 : 1100-1108. Wickham T.H. and Practices and accountabiligy for better water A. Valera 1978 management in Southeast Asia, IRRI, Los Banos, Phil. Wolda, H. 1977 Fluctuations in abundance of some homoptera in a neotropical forest. Geo-Eco-Trop. 3:229 - 257. Wold a, H. 1978 Fluctuations in abundance of tropical insects. Amer. Natur. 112 : 1017 - 1045. Wright, S. 1978 Evolution and the Genetics of Populations. Vol. 4 : Variability within and among Natural Populations. Univ. of Chicago Press, Chicago. Yabuno, T. 1961 Oryza sativa and Echinochloa crus-galli var. oryzicola. Seiken Ziho l£ : 29-34. Yoshida, S. 1978 Fundamentals of Rice Crop Science. IRRI, Los Banos, Philippines. Zadoks J.C. and The role of crop populations and their P. Kempmeijer 1977 deployment illustrated by means of a simulator EPIMUL 76. Annals N.Y. Acad. Sci. 287 : 164-190. Zandstra, H.G., A Methodology for On-farm Cropping Systems E.C. Price, Research. IRRI, Los Banos, Philippines. J.A. Litsinger, R.A. Morris, 1981 360