DECISION-MAKING IN THE CONTROL OF VEGETABLE
PESTS IN THE THAMES VALLEY
BY
MD. OUSOH BIN MAMAT
Dip. Agric. (Malaysia), B.Sc. (L.S.U), M.Sc. (P.S.U)
Thesis submitted for the degree of
Doctor of Philosophy of the University of London and
for the Diploma of Imperial College
Department of Pure and Applied Biology,
Imperial College,
Silwood Park,
Ascot,
Berkshire OCTOBER 1984- -ii -
ABSTRACT
This thesis investigates the problem of pest manage ment on vegetable crops in the Thames Valley area. Various philosophical approaches to problem solving research are discussed, revealing the underlying principles of each approach. A decision-analysis approach is employed in this investigation, starting with a descriptive analysis of the vegetable pest management problem, using information gathered from the literature, personal discussion , farm record exam ination, and intensive studies on a few selected farms.
To obtain further information on current pest management problems in the Thames Valley, an interview survey of 20 vegetable growers in the area was conducted. One outcome of this survey was to reveal the importance of aphid pests in the area, especially on autumn cabbages. To investigate this problem in more detail, an intensive study was con ducted in growers' fields to examine the factors thought to be responsible for the aphid problem in the area. This included monitoring aphids, their parasites and predators; assessing the percentage kill of aphids; assessing spray droplet coverage on plants; and calibrating growers' sprayers.
Finally, the findings of these investigations are used to suggest recommendations for future aphid control in autumn cabbages and further complementory research to this thesis. i ii
TABLE OF CONTENTS
Page
ABSTRACT ...... ii
PREFACE ...... ix
SECTION Is DECISION-ANALYSIS APPROACH TO VEGETABLE
PEST MANAGEMENT PROBLEMS ...... 1
CHAPTER Is AN INTERPRETIVE REVIEW OF VARIOUS
APPROACHES TO AGRICULTURAL RESEARCH ... 2
1.1. Introduction ...... 2
1.2. The IRRI On-Farm Cropping System Approach 6
1.3. The CIMMYT On-Farm Farming System Research
Approach 9
1.4. The Decision-Analysis Approach to Pest
Management Research 14
1.5. Conclusion ...... 19
CHAPTER 2: VEGETABLE PRODUCTION IN UNITED KINGDOM
AND THAMES VALLEY ...... 21
2.1. Introduction ...... 21
2.2. Vegetable Production in the United Kingdom 22
2.3. Changes in the United Kingdom Vegetable
Sector ...... 22
2.4. Vegetable Production in the Thames Valley
Area ...... 27
CHAPTER 3: DESCRIPTIVE ANALYSIS OF THE VEGETABLE
PEST SYSTEM ...... 30
3.1. Introduction ...... 30
3.2. The Structure of the System ...... 30
3.2.1. The components of the system ...... 30
3.2.2. Cropping schedule/cropping system ... 33 iv
3.3. The Dynamics of the Vegetable Pest System 35
3.3.1. Time profile for some of the major pest
components ...... 35
3.3.2. The effects of pests on vegetables ... 38
3.3.3. The effects of parasites and predators on
vegetable pests ...... 40
3.3.4. The effects of pesticides on components
of the vegetable pests ...... 40
3.4. Management and the Vegetable Pests System 43
3.4.1. The objectives and constraints of a
vegetable grower ...... 44
3.4.2. Grower’s perceptions of vegetable pests 45
3.4.3. Control measures for vegetable pests ... 46
3.5. Conclusion ...... 47
SECTION II: INTERVIEW SURVEY OF THAMES VALLEY
GROWERS ...... 50
CHAPTER 4: INTERVIEW SURVEY OF THAMES VALLEY
GROWERS: BACKGROUND INFORMATION ... 51
4.1. Introduction ...... 51
4.2. Aims and Operation of the Survey ...... 51
4.3. Results and Discussion of the Survey ... 55
4.3.1. The vegetable Farm Agroecosystem ...... 55
4.3.1.1. Farm size ...... 55
4.3.1.2. Experiencein vegetable growing ...... 57
4.3.1.3. The predominant soil types on vegetable
farms ...... 58
4.3.1.4. Types of vegetable grown ...... 5 9
4.3.1.5. Methods of plantingvegetables ...... 66
4.3.1.6. Crop irrigation practices ...... 70 V
4.3.1.7. Crop rotation practices ...... 71
4. 3.1.8. Marketing outlets ...... 73
4.3.2. Conclusion ...... 73
CHAPTER 5: INTERVIEW SURVEY OF THAMES VALLEY GROWERS:
PESTS AND PEST CONTROL ...... 76
3.1. Introduction ...... 76
5.2. Major Pest Problems of Vegetables in Thames
Valley 76
5.3. The Reasons Why Pests Become a Problem ... 80
5.4. Crop Protection Practices Currently Adopted
by Growers ...... 82
5.4.1. Clubroot of brassicas ...... 82
5.4.2. Fungal diseases of leeks and onions ... 85
5.4.3. Cabbage aphids 85
5.4.4. Cabbage rootfly ...... 90
5.4.5. Caterpillars ...... 90
5.4.6. Groundsel and couch grass ...... 93
5.5. The Cost of Current Pest Control Programmes 93
5.6. Growers Crop Inspection Practices ...... 94
5.7. Problem Faced by Growers in Carrying out
their Crop Protection Programmes ...... 95
5.8. Grower's Opinions on their Present Crop
Protection Programmes Could be Improved ... 98
5.9. Sources of Advice and Information Used by
Growers ...... 98
5.10. The Problems of Cabbage Aphids ...... 102
SECTION III: INVESTIGATION OF FACTORS DETERMINING
THE PROBLEM OF CABBAGE APHIDS ON
COMMERCIAL FARMS IN THAMES VALLEY ... 104 CHAPTER 6: POPULATION STUDIES OF THE CABBAGE
APHIDS, THEIR PARASITES AND PREDATORS
IN COMMERCIAL CABBAGE FIELDS ...... 105
6.1. Introduction ...... 105
6.2. Biology of the Aphids ...... 106
6.2.1. The cabbage aphid ...... 107
6.2.2. The peach-potato aphid ...... 112
6.2.3. The potato aphid ...... 113
6.3. Population Studies of the Cabbage Aphids,
their Parasites and Predators in Commercial
Cabbage Fields ...... 114
6.3.1. Material and methods ...... 115
6.3.2. Results and discussions ...... 117
6.3.2.1. General trends in the development of
live aphid and mummy population in a
cabbage field without chemical control 117
6.3.2.2. Trends in the number of live aphids and
mummies in commercial cabbage fields
with chemical control ...... 121
6.3.2.3. Changes in the percentage composition of
alatae, apterae and nymphs in relation to
crop stages and chemical control
activities ...... 128
6.3.2.4. Field counts of JD. rapae and their rate
of parasitism ...... 130
6.3.2.5. Field populations of predators ...... 136
6.3.2.6. The aerial population of alatae and the
adult parasites and predators of aphid
in cabbage fields 140 Vll
6.3.2.7. Changes in the percentage of the cabbage
plants infested with aphids in relation
to crop stages and chemical control ... 146
6.4-. C o n c l u s i o n ...... 150
CHAPTER 7: INVESTIGATION INTO GROWERS' CONTROL
STRATEGIES ...... 153
7.1. Introduction ...... 153
7.2. Assessment of Field Percentage Kill of
Aphids, Parasites and Predators Achieved by
Commercial Vegetable Growers ...... 153
7.2.1. Method of study ...... 154
7.2.2. Results and discussion ...... 155
7.2.3. Conclusion ...... 158
7.3. Aphid Distribution Within a Cabbage Plant
at Various Stages of the Crops ...... 158
7.4. Assessment of Spray Coverage on Cabbage
Plants in Commercial Vegetable Farms ... 161
7.4.1. Materials and methods ...... 162
7.4.2. Results and discussion ...... 165
7.4.3. Conclusion ...... 167
7.3. Calibration of Conventional Tractor Mounted
Medium Volume Sprayers Used by Commercial
Vegetable Growers ...... 168
7.3.1. Calibration method ...... 170
7.5.2. Results, discussion and conclusion ... 171
7.5.2.1. Types and age of nozzle ...... 171
7.5.2.2. Number of nozzles per boom and swath ... 173
7.5.2.3. Height of nozzles ...... 173
7.5.2.4. Actual volume sprayed and the percentage
error made ...... 174 v iii
7.6. Conclusion 175
SECTION IV: CONCLUSION ...... 180
CHAPTER 8: RECOMMENDATION FOR FUTUREAPHID CONTROL 181
8.1. Introduction ...... 181
8.2. The Timing of Chemical Application forAphid
Control ...... • 182
8.3. The Coverage of Spray Droplets on Cabbage
Foliage ...... 190
8.4. The Methods of Chemical Application
(Granules versus Spray) ...... 192
8.5. The Choice of Chemicals ...... 194
CHAPTER 9: RECOMMENDATION FOR FURTHER RESEARCH ... 198
9.1. Introduction ...... 198
9.2. Recommendation on Further Complementary
Research ...... 199
9.3. Appraisal of the A p p r o a c h ...... 199
ACKNOWLEDGEMENTS ...... 202
REFERENCES 204
APPENDIX 1 218 ix
PREFACE
There are many Integrated Pest Management (IPM) projects that are being advocated for farmers in the third world as well as in the developed countries. However, most of th (Norton, 1982a)* The latter approach is employed in this thesis to investigate vegetable pests and their management problems in the Thames Valley. The thesis consists of 9 chapters, within 4- sections. The first section consists of 3 chapters. The first chapter Valley where we carried out this study. The third chapter provides the initial descriptive analysis of vegetable pest management problems based on the information gathered from available literature and from various discussions held with personnel from the Agricultural Development and Advisory X Service (ADAS) and the National Vegetable Research Station (NVRS), as well as with some of the prominant individuals from the vegetable growing sector in the Thames Valley area. Since the initial descriptive analysis in section one is far from adequate, especially with regard to current pest problems and management practices in a localised area such as the Thames Valley, an interview survey of 20 Thames Valley vegetable growers was conducted. The aims, operations, results and discussion of this interview survey are discussed in detail in section two, which is divided into two chapters. Chapter four provides the background information and chapter One outcome Of the survey was to highlight the importance of aphid pests in the Thames Valley area, especially on cabbage, and the possible reasons why they are a problem. Leading on from this finding , the next stage in research was to undertake an intensive investigation in growers'fields to study the factors thought to be responsible for aphid problemsin the Thames Valley area. This included monitoring of aphids, its parasites and predators; assessment of the percentage kill of aphids; assessment of spray droplet coverage on the plants; and the calibration of the growers' sprayers. The aims, methods, results, discussion, and con clusion of each investigation are discussed in detail in section three which is divided into two chapters. Chapter six discussed the population studies of the aphids, their parasites and predators in commercial cabbage fields, and xi chapter seven covers the investigation into growers* control strate gies. The main purpose of these investigations was to explain why the problem exists and also to expose some of the weaknesses of current aphid control programmes as adopted by commercial vegetable growers in the Thames Valley. Based on the findings of section three, recommendations pertaining to future aphid control and on further research to complement the work of this thesis are formulated and discussed in section four, which is divided into two chapters. Chapter eight discusses the recommendations on future aphid control which focus on four major aspects of aphid control programmes: (1) the timing of chemical application for autumn cabbages; (2) iproving the spray droplets coverage on the K plants; (3) chemical control stratergy, that is granules versus spray; (4-) the choice of chemicals. Chapter nine discusser the recommendations for further research to comple ment the work of this thesis, and finally the appraisals of the decision-analysis approach to pest management as adopted in this thesis. 1 SECTION I DECISION--ANALYSIS APPROACH TO VEGETABLE PEST MANAGEMENT PROBLEMS 2 CHAPTER 1 AN INTERPRETIVE REVIEW OF VARIOUS APPROACHES TO AGRICULTURAL RESEARCH 1,1. Introduction From the mere gathering of nature's produce, early man began to cultivate the land thus inventing agriculture to increase and assure a supply of food. During the process of agriculturalisation, man attempts to gain control over his environment. As men's knowledge of the agricultural system has increased, his capacity to modify it has also increased, leading to further advances in man's knowledge system. In this knowledge system, science grows and techno logy is generated. According to Bunting (1979) there are five principle components to the knowledge system; (1) the stock of accumulated knowledge, in libraries and the minds and memories of people; (2) research, as a means of extending it by acquiring new knowledge; (3) development, as a means of preparing it for use on a practical scale in which new technology is produced and tested; (4-) education and training as a means of communicating knowledge, skills, and technology, as well as for teaching people how to apply them; and (5) application, which is the pratical means of using or applying the knowledge or technology to particular purposes. Research alone, the universal panacea to so many agricultural problems, achieves little if the other com ponents of the system are lacking or cannot take up the new 3 knowledge or technologies it produces. Research can be broadly defined as a process of in vestigation by which we increase our knowledge of why the world is as it is and how it might be changed (Dillon and Hardaker, 1980). There are two types of research; (1) basic research, i.e research undertaken without any specific purpose of resolving a particular problem such as research undertaken in basic sciences, and (2) applied research, i.e research undertaken specifically for the purpose of resolving a particular problem (Dillon and Hardaker, 1980). In con ducting applied research, the researcher can either adopt the commodity/disciplinary approach or the system approach. The former is the more traditional scientific approach where the problem solving research is being compartmentalized and confined within the scope of each commodity or discipline. The latter covers a much wider scope, crossing the boundries of each commodity or discipline and therefore is multidis ciplinary in approach. The application of a systems approach to agricultural research is a recent development (Ruesink, 1976). Its origins lie in the transfer of system science employed widely in the analysis of physical systems, especially in the various engineering disciplines (Getz and Gutierrez, 1982). The phrase "system analysis" has been used in so many different contexts that, in biology, there is considerable confusion as to its correct meaning (Getz and Gutierrez, 1982). In a biological context, Watt (1966) defined a system as con sisting of "regularly interacting and interdependent com ponents forming a unified whole". Therefore, a biological system may be conceived of at the cellular, organismic, population, and community levels. In agricultural manage ment it is more useful to consider systems upto the ecosystem level. Thus, in the agroecosystem context, system analysis can be defined as "the application of those tech niques (both qualitative and quantitative) that enhance our understanding of the interactions between components of the crop-pest system and their relationship to the environment and management practices" (Getz and Gutierrez, 1982). Many authors (Haynes, 1971; Stark and Smith, 1971; Stark, 1973a and b; Geier ejt a_l, 1973; Huffaker and Gutierrez, 1974; Giese ejt a^L 1975 ; Metcalf and Luckmann, 1975; and Conway £t al^, 1975) have discussed the advantages of approaching pest management through the systems approach. In depth literature reviews of this approach are described adequately elsewhere (Watt, 1962; Ruesink, 1976; Norton and Holling, 1979; Huffaker, 1980; Levins and Wilson, 1980; Haynes and Gage, 1981; and Getz and Gutierrez, 1982). The material presented in those reviews suggests that nearly all of the effort put into a system approach to pest manage ment has concentrated on developing models to describe the existing system. Very little effort seems to have been applied to encompass all aspects of pest management, including acquiring information from the agroecosystem, decision making, and taking action to manage the pest situation. 5 At the same time as these system approaches to pest management have been developing, there has been some change in the overall approach to problem solving in agriculture especially when dealing with small farmers in the third world. Chambers (1980) calls for the need to change the values and behaviour of the researcher himself, to value learning from and with other disciplines, and from and with small farmers; to value equity as well as production; to seek out methods that are cost effective, if unconven tional in identifying research priorities; and to conduct more research with farmers as colleagues, in order to address technologies that are acceptable to small farmers. At the research organization level, an awareness of the need for a change began to develop in the early seventies. When the International Rice Research Institute (IRRI), the Centre for Tropical Agriculture (CIAT), the International Institute for Tropical Agriculture (IITA), the International Crops Research Institute for the Semi- Arid Tropics (ICRISAT), the International Livestock Centre for Africa (ILCA), and the International Wheat and Maize Improvement Centre (CIMMYT) began to study the existing farming systems and the locale, specific ecological diver sity within the regions in which they worked (Bunting, 1979). Three distinct system approaches to problem solving research of relevance to pest management can be identified; the on-farm cropping system approach, developed at IRRI, the on-farm farming system approach of CIMMYT, and the 6 decision-analysis approach to pest management (Norton, 1982a). The underlying principles and methodology of each approach are discussed briefly below. 1.2. The IRRI On-Farm Cropping System Approach. Although the advances of the green revolution in agricultural technology, based on high yielding rice varieties (L'itsinger et^ a_l, 1980), greatly enhanced the potential for food production on Asian farms, the reali zation of high yields and profits, particularly for small- scale farmers have been slower than expected. The mere adoption of high-yielding varieties does not insure farmers of the full potential yields and profits possible in a given o enviornment. To obtain the full benefit from high yielding varieties, farmers need to properly manage them by the correct use of crop production practices, collectively termed "component technology". Often the component tech nology has been forced on farmers as a package of practices in nationwide production programmes in Asia. The discrepancies between yields obtained at experi ment stations and those in farmers fields have been termed the "yield gap" (IRRI, 1979). The constraints that cause this yield gap are of two types: biophysical and socioeco nomic (Litsinger ejt a_l, 1980). This idea of a yield gap and its contributing constraints has stimulated research workers to reevaluate the process of technology development for small-scale farmers in Asia (Zandstra et^ _al, 1981). The outcome is embodied in the cropping systems approach 7 (Zandstra, 1977). The on-farm cropping system approach involves local testing and verification of crop production technology on the farm by a multidisciplinary team of researchers. This is described in detail by Zandstra et (1981). The approach focuses on (1) the description and classification of the environment, (2) the design of improved cropping systems, and (3) the testing of these cropping systems on individual farms, and on methods for the formulation of production programmes (Figure 1). The end product is a package of practices adapted to the production programme’s biophysical and socioeconomic environment. The basic philosophy behind the on-farm cropping system approach is that the productive base of a cropping system-plant growth is influenced by management and the socioeconomic environment, which are both modifiable (Zandstra et al_, 1979). Therefore, when a new technology that has been produced does not fit into the biophysical and socioeconomic environment, the researcher then seeks to change the environment by providing institutional support to encourage farmer adoption of the new technology. This is the interventionist approach to technology development (Zandstra ejt 1979). While this approach may work in some cases, in others it may very well fail. For instance, in very poor countries, where the required institutional support (personnel, funds 8 Figure 1, Components of the on-farm cropping system research approach (Source: Zandstra et al, 1981) 9 etc.) has not been met, or where the resource limitations of the small farmers are too difficult to remove, new technology is unlikely to be adopted by small farmers. This, together with the lack of farmer's participation in the problem identification stage, which may result in unsound understanding of the system and inaccurate diagnosis of the problem, appears to be the main criticism of the approach. 1.3. The CIMMYT On-Farm.Farming System Research Approach. From the experience gained in their involvement with agricultural development projects in Central America and Eastern Africa, workers at the International Wheat and Maize Improvement Centre (CIMMYT) developed an approach based on recognising the importance of effective problem diagnosis in any agricultural development research. Collinson (1984) argues that the closer farmers are to subsistence, the more important is effective problem identification, because farmers themselves make the deci sions about what is good for them and would decide whether to adopt innovations in the light of the economic circum stances within which they operate. Eventhough there are many problems that can be observed on any farm, all reflecting eventual development opportunities, Collinson (1984) argues that small farmers will only be willing to re-allocate limited resources to implement appropriate solution to those current problems which most inhibit a better realization of their priorities. Therefore, in order to identify those problems that farmers would be 10 most interested to solve, there is a need for CIMMYT to understand farmers' priorities and how current management practices limit farmers/ satisfaction . Based on (1) an understanding of the small farm as a system, (2) small farmer rationality, (3) sources of variation between farming systems, and (4) how small farmers change, CIMMYT has developed the farming system research approach to problem diagnosis in agricultural research and development projects. Farming system research refers to the use of a system perspective to identify tech nologies appropriate to local specific farm situations or systems (Collinson, 1984). It applies farm management principles at the system level, allowing the cost effective use of very scarce professional manpower, usually an inter disciplinary team work of an agronomist, a social scientist, and an animal scientist. The conceptual model of a farming system (Figure 2) forms the basis of the approach which is applied in four stages (Collinson, 1984): (1) Understanding the main management challanges for local farmers by investigating the circumstances of their production environment. (2) Describing what farmers are doing and how they do it, to meet their priorities in the face of this environment. (3) Understanding why they do these things in these ways to meet their priorities. (4) Identifying constraints that prevent them from 11 Figure 2. A conceptual model of a farming system (Source: Collinson, 1984) 12 achieving their priorities more effectively. The key to the approach is: (1) Describing the three most visible facets of the con ceptual model shown in Figure 2, That is, local cir cumstances, farmers* activities, and farmer's pro duction methods. (2) Creating hypothesis on the less visible facets of the model, for example about the constraints that prevent farmers from achieving their objectives, and about the possible solutions to these constraints, and (3) Subsequently undertaking research, field trial etc. to verify these hypothesis. A low cost and rapid method for implementing the farm system research approach has been developed by CIMMYT, and discussed in detail by Byerlee ejt (1980). Basically, there are four sequences: (1) Definition of recommendation domains. (2) Understanding farmer circumstances. (3) Description and understanding of the farming system - Informal survey. (4) Description and understanding of the farming system - The formal survey. Generally, to complete this sequence of methods will require a minimum of six weeks, plus some lead times to prepare the community for the activities of research among them (Collinson, 1984). After the research team has gained an understanding of the system, it will then review materials and methods for the project and the output from national or 13 international technical research. It identifies those material and methods which appear potentially relevant as solution to identified management compromises, as more c efficient alternatives to pratices absorbing high levels N of limiting resources, or as interventions to improve productivity by taking up only slack resources. The re search team cast their net as widely as possible, seeking several alternative strate gies for the solution of any single problem. The team then follows through a pre-screening process, essentially as an ex ante evaluation of the appropriateness of each possible solution to the local farm situation. For technologies which pass through this filter, the technical scientists must decide, whether they think the relationship found experimentally will hold when decidi«j what experiment is required. Where they have a low level of confidence in the relevance of the experimental findings to local con ditions, they may feel it necessary to run a relatively formal experiment to identify the relationship locally, under farmer conditions. With a high level of confidence, they will proceed straight to a farmer managed comparison of the new existing technology. The higher the confidence, the more rapidly will the on-farm research programme take technologies into the extension and diffusion process. The idea of the farming system research approach is derived from the earlier concept of appropriate technology by Schumaker, (1973) and the concept and methodologies used 14 in village studies carried out in Northern Nigeria by David Norman (1974). Eventhough the farming system research approach has gained some acceptance as the route to appro priate technology for small farmers in the less developed countries, it is faced with similar problems to those in volved in Integrated Pest Management programmes. Some of the problems, for example, shortage of trained personnel, could very well lead to difficulties in maintaining the credibility of the approach due to poor implementation and consequent disillusion with the approach. Futhermore, according to Collinson, (1984), a better exploitation of such an approach has yet to be explored and acceptance by the reluctant administrators of the developing countries has yet to be realised. 1.4# The Decision-Analysis Approach to Pest Management Research. b Susequent to the public awareness of the ill effects of the pesticides, there has been an active reappraisal of the basic philosophy and methodology of pest control over the last decade • A renewed emphasis on developing or improving alternative pest control tactics has fostered the concept of "Integrated Pest Control" (IPC), which is based on sound ecological, sociological and economic principles (Smith and Reynold, 1966). This philosophy of IPC has gained wide acceptance as an alternative to what is seen as the^discri minate use of pesticides. However, successful implementation of IPC remains dis 15 appointingly low and appears to have taken place only where all other alternatives have failed (Way, 1971), Norton (1982a) suggest that a fundamental reason for this is the innate difficulty of developing and implementing an IPC approach that is likely to be adopted by farmers. This difficulty arise from the complexity of the problem being tackled, and the lack of effort to appreciate the full nature of the problem especially from the farmer's standpoint. More often than not IPC package s produced do not address the farmer's needs but what the policy makers and researchers feel is appropriate. In attempting to address this problem, Norton (1982a) suggested an approach involving a screening procedure that serves to guide research and extension effort towards the most promising programmes according to the real needs of farmers. The purpose of this screening procedure is to provide a systematic, yet flexible means of focusing on those ecological, technical, socioeconomic, and institu tional features of the problem most likely to affect success. The major components involved are shown in Figure 3. This approach is in many respects similar to the farming systems approach in concept but focusing particularly on crop protection/pest control problems. Initially, a descriptive analysis of the regional and the farm system is carried out during a workshop-style discussion attended by interested parties, where an attempt is made to structure the problem that is, to identify the relevant decision-makers, their 16 Figure 3. The suggested procedure for decision- analysis approach (.Source: Norton, 1982a). 17 possible actions and objectives, and to the region and of the farm, that affect pest attack and damage. This descrip tion of the regional system is largely concerned with dynamics of pest development in the region and how govern ment and private decision-making may affect it. On the other hand, the description of the farm system encompasses the major ecological and technical relationships, and the major decision-making components at the farm level, such a descriptive analysis (details can be found in Norton, 1982) could serve a number of functions: (1) Provide a basis for choosing between alternative IPC programmes. (2) By '’modelling" relevant regional and farm systems, it provides an explicit context in which specific IPC studies can be made. (3) In providing a framework for incorporating new in formation as it becomes available, a successively more detailed and useful model can be obtained (Figure 3). (4) When simple, descriptive techniques, that avoid esoteric jargon, are used during a meeting, greater communication between entomologists, plant pathologists, agronomists, and economists is likely to result. (3) It helps to identify key questions. Thus a major purpose of the descriptive analysis is to raise key biological, ecological, technical, and decision making questions which fall into two catagories: missing in formation on the ecological, technical, and decision-making processes, and speculative questions, concerning "what would 18 happen if ...... "problems. With missing information, parti cularly that which prevents further analysis, empirical and or experimental study is often the obvious next step. In the case of less important pieces of information and parti cularly for speculative questions, qualitative analysis can be used to provide valuable insights on priorities for research and extension. The qualitative analysis of key questions draws on general theories, principles, concepts and models of pest control, as well as on the specific experience gained from previous programmes, to provide a screen for assisting decisions on future courses of action. Here, each of the key questions would be clarified and analysed to set research priorities and to decide on which characteristics or vari ables to concentrate. Once key questions have been identified and ordered by priority, the aim of research is to provide answers. Ideally the research programme is determined by a flexible, interdis ciplinary team, capable of investigating those questions of most relevance to the development and implementation of IPC programmes. As research results become available, the whole process is repeated, improving the descriptive analysis, raising further key questions and so on, hopefully leading to the development of programmes which will be acceptable to farmers. The appeal for such an approach lies in its dynamic and 19 flexible format, and the powerful descriptive and qualitative analysis techniques that it employs. In practice, it could be employed for any magnitude of problems, from an academic thesis to large agricultural development projects. The main problem with this approach lies in getting the right institutional framework and closer cooperation between research and extension activities. 1.5. Conclusion. All of the three approaches discussed above have their own merits and weaknesses. The IRRI cropping system approach has been adopted for a number of agricultural projects in South and Southeast Asian countries where the farmers are quite resourceful and wealthier than their counterparts in some of the poorest countries in Africa. On the other hand the CIMMYT farming system approach is more appropriate for agricultural development projects in the poor countries of Central America and Africa where a high percentage of the farmers are still at the subsistence level. The more specific decision-analysis approach to pest manage ment has yet to see a wider adoption in large agricultural projects. In this thesis an approach has been used based largely on the farming systems and decision-analysis approach to investigate pest problems of vegetable growers in the Thames Valley. In part, the purpose has been to see how this approach can be applied to a specific problem area. Also by using this approach it is hoped that the results of the 20 research will be of relevance and useful to growers as well as to the various agencies involved in the vegetable sector. 21 CHAPTER 2 VEGETABLE PRODUCTION IN UNITED KINGDOM AND THAMES VALLEY. 2.1. Introduction. The concepts and philosophies that lie behind various approaches to agricultural problem solving have been dis cussed in the previous chapter. In this chapter, an attempt is made to examine the vegetable production at national and regional level, focusing on the economic importance and advances made in the vegetable sector since 194-0. 2.2. Vegetable Production in the United Kingdom. Vegetable production is an important sector in British agriculture. As field vegetable represent 4-% of total agricultural output, they are at a par with wheat, poultry, fat sheep and lambs (Bleasdale, 1978). In terms of the output of the arable sector, vegetables account for 14-%, achieved on only 5% (202,34-0 hectares) of the arable acreage. The current value of vegetable production is £662 million (1983) (Anon, 1984). Although in volume terms, the United Kingdom is the third largest producer of fruits and vegetables within the European Community (EC), only 4-0 to 30% of total consumption is home produced (Anon, 1983a). The deficit is met by imports worth £1.6 r* billion, of which a quater were of temperate types, mainly A. from France, the Netherlands, Spain and Italy. 22 Although a great diversity of vegetable crops are grown throughout the United Kingdom, the major vegetable growing count ies are Lincolnshire, Norfolk, Lancashire, Cambridgeshire, Kent, Suffolk, Bedfordshire, Humberside, Hereford, Worcester, and Cornwall (Figure 4-). The highest acreage of vegetables grown for human consumption in England (excluding potatoes) was 210,800 hectares in 1977 (Figure 5). Since then, the acreage has declined to 137,000 hectares in 1983 representing a fall of 37% since 1977 (Anon, 1979; 1982; 1983b). This fall in acreage has been attributed to the elimination of small acreage market gardens and to the dramatic increase in yield per hectare (30 - 70%) at a time when consumption only increased by 12 - 13% (Anon, 1983a). For example, the national average yield of cauliflower increased from 14.6 t/ha in 1976 to 19.1 t/ha in 1982, onions from 20.3 t/ha to 31.2 t/ha, green peas from 3.2 t/ha to 4.6 t/ha, and carrots from 29.8 t/ha to 49.6 t/ha, and many others (Anon, 1983c). 2.3. Changesin the United Kingdom Vegetable Sector. Before 1940, there had been little change in the vege table sector in the United Kingdom where production was mostly by market gardeners in such recognised areas as Evesham, Bedford, Essex, Middlesex, Bucks, Surrey and Kent k, together with Lancashire, Chesire and West York in the North and Cornwall in the South-west. All production at that time was for "fresh", nearby markets. In 1940, two major developments began, which became the forerunners of other revolutionary changes. Those developments were the 23 Figure 4. Major vegetable growing counties (1 = Lincoln shire, 2 = Norfolk, 3 = Lancashire, 4 = Camb ridgeshire, 3 = Kent, 6 = Suffolk, 7 = Bedford- shire, 8 = Humberside, 9 = Hereford and Wor cester, 10 = Cornwall) and Thames Valley area rfr). 2 5 0 i 2k Figure 5. Acreage of field vegetables (excluding potatoes) grown for human consumption in England (Source; MAFF June Census). 25 use of chemical weed control (Sulphuric acid in onion crops) and the development of the Bean tractor/tool carriage, a universal piece of equip ment with interchangeable tools for drilling, hoeing, spraying etc. Both developments re duced labour needs significantly (Anon, 1983d). However, these developments preceded three even greater changes: insect pest control by synthetic chemicals (DDT, BHC, and Schradan in 194-7-8); almost complete mechanization of vegetable production; and the tremendous switch from market gardening to large field-scale production. This last development has taken place partly in the last ten years. An ever increasing proportion of vegetable output has been produced by large-scale growers (greater than 300 acres), making use of more advanced production techniques, and grading, packing and distributing their produce to meet the requirements of the modern retailer - the supermarket. Many vegetables are now sold in competi tion with highly organised imports from the EEC and other areas of the world. This changed situation has brought about large-scale direct selling to supermarkets and dis tributors; a reduction in sales of vegetables to wholesale markets; and the decline of the small scale grower, unable to compete and meet these demands. Therefore, while the ability to grow a good product is still essential, it has become of secondary importance to preserving the shelf-life, quality and the distribution of the produce (Anon, 1983d). As a result large vegetable 26 production and marketing groups have developed and pro duction has concentrated in East Anglia, Kent, the Thames Valley, the West Midlands and Cornwall. Today vegetables are sold "fresh" and for prepacking, canning and freezing, some such as Brussels sprouts, onions and dried peas^even been exported. The present status of vegetable production has been made possible by many technological advancements over the last ten years, ranging from soil preparation to harvesting and storage. A great deal of research on vegetable manu ring by the NVRS has simplified and made possible more accurate determination of nutrient levels for specific crops (Greenwood, 1980). Precision drilling of seeds has been further developed with the aid of vacuum seeders, precision belt-seeders, and fluid drilling while modular transplanting systems have had a significant impact (Royle, 1980; Norman, 1984). The development and use of vacuum-cooling and ice-bank cold storage, removed the field heat from the vegetables before packing and tran sportation, and equally the much publicised "cold-chain" system is rapidly being put into practice for supplying the supermarkets (Anon, 1983e; Dennis, 1983; Openshaw, 1983; Peters, 1983). In vegetable processing, harvesting machinery has advanced from static to mobile harvesters, including complete pea harvesters and multi-row bean harvesters. Finally, computers are increasingly being introduced into the vegetable industry, not only for environmental control needs such as irrigation, crop 27 drying, cooling and storage but also for management purposes including data storage and analysis, and for ward planning (Norman, 1983b; Anon, 1983d). 2.4. Vegetable Production in the Thames Valley Area. The Thames Valley vegetable production area includes a those areas along the river Thames and its tributaries (see Figure 4) within the counties of Greater London, Surrey, Berkshire, Buckinghamshire, and Oxfordshire (Tiffin, D., personal communication). Commercial vege table production in the Thames Valley began in the 1920's when several retailers at Covent Garden market spread eastward from areas around London (Barker, 3.3., person al communication). Production in the Thames Valley increased until 1970, when more than 5,000 hectares of field vege tables were produced (Figure 6). Since then, the acreage has declined steadily to 3,700 hectares in 1981, reflecting a similar decline in acreage in the rest of England and Wales. This drop in acreage in the Thames Valley is most pronounced in brassicas (2,000 hectares in 1970 to 900 in 1981), lettuce (620 hectares to 380), and beetroots (250 hectares to 50). The large drop in the brassica acreage was mainly due to a loss of Brussels sprouts and cauli flowers to other vegetable growing areas such as Lincoln shire and Cornwall. The reason is that the smaller scale producers in the Thames Valley are unable to grow these crops competitively on a large scale. However, a few crops have increased in acreage in the Thames Valley, for example, dried peas, leeks (MAFF October census) and Hactares (000) Figure 6. Acreage of field vegetables (excluding (excluding vegetables field of Acreage 6. Figure (C) MAFF June Census Census June ). MAFF oaos fr nln ad ae (A), Wales and England for potatoes) al gop i te hms aly (Source; Valley Thames the in groups table hms aly B, n idvda vege individual and (B), Valley Thames 28 0—0 ~Jf r - Peaso-o Lattuca Brassicas 29 onions. The results of these developments in vegetable pro duction in the Thames Valley has been for the number of vegetable types grown to decline from 4-0 to 20 or less, and for growers to specialize in fewer but more profitable vegetable types. Another development is that certain Thames Valley growers have organised a marketing co operative (Thames Valley Marketing) to improve the pre sentation of their produce (such as the Yellow Box Scheme) and to safeguard the quality of their produce on the market (Anon, 1980). To increase their collective political voice in the National Farmers Union (NFU) and to provide outdoor vege table growers in the Thames Valley with a specialist service, an association, the Thames Valley Vegetable Growers (TVVG) was formed in 1972. In 1976, a full-time secretary was appointed , based at Home Farm, Hurtmore, Godaiming, Surrey. Today, there are 4-2 growers in the TVVG and its area extends from Oxford in the west to Thanet in the east, and Godaiming in the south. In essence the area covers the Thames Valley with 3 members in Essex, 3 members in Kent, with the remainder being south of the river (Hill, M. M., personal communication). 30 CHAPTER 3 DESCRIPTIVE ANALYSIS OF THE VEGETABLE PEST SYSTEM. 3.1. Introduction. Following the brief introduction to vegetable production at the national and regional level of chapter 2, let us now examine the vegetable pest system, using descriptive analysis techniques (Norton, 1976, 1979, 1982a). This descriptive analysis is based on literature reviews and various discus sions held with workers at the NVRS and ADAS as well as with various vegetable growers in the Thames Valley. The aim of this descriptive analysis is to appreciate the context of vegetable production within which the problems of pest control are investigated. The description proceeds to the format of Figure 7. 3.2. The Structure of the System. 3.2.1. The Components of the System. The vegetable farm agroecosystem is complex due to the great diversity of vegetable types and of the environments in which they are grown. However, it can be oversimplified and represented in Figure 8, showing the various components and significant interactions. The boundary of the vegetable farm agroecosystem is the boundary of the individual farm. Across this boundary, management inputs are fed into the system resulting in outputs which are removed from the system 31 Figure 7. Major components involved in the descriptive analysis of the vegetable pest system 32 Figure 8 The vegetable farm aqroecosystem 33 to the markets. Table 1 represents a detailed structure of the various components and subcomponents of the vegetable farm agroeco system. Basically the inputs are cat gorised as natural (already in the system) and management (being introduced into the system all the time). Organisms marked with a double asterisk are considered to be^major importance, those with a single asterisk of secondary importance and those without an asterisk are occas ional in nature. However, occas ional pests could become a major problem if management practices change or as a result of resistance to pesticides and the absence of population regulating factors. 3.2.2. Cropping schedule/croppinq system. In general, vegetable growers practice continuous crop- ping schedules where the planting and harvesting intervals throughout the four seasons depend very much on the demand for their produce and the supply contracts they have to fulfil with the supermarkets or processing factories. Two types of seasonal varieties are grown: the summer/autumn harvest varieties, planted in April to Guly; and winter/ spring harvest varieties planted in Gtrly to August. Each planting period for each type of vegetable maybe scheduled to have a range of planting dates to provide continuous harvesting. The actual cropping and crop rotation schedule varies from farm to farm. Usually the rotational sequence is Table 1. The components and subcomponents of the vegetable farm agroecosystem. Inputs Crops Soil Season Weather Insect pests Di sease Weed Parasite Predator Output Management inputs Seed Cabbage Light Winter Cold ♦♦Cabbage ♦♦Clubroot ♦♦Shepherds ♦♦Idiomorpha ♦♦Bembidion lampros Leave Fertilizer Cauliflower Medium Spring Warm aphid ♦Alternaria purse ♦Phygaduenon ♦Harpalus aeneus Head Pesticide Brussels sprout Heavy Summer Wet ♦♦Peach-potato leaf spot ♦Charlock trichops H. rufipes Curd Machinery Chinese cabbage Autumn Dry aphid Dry rot ♦♦Groundsel ♦Aleochara Ferania melanarius Fruit Labour Savoy Calm Potato aphid ♦♦Butt rot Nightshade bilineata Trechus Seed Planting time Calabrese Windy ♦Lettuce root ♦♦Downey mildew Fathen Strongwellsea quadristriatus Root Harvesting time Radish aphid ♦Grey mould ♦♦Couch grass castrans ♦♦Chrysopa spp. Button Cropping schedule Swede ♦ ♦Lettuce Aj>lud Big vein virus Chickweed ♦♦Apanteles ♦Coccinella etc. Crop rotation Turnip ♦Cabbage white ♦Tip burn Polygonium glomeratus 7-punctata Lettuce butterfly ♦♦Leaf blotch Fumitory ♦Pteromalus ♦Propylea Natural inputs Leeks ♦Cabbage moth ♦♦White tips Knotgrass glomeratus 14-punctata Crops Salad onion Dimond-back ♦♦Foot rot ♦Corn marigold P. puparium Adalia bipunctata Soil Dry bulb onion moth ♦♦Rust ♦Kew weed Herogenes ♦♦Syrpbus balteatus Sunlight Beans ♦♦Turnip moth ♦White rot ♦♦May weed cerophaga ♦S. luniger Water Peas Cabbage white Botrytis etc. H. fenestralis Metasyrphus Weather Carrot fly leaf rot **Diaretiella corollae Season Parsnips Cabbage flea Turnip mosaic rapae Sphaerophoria Insect pest Celery beetle virus Aphidius scripta/merthastri Disease Sweetcorn ♦♦Onion fly ♦Cauliflower matricarise Anthocoris nsmorum Weed Parsely ♦♦Carrot fly mosaic virus etc. etc. Nematode Courgette ♦♦Onion fhrips Ring spot Snai1/slug Asparagus Frit fly etc. Bird/rodent Fennel ♦Bean aphid Parasite Rhubarb ♦♦Cabbage root Predator etc. fly Pollinator etc. Decomposer 35 lettuce followed by brassica and then followed by leeks. Again the actual break for each vegetable type to be grown on the same plot varies from farm to farm. For example, brassicas may have a break ranging from i to 3 years. Figure 9 shows the crop periods for some of the vegetable types grown in Thames Valley which range from 2.5 months for crops such as Chinese cabbage or beans to 6 months in the case of leeks. 3.3. The Dynamics of the Vegetable Pest System. The dynamics of a system is the changing nature of the system through time and the various interactions between the subcomponents. 3.3.1. Time profile for some of the major pest components. The time profile shown in Figure 10 illustrates the life cycle of various organisms that are of major importance. This biological description can help to identify when control measures might have an impact on non-target organisms, the damage caused by pests and the time when natural enemies might have an impact. It also indicates the carry-over effects from year to year. For instance, disease innoculum associated with clubroot, downey mildew, and fungal diseases of leeks and onions can carry over in soil as well as in crop debris. The question marks in the time profile of Bembidion lampros, which is an egg predator of cabbage rootfly, represent missing information. 1 9 8 0 ------* * ------1 9 8 1 J F M A M J J A S O N D J F M A M J J A S O N D 1 1 1 1 1 S DD T i H | 1 Spring green • 1 1 d = ! = i = U = 1 1 » 1 1 l • 9 S DDT H i « Summer cabbage i 1 1 l 1 1 i t = 1 .. 1 = * = = = < i 1 1 i 1 S DD T H 1 i Autumn cabbage 1 1 1 1 1 • *= 1 -- 1 * 1 1 « 1 t S DD T H 1 Winter cabbage i 1 1 < = 4 = » ■ = = * = = 4 1 t 1 1 i s DD T H i 1 1 Savoy i 1 1 i *=— ' 1 1 --- I =1 i 1 1 1 1 ES LWS EST MCS LWST MCT ECH M C H J i Brussels sprout 1 , i ----- 1------* F = -1------I — 1 -- i i • < i i s DD T H • 1 1 1 i Calabrese i i i i t = = # = 1 — *• = 1 i i « t i 1 DD H i i 1 Chinese cabbage i i i 1 ' — ( = 4 i i 1 i t s T H • • Summer lettuce • 1 { = =- 1 = M = = 4 • i • ■ 1 « • 1 s T H i • Autumn lettuce t 1 i • 1== 1 1 ■= = 4 • i i < 1 « • i t s DD T H • • Leeks i 1 l 1 i 1= i = 4 - Figure Digram showing cropping period for each vegetable within a 2 year continum ( 5 = sowing, DP = direct drilling, T = transplanting, H = harvesting, ES = early sowing LSW = late winter sowing, MCS = main crop sowing, EST = early sowing transplanting LWST = late winter sowing transplanting, MCT = main crop transplanting, ECH = early crop harvesting, MCH = main crop harvesting. 37 Organ!sms F HA M 3 3 A S 0 11 Cabbage rootfly verwlnterlng pupae Adult (1st genera-/ Adult (2nd gene- Overwintering n soil tlon) / ration) pupae In soil Eriulsclila brasslcae larvaepupae J / larvae pupae soil root soil / soli root soil Cabbage aphid Spring colonies /Summer coloniesj Autumn Overwinter as Emergence / Apterae / colonies egg or as nymphs, Brevlcorvne brasslcae apterae and APtCAlatae / alatae In South on bratslcas / on brasslcas/ England on bra / / brassicas sslcas and other Cruclfars Peach-potato aphid Spring colonies / Summer coloniesJAutumn Overwinter as Emergence / Apterae / colonies egg or as nymphs, Hvzus perslcae Apterae / Alatae / Apterae apterae and alatae Alatae / / Alatsa lh Iduth tnglahd on brasslcas, le on wlntwr nost j on cabbage, lettuoe, pot- ttuce, potato and j atoes, sugarbeet, and etc. etc. Lettuce aphid Spring colonies /summer colonies/ Autumn Overwinter as eggs Overwintering J Apterae /Apterae on currents and Nasanovla rlblsnlgrl eggs hatch / Alatae / Alatae gooseberries Apterae / / Alatae / on lettuce / on lettuce winter host J / Turnip moth Larvae over Larvae eme- pupae Adult emerge (1st /Adult emerge (2nd winter In rge and con generation) generation) Aqrot15 seqctum soil tinue feeding on crops at the Larvae „Pupae/ , Larvae base of the in soil near In soli near host plant host crops crops Onion fly Overwintering pupae Overwintering in soil pupae In soil Hy1cmy1 a antlqua Egg Larvae pupae Soil onion soil Carrot fly Pupation completed Adult emerge Adult emerge Pupate at var in carrot (1st genera (2nd generation) ious time during Ps11 a rosae tion) Egg the winter ^99 Larvae Larvae in carrot Pupae in soil near carrot plants Clubroot of brasslcas Resting /Zoospores /Sporulation/Reinfect lOisinteg- Resting spo spores /infect root/zoospores /and devel-/rating res overwin Plasmodlophora germi-/ hairs / /op clubroot/clubroot ter in soil brassIcae nate / / / ______/ Oonney mildew of Oospores (Infect (Produce (Reinfect (Sexual repro- Oospores lettuce germlnate/lettuce/more zo-lother le-fductlon (anth overwinter in soil /leaves /ospores /ttuce /ridia ♦ oogonla In soil and Premia lactucae I produce oos crop refuse I Ppores ) I tilomorpha rapae 'Adult emerge and parasitised Larva drop Pupae over 1st and 2nd lnstar larvae of/ off from 2nd winter in cabbage rootflyof 1st anp generation In soil 2nd generation rootfly lar vae and pu pate In soil Apanteles qlomeratus Adult emerge and parasitised 'Larva in ca Pupae over larvae of 1st and 2nd gener-/ bbage white winter in ation cabbage white butter-. butterfly soil fly larvae of 2nd generation drop and pu pate in soil Claret lei 1 a rapae Adult emerge and parasitised all stages Overwinter as of cabbage aphid and peach-potato aphid pupae in aphid mummies Bembldlon 1ampros Adults emerge and prey dlssappear in Duly Overwinter? on cabbage rootfly eggs Where? on what? where? Syrphus ba11eatus /Adult appear, lay eggs among aphid Adults migrate colonies on brasslca plants, larvae to warmer part feed on aphid colonies and pupate on of England or brasslca plant may overwinter as larvae or pupae In soli Vegetable crops Harvesting ^/'Summer/Autumn crops Harvesting Wintwr/spring crops 'Planting '"Pi an 11 ng Figure 10. Time profile for major components of the vegetable farm aqroecosystem showing the life stages and sites of the major Insect pests, diseases, parasites, and predators throughout the year. 38 3.3,2. The effects of pests on vegetables. Table 2 shows the damage that can be inflicted by vege table pests. Weeds are not included because they are generally considered as competitors with vegetable plants for nutrients, sunlight, and water. In terms of revenue obtained, there are 2 components to crop loss caused by pests; (1) a reduction in the amount of acceptable produce; and (2) a reduction in the quality ("comestic" effects) or size of the produce, which sells at a lower price. Pests that damage the produce directly, usually have the lowest economic threshold (defined by Stern et_ jQ, 1939 as "the density at which control measures should be determined to prevent an increasing pest population from reaching the economic injury level") for control action to be taken. However, in most vegetable crops, the requirement of the market especially supermarkets and processing factories is such that economic thresholds are not feasible. This often leads to growers resorting to prophylactic chemical control for most vegetable pests. For most vegetables, yield loss determination can be very complex when pests inflict both direct and indirect damage, such as the damage caused by cabbage rootfly on Brussels sprouts. As a result, information relating to damage and yield relationship appears scarce for many vege table pest situations, and especially for occas ional pests. 39 Table 2. Damage matrix showing which components of yield are a ffected by the major Insect pests and diseases (pest component). Each asterisk indicates that a damage effect on plant can occur. Insect pests BRASSICAS LETTUCE LEEKS/ONIONS PEAS/BEANS UMBELLIFEROUS and diseases R S L HCBRSLRSLBB L F L R3/ Cabbage aphid * * * * * Peach-potato aphid * * P otato aphid * L ettu ce root aphid # L ettu ce aphid * Cabbage white b u tterfly * Cabbage moth * * Diamond-back moth #* Turnip moth * * * Cabbage white fly * * * * Cabbage flea beetle * Onion fly * * * Carrot fly * Onion th rips * Bean aphid * * Cabbage ro o tfly * * * Clubroot * Alternaria lef spot * Dry rot * Butt rot * * Downey mildew of le ttu c e * Grey mould of lettu ce * Big vein virus * Internal tip burn * * Leaf blotch * White tip * Foot rot * * Rust * * White rot * * Botrytis leaf rot * Turnip mosaic virus * Cauliflower mosaic virus * Ring spot of cabbage * a/ _ R = root, S - stem, L - J e a v e s, , H = head, C = curd, B = button, BB = bulb, F = fruit. 4 0 3.3,3. The effects of parasites and predators on vegetable pests. -r The relationship between parasites/predators and athropodA pests are shown in the interaction matrix of Table 3. The extent of parasitism or predation of the majority of the species listed, in regulating its prey or host populations are not well understood. However, some of the parasites and predators of cabbage aphid and cabbage rootfly have been studied in some detail by several authors (Wright ejt jrl, 1960 a and b; Coaker and V/orall,A 1961; Chua, 1976; Akinlosotu, 1973; Hafez, 1961; Paetzold and Vater, 1966; Pollard, 1971; atb Herakly and El Ezz, 1970; George, 1957; Daiber, 1971; Hughes A and Mitchell, 1960). For cabbage rootfly, it has been shown that no single mortality factor is responsible for main taining the constant number of adults that occur from one generation to the next (Hughes and Mitchell, 1960). In theory, there is some potential for biological control in vegetable crops, although in practice, vegetable growers seldom, if at all, place any importance on biological control in their current control programmes. This is likely to be the result partly of the dynamic nature of the vegetable agroecosystem and partly due to the uncertainty of the bene fits associated with biological control effort. 3.3.4. The effects of pesticides on components of the vegetable pest. Table 4 shows the harmful effects of insecticides, fungi cides and herbicides on target and non-target organisms. 41 Table 3. The parasites and predators host/prey relationships. Cabbage White cabbage Aphid Diamond-back Onion Carrot rootfly butterfly moth fly fly P a ra site s Tdionorpha rapae La/ Phyqaduenon trichops L Aleochnra bilineata P P Empusa muscae A Strongwellsea castrans A Apentales glomeratus L Pteromalus puparium P Hemitalis melanarius L Heroqenes cerophaqa L H. fenestralis L Diaretiella rapae A+N Aphidius m atricariu s A+N Entomogenous fungi E+N+A Aphaereta cephalotes L Dachnusa spp. P Predators Bembidion lampros (A)*3/ E Harpalus aeneus (A) E H. rufipes (A) E Feronia melanarius (A) E Trechus quadristriatus (A) E Syrphus b alteatu s A+N S. luniqer (L) A+N Itetasyrphus co ro lla e (L) A+N Sphaerophoria scripta/ m erth astri (L) A+N Coccinella 7-punctata (A+L) A+N Propylea 1^-punctata (A+L) A+N Adalia bipunctata (A+L) A+N Anthocoris nemorum (A+N) A+N Chrysopa spp. A+N 3 / Prey stage: E = egg, A = adult, N = nymphs, L la = r v a e , and P = pupae. Predatory stage: A = adult, L = larvae, and N = nymphs. 42 Table 4. The Impact of pesticides on the various components of the vegetable farm agroecosystem (asterisk Indicate harmful effects) Herbicides Pre- Post Insecticides Fungicides emerqence emergence u O O Most of the insecticides commonly used in vegetable crops besides killing the target organisms are also harmful to non-target organisms such as parasites, predators, pollinators, and decomposers. As expected, broad spectrum insecticides, such as permethrin, triazophos, and mevinphos, are shown to have harmful effects across a wider range of non-target organisms compared with more specific insecticides, such as chlorfenvinphos granules. Most commonly used fungicides and herbicides, even(though i harmless to parasites and predators, are quite destructive to soil microorganisms such as the nitrification bacteria, denitrification bacteria, legume nodulation bacteria, free living nitrogen fixation bacteria, Actinomycete, cellulolytic and other soil fungi. However, few herbicides have any great or prolonged adverse effects on the total bacterial com ponents of the soil, and in the case of fungicides, the posi tive effects far overweigh the negative effects (Hill and Wright, 1978). As a general rule, all species of fungi are immediately suppressed by fungicides to a variable degree, and the recovery from such effects maybe relatively rapid or slow, depending on prevailing soil conditions. The spectrum of species in the soil after treatment maybe con siderably altered, and may persist for a long period (Hill and Wright, 1978). 3.4. Management and the Vegetable Pest System. Information on the management of vegetable farms, as practiced by Thames Valley growers, is not available in the literature. The interview survey of Thames Valley vegetable growers, discussed in detail in the next section attempts to provide some of this information. In the meantime, it is useful to consider the major factors affecting the management of a vegetable farm and which affect the decisions made. Norton (1976) argues that crop protection decisions are determined by four main factors: (1) the farmer's objectives; (2) his perception of pest attack and the damage it can cause; (3) the control measures available to him, and (4-) the decision rules by which he operates. Mumford (1978) has discussed some of these factors with relevance to sugarbeet pests, and Lane (1981) to oilseed rape pests. The relevance of these factors for vegetable pest control are discussed briefly below. 3.4-.1. The objectives and constraints of a vegetable grower. In general, vegetable growers are likely to have more than one objective that will affect their pest control deci sions, and these objectives are likely to be different -fi\e depending on whether^grower has contracted the bulk of his produce to processing factories or is supplying the "fresh" market. Nevertheless, the objectives of most growers will include one or several of the following: (1) Maximizing yield of acceptable produce for the market. (2) Achieving a certain level of revenue in any one year. (3) Minimizing the cost of production. 45 (4) Minimizing the use of pesticides. The emphasis given to any of these objectives by the indivi dual grower will depend very much on his perception of the problems and its solutions, his attitudes towards the environ ment and moral values, as well as his social commitments. Similarly, the constraints to which a grower is subject to may include one or several of the followings: (1) Limited resources such as capital, land and labour. (2) Limited knowledge of the pests and their methods of control. (3) Restrictions governing the use of pesticides on ground of hazards and pollution. (4) Clash of farm routine with growers social interest and obligations The degree overjwhich the growers are able to manipulate his constraints depend very much on his ability to reallocate his limited resources to the new situation, and also on his experience in vegetable growing. A very experienced grower maybe able to overcome several of the above constraints as compared to those who are relatively new to the business. 3.4.2. Grower's perceptions of vegetable pests. Tait (1983) surveyed the vegetable growers of Bedford shire and Lincolnshire and found that these growers tend to perceive aphids as the biggest threat to their crops, while caterpillars and cabbage rootfly were regarded as second and third important. Clearly, grower's use of pesticides is likely to be influenced by the extent to which they are aware 46 of pest problems on their crops, and the relative importance which they attach to each pest. Tait (1983) also found that some growers were more worried by pests which could not be seen until the damage was done, such as cabbage rootfly or to a lesser extent aphids. Others were more concerned when either the pest or its effects could be clearly seen, as for caterpillars. Tait also investigated the levels of infest ation that growers would tolerate for aphids, caterpillars, and cabbage rootfly, and she found that 96% of the growers would not tolerate any loss of crop due to damage by the cabbage rootfly, while in the case of aphids and caterpillars, about 30-60% of the growers would tolerate a certain degree of infestation (upto 20% of the plants infested). Such a low tolerance for pest damage reflects the quality concious- ness of the growers in the surveyed area, where a high per centage of the crops grown are contracted to processing factories, which demand the highest quality produce. 3.4.3. Control measures for vegetable pest. The control of vegetable pests depends almost entirely on the use of synthetic pesticides. Such dependence on pesticides, virtually excludes all other control methods (Wheatley, 1971). To investigate insect pest control, Tait (1983) carried out an insecticide usage survey on vegetable brassica crops in the "Holland" area of Lincolnshire, where VI she found that demeton-S-methyl was predominatly used for A» the control of aphids, DDT for the control of caterpillars, and dieldrin or aldrin root dips for cabbage rootfly. She also observed that, in general, the chemicals used for 47 particular pests in the area were approved for that purpose by the Agricultural Chemical Approval Scheme (ACAS). How ever, these findings may or may not reflects insecticide usage in other vegetable growing areas because the level of pest control required varies. Also, since the survey was carried out in 1978, obviously some of the chemicals are already outdated. Figure 11 shows some of the decisions that a vegetable grower has to make in choosing a particular control strate gy. In choosing between these alternatives, a grower usually employs certain decision rules based on his objectives and constraints. 3.5. Conclusion. This section has attempted to describe the vegetable pest system based on available information in the literature and from discussions held with various agencies and with vege table growers. The main aim has been to provide a general insight into the background of the vegetable industry and to set a framework for further descriptive analysis in the following section. From the initial descriptive analysis, described above, it is clearly evident that information pertaining to the actual management practices of the vegetable farm, especially in localised areas such as the Thames Valley are grossly lacking. Without such vital information, it is not possible at this stage to clearly identify the key questions relating to pest control problems in the area on which further research (a ) Decision before planting Mul tiple Monocropping ^ Non-intensiveiv^^r c r o p p i n g s c h e d u l e ' In te n s i v e c r o p p i n g schedule y ~~ High crop \ s t a n d < L o w c r o p stand — Resistant variety \ Multivars < Susceptible variety Full cultivation Minimal cultivation Direct sowing Transplanting Peat block^— Preemergence \ pest control Post emergence pest control (b ) Decision after planting ^Integrated control < <---- Biocontrol + cultural control < Pesticides «-— Opportunistic < ^Prophylactic^^ Adaptive -^Pest mon i toring crop inspection Low control threshold/Frequent Hii|h control threshold /Infrequent Figure 11. A decision tree for cultivation and pest control oF vegetable crops can be based. Therefore, an interview survey of vegetable growers in the Thames Valley was proposed with the aim of obtaining further information on the current pests and management problems in the area. 50 SECTION II INTERVIEW SURVEY OF THAMES VALLEY GROWERS 51 CHAPTER 4 . INTERVIEW SURVEY OF THAMES VALLEY GROWERS: BACKGROUND INFORMATION. 4.1. Introduction. In the previous section, we discussed the production of vegetables both at the national level and in Thames Valley, and described the major components of vegetable pest pro blems, as obtained from the available literature and from discussions with a number of people involved in the sector. In this section, to continue the descriptive analysis, an attempt is made to determine what actually happens on farms! This is achieved through an interview survey of vegetable growers. Some of the factors to be looked at in this inter view survey are shown in Figure 12. Beside the aims and operations of the survey, Chapter 4 will discuss the results of the survey pertaining to background information and Chapter 5 will discuss those results pertaining to pests and pest control. 4.2. Aims and Operation of the Survey. The interview survey was designed to achieve the following objectives: (1) to identify the key pests and management problems existing in the Thames Valley area as perceived by growers; (2) to determine the crop production and crop protection practices currently adopted by growers; and (3) to determine the factors affecting the crop pro duction and crop protection decisions. The overall purpose Figure 12. Some of the factors to be looked at in the Thames Valley Pest Problems Survey 53 was to obtain a better appreciation of grower's problems, allowing a more meaningful pest control research programme to be adopted. The interview survey was conducted in the months of January and February, 1983. This particular period was chosen because it coincided with minimal cropping activities. The respondents were selected from the full membership list of TVVG for 1982. Of the full list of 33, 13 were rejected for the following reasons - 5 were in Kent, 3 in Essex, and 3 in Hants and 2 declined to be interviewed. Of those farms visited the owner/manager was interviewed using a question naire. On average, the interview lasted for about an hour. Prior to this survey, a preliminary examination of records and a detailed analysis of six selected farms had been carried out from June to August, 1982. Information Godaiming for corrections and comments. At the same time, a pilot survey of some growers was carried out in late December to test the questionnaire. It was amended to in clude comments from the relevant agencies above as well as to meet the time limit of the inter.view and to increase the effectiveness of the questionnaire in obtaining the infor mation required from growers. After eleven drafts, the 54 final form of the questionnaire (Appendix 1) was ready. The TVVG, who formally approved the survey, then circulated a letter to the growers to be interviewed, informing them of the objectives of the survey and asking them for their full cooperation in the project. Two weeks after these letters were sent out (First week of Oanuary, 1983), appointments for an interview with individual growers were made via phone calls. The questionnaire was divided into 3 sections: (A) back ground information to vegetable production in the Thames Valley area; (3) major insect, disease and weed problems, current control methods and decision making; (C) source of advice and information used to make decisions (see Appendix 1). Section A, consisting of questions 1-13 is designed to obtained a description of the vegetable system (i.e the farm, grower, crops grown, cropping practices, and marketing outlets for the produce) and also begins the questioning T with fairly straightfoward factual questions which the grower could readily answer, so as to build the grower's confidence and make him feel at ease. Section B - questions 14-21 are more specific questions, designed to extract in formation on the major pests and the reason why they are a problem, as perceived by the growers, the crop protection practices currently adopted, the estimated cost of current control programmes, crop inspection, the problems of imple menting current control programmes, and the growers' opinions on how current control programmes could be improved. Section C, with question 22 and 23 only, relates to the 55 sources of advice and information which are most sought by the growers to make decisions. You will notice (see Appendix 1) that in question^, 6, 7, 11, 15, and 22, the word CARD is inserted at the end of these questions. This is to remind the interviewer to hand out a prepared card of each question to help the grower recall the possible options listed. 4.3. Results and Discussion of the Survey. The results and discussion of the survey are discussed under 3 main headings in 2 chapters. Chapter 4 covers the results of questions 1-13, and chapter 5 from questionsl4 to 23. 4.3.1. The Vegetable Farm Aqroecosystem. This part of the discussion is based on the grower's "n. responses to questions 1-13 of the questionaire. K. 4.3.1.1. Farm size. From the survey, and from talking to individual growers, vegetable farms in the Thames Valley area can be classified into 3 catagories: (1) small farms, with an acreage of less than 100 acres; (2) medium farms, acreage between 101 to 299 acres; and (3) large farms, where the acreage is larger than 300 acres. Figure 13 shows the number of growers surveyed whose acreage falls under these 3 catagories. Farm size ranges from 20 to 700 acres, about 50% of the farms being 56 149 199 249 299 349 399 above Acres Catagory 1----|------2------|------3------ Figure 13, The size of vegetable farms in Thames Valley. small farms, 25% medium farms, and another 25% in the large farm catagory. The average acreage of small, medium and large farm catagories are 52.1, 150.8, and 402.2 acres respectively. (a) Small farms. The results indicate that the majority of growers are in this catagory. This is often a family unit farm, using intensive (family) labour and specializing in crops which require high labour inputs such as spring onions, radishes, beans, courgettes, etc. The bulk of the farm produce is sold in their own farm shops or as pick-your-own produce. (b) Medium farms. This catagory is usually characterised by intensive cropping with severe constraints on land and capital re sources. Growers in this catagory often seem to be con 57 fronted with tremendous management pressures, with land and capital constraints leaving little room for manouver. These growers were particularly sensitive to price fluctuations of inputs and outputs. This appears to have led to the use of an insurance type of policy with regard to pest control to ensure their capital investment is covered, otherwise they would be out of business. (c ) Large farms. Farms in this catagory are less affected by the ups and downs of business because they are endowed with land and capital resources, allowing some flexibilty in resource allocation and other business ventures. Also, they have the bargaining power to obtain a supply contract with the big supermarket chain stores within or outside their local areas A-.3.1.2. Experience in vegetable growing. The experience of growers in growing vegetable crops on the same farm is shown in Figure 14-. About 55% of the growers have more than 30 years of vegetable growing ex perience. Only 15% of the growers have less than 10 years experience and could be considered as newcomers to the busi ness. This would indicate that most of the Thames Valley vegetable growers are very experienced in husbandary, pest control and marketing of vegetable crops. On the other hand it also reveals that the majority of farms in the area have been under vegetable cultivation for a long time, especially as many farms have been handed down from several generations 58 to u Years experience Figure 14. Vegetable growing experience of Thames Valley growers. This implies that some of the current disease problems in the area may largely be the result of the accumulation of disease organisms over so many years of continuous vegetable cropping on the same piece of land. 4.3.1.3. The predominant soil types on vegetable farms. The predominant soil type ranges from fine sandy loams to medium loam with gravel (Figure 15). About 30% of the farms were of a sandy loam, which was considered to be ideal for the great majority of vegetables grown in the area. Another 40% of the farms in the area were either sandy loam with gravel or medium loam which were considered satis factory for vegetable growing but would cause greater wear and tear to farm machinary. The other 30% of the farms were either of fine sandy loam or medium loam with gravel which were considered acceptable soil types on the extreme end of the range. The importance of having the right soil type for vege 59 table farms rests on the implications for crop growth, culti vation, pest control, and drainage. Medium loam soil with 30 1 .Figure 15. The predominant soil types on vegetable farms of Thames Valley. gravel for instance, can pose problems with direct drilling. Heavier soil types, such as clay loam are often associated with serious clubroot problems. On the other hand, if the soil is too light, it may pose problems with the retention of soil moisture, fertilizer and granular insecticides at the root zone layer- Farms around meadows or surrounded by canals or rivers are sometimes flooded at any one time of the year and heavy soil may take a long time to drain out. 4-.3.1.4-. Types of vegetables grown. Altogether there are 4-5 types of vegetables grown (Table 5). The brassicas, with 19 different types of vegetables, is the largest group among which early summer, summer, autumn, and winter cabbage are the most frequently grown crops. On the other hand, radish, swede, turnip and kohlrabi are only grown by a few growers. The next largest group to brassica is 60 Table 5. L ist of vegetable types grown in the Thames Valley. Vegetables No. of Growers Vegetables No. of Growers (out of 20) (out of 20) Nr a a si c o st Leeks & Onions 1. Early Summer Cabbage 19 24. Leeks 20 2. Summer Autumn Cabbage 19 25. Salad Onions 16 3. Winter Cabbage 19 26. Dry Bulb Onions 6 4. Spring Greens 16 Peas & Beans 5. Ca labrese 16 27.Broad Beans 13 6. Savoys 15 28. Runner Beans 10 7. Kale 15 29. Green Peas 6 8. White Cabbage 14 30.French Beans 5 9. Sprouting Broccoli 14 31. Dry Peas 1 10. Autumn Cauliflower 11 Carrots and Parsnips 11. Summer Cauliflower 10 32. Parsnips 18 12. Winter Cauliflower 10 33. Carrot 12 13. Brussels Sprouts 10 M iscellaneous Crops 14. Red Cabbage 10 34.Spinach 16 15. Chinese Cabbage 9 35. Sweetcorn 16 16. Radish 6 36. Marrow 16 17. Swede 4 37. B eetroots 11 18. Turnip 3 38. Courgette 11 19. Kohlrabi 1 39. F ennels 10 L e ttu c e : 40. Parsely 7 20. Crisp Lettuce 20 41 . Rhubarb 6 21. Butterhead Lettuce 19 42. Celery 4 22 . Cos Lettuce 15 43. C eleriac 4 23. Iceberg Lettuce 4 44.Asparagus 1 45. Mint 1 61 lettuce, with crisp lettuce being grown by all growers in terviewed. Leeks and onions are another important group, with leeks grown by all growers interviewed, and established itself as the main winter vegetables in Thames Valley. The other two groups, carrot and parsnips, and peas and beans are of minor importance because of their decreasing popula rity with Thames Valley growers. There are about 12 minor types of vegetables grown in Thames Valley area grouped under miscellaneous of which spinach, sweetcorn, courgette and marrow are most frequently grown. The number of vegetable types grown on individual farms in the Thames Valley ranges from 11 to 40. The majority of farms (55%) grow between 21-30 different types of vegetables (Figure 16). In general growers tend to specialize in less than 20 vegetable types. By specializing in a few crops, the strains of management are reduced and the larger acreage of each crop, thus enables the unit cost of production to be reduced. Types of vegetables Figure 16. The number of vegetable types grown per farm in Thames Valley. 62 The importance of each group of vegetables can be ex pressed in terms of the percentage of the total vegetable acreage on each farm occupied by each vegetable group. From the survey it was found that ^0.20% is devoted to brassicas, 16.35% to lettuce, 17.35% to leeks and onions, 3.15% to peas and beans, 5.85% to carrots and parsnips, and 17.5% to miscellaneous crops (Figure 17). This mean that brassicas, onions beans parsnips neous crops Figure 17. Mean percentage of total vegetable acreage per farm devoted to the different groups of vegetables. lettuce, and leeks and onions constitute about 75 % of the total vegetable acreage on the average farm. Besides vegetables, 70% of the growers interviewed grow field crops or cereals (Table 6). Potatoes are most fre quently grown, soft fruits, such as strawberries, goose berries, cranberries, blackcurrents, and raspberries are also grown frequently, especially by small growers, to add variety to the produce sold in their farm shops or for 63 "pick-your-own" outlets. On the other hand, cerealsare grown on large farms as a break crop in the crop rotation programmes. Table 6. Crops beside vegetables grown by Thames Valley vegetable growers. Crops No. of growers Average acreage/ (out of 14-) farm (1982) 1. Potatoes 10 7.5 2. Soft fruits 8 13.5 3. Cereals 3 270.2 4. Oilseed rape 1 125.0 5. Apples 1 8.0 6. Wall flower 1 3.0 Some of the vegetables grown in Thames Valley area have only been grown recently by some growers (for the last 3 years). Vegetables such as Chinese cabbage, fennel, Cala brese, and iceberg lettuce are the most popular recent adoptions (Table 7). On the other hand, some growers have stopped growing certain types of vegetables. Notable amongst these are (i) beans, due to a decrease in demand, labour problems for picking and poor returns from the crops (Table 8); (ii) beet root and celery, since growers in the Thames Valley could not Table 7. Types of vegetables recently adopted and the reason for adopting them Types of No. of growers Main reason given for growing them • vegetables (out of 20) 1. Chinese cabbage 9 Give good returns due to increase in demand. 2. Fennel 6 Give good returns due to increase in demand. 3. Calabrese 6 Give good returns due to increase in demand. 4. Iceberg lettuce 4 Give good returns due to increase in demand. 5. Sweetcorn 3 Give good returns due to increase in demand. 6 . Summer cauliflower 3 To extend cropping season. 7. Dry bulb onion 3 Able to grow them very well via multiseeded peat. 8. French bean 3 Increase range of produce for wholesale market. 9. Courgette 2 Increase range of produce for farm shop. 10. Brussela sprout 2 Increase :range of produce for farm shop. Table 8. The types of vegetables which some growers have stopped growing Types of No. of growers Main reason why stopped growing. vegetables______(out of 18)______ 1. Beans 10 Poor returns plus labour problem for picking. 2. Beetroot 6 Uneconomical to grow on small scale. 3. Celery 6 uneconomical to grow on small scale. 4. Summer cauliflower 6 Clubroot disease problem plus unsuitable climate 3. Chinese cabbage 4 Poor quality obtained due to disease problems. 6. Turnip 4 Uneconomical to grow on small scale. 7. White cabbage 3 Clubroot disease problems plus storage problems. 8. Dry bulb onion 3 Cannot get good quality skin. 9. Radish 3 High cost of labour. 10. Red cabbage 3 Clubroot disease problem plus poor returns. 11. Cos lettuce 3 Uneconomical to grow due to poor returns. 12. Swede 3 Cabbage rootfly problem plus poor returns. 13. Cougette 2 Picking problems plus high wastage rate. 14. Sprouting broccoli 2 Clubroot problem. 15. Butterhead lettuce 2 Best grown in glasshouse. 66 compete with growers in other areas, who could grow these crops on a large, field scale through mechanization; and (iii) crops such as summer cauliflower, Chinese cabbage, white cabbage, red cabbage, and sprouting broccoli, because of clubroot disease problems. In the future, the majority (90%) of the growers inter viewed thought theywould be growing fewer vegetable types. Only 10% of the growers thought the number of vegetable types they would be growing would remain the same. No growers thought they would grow more vegetable types in the future. The main reason for growing less vegetable types in the future is the need to specialize in a few, profitable vege table types, taking into account changes in approach to marketing, marketing outlets, and competition with growers in other areas (Table 9). Those who said they would grow the same number of vegetable types in the fu ture cited the lack of acreage for new entries, and requirements for the broad demand for farm shop sales as their main reason. 4.3.1.5. Methods of planting vegetables. Concentrating on the 3 main vegetable groups (Brassicas, lettuce, and leeks), there are 4- ways in which these vege tables are planted into the field: (1) bare-root trans planting; (2) peat block transplanting; (3) direct drilling; and (4-) module transplanting. The sequence in which these innovations have become available is in the order mentioned, with module transplanting being the latest planting tech nique. Figure 18 shows the number of growers adopting the Table 9. The reasons why vegetable growers would grow less types of vegetables in the future. No. of growers Reasons giving reason ______(out of 18) 1. The need to specialize in fewer but profitable types of vegetable due to 8 changes in marketing approaches, marketing outlets, and competition with growers in other areas. 2. Losing the types of vegetables that they grow on a small scale to large 3 specialized growers in other areas who can grow them on a larger mechanised basis. i 3. Managing to many types of vegetables on one farm is becomming more 2 0\ complicated and unmanageable. i 4. Very few new types of vegetables are available that are economically viable. 2 5. Acute disease problems such as clubroot has resulted in a whole range 1 of brassicas being abandoned. 6. Shortage of farm labour has resulted in labour intensive types of 1 vegetables being abandoned. 7. Marketing cost of some types of vegetables has increased several fold 1 without a reciprocal increase in the price of the vegetable, resulting in those vegetable types being abandoned. rowers adopting aiu patn mtos o bascs ltue ad leek and lettuce, brassicas, for methods planting various rp. h ms pplr ehd f lnig rsia is brassicas planting of method popular most The crops. planting. Only some growers adopt module transplanting, transplanting, module adopt growers some Only planting. trans bare-root and block peat by followed drilling, direct oty n ag frs n o a ra bss Hwvr some However, basis. trial a on and farms large on mostly by direct drilling. Module transplanting in lettuce crops crops lettuce in transplanting Module drilling. direct by extent lesser a to and transplanting block peat by planted in transplanting module tried not have who growers the of dicated they would do so in the future. Lettuce was mainly mainly was Lettuce future. the in so do would they dicated ek ae any lne b te rdtoa br-ot trans bare-root traditional the by planted mainly are Leeks the in years 2 last the within introduced been just has lnig ehd n b drc diln. oe rwr still growers Some drilling. direct by and method planting it. tried have farms large only far so and Valley, Thames use peat block transplanting, though the majority have have majority the though transplanting, block peat use ean o n future. in so remain would probably and progressed not has leeks in module of adoption transplanting The cost. high its to due it abandoned iue 8 Aoto o br-ot rnpatnI peat L transplantingI bare-root of Adoption 18. Fioure block transplantinq transplantinq block etc ad ek rp i Tae Valley. Thames in crops leek and lettuce tUftHin transplantinq module and brassica, 68 llllIU, iet drilling direct «P o -C planted indicate that most of the brassicas and leeks are direct direct are leeks and brassicas the of most results that These indicate 20. Figure in shown is farm, each on planted planted via peat block transplanting but there is a general general a is there but transplanting block peat via planted method bare-root the occurs, transplanting where but drilled il on ie a t drc diln i te er t come. to years the in drilling direct to way give soon will mostly still is Lettuce crops. two those in pruminates still concensus among the growers that peat block transplanting transplanting block peat that growers the among concensus advantages. In the opinion of the majority of growers, growers, of majority the of opinion the In dis advantages. and advantages have drilling direct and block peat Both iet rlig s es xesv ad es ie consuming time less and expensive less is drilling direct 70 70 " “ 0 6 50- 50- 40 - 10- 20- 30 “ 0 -■ Figure 19. Average percent of the total acreage planted planted acreage total the of percent Average 19. Figure The area planted by each method, out of the total acreage acreage total the of out method, each by planted area The Brassicas ot rnpatn! , et lc trans |, block peat transplanting! root planting (bare- methods planting each by farm each on rnpatn ) o bascs lettuce brassicas, ) for transplanting n les n hms Valley Thames in leeks and I drc drilling , direct etc Leeks Lettuce 69 I H > anc* 70 compared with the peat block method. However, with direct drilling, uniform plants are difficult to achieve and greater weed problems arise due to the firm soil requirement of the direct drilling method, which is achieved through minimum or shallow tillage. Non uniform crops would result in a high percentage of crop wastage due to oversize or out of shape plants not being harvested. To overcome these problems, some growers have recently begun experimenting with module transplanting, which origi nated from California. Some of the larger growers who have tried this new technique have already indicated that they will switch from direct drilling or peat block to module transplanting, especially in brassica crops in the future. They claim that module transplanting is as fast as direct drilling plus it has the added advantage of uniform plants. However, they concede that the technique needs a capital outlay for the module machine which many small farms cannot afford. 4.3.1.6. Crop irrigation practices. Irrigation systems are an integral part of vegetable farms in the Thames Valley. All of the growers interviewed said they irrigated their crops during certain critical stages of the cropping period. There are 2 systems that are commonly used in the area: the sprinkler system and the drip system. The sprinkler system appears more popular because of its superior efficiency but it is more expensive s and has a more elaborate layout than the drip system. 71 About 75% of the irrigation requirement on a farm is devoted to transplanting or is applied at drilling time. It is a common practice in the area, immediately after planting, for the sprinkler system to be switched on to wet the trans planted seedlings and the soil for about 3-4- hours. This is done to keep the transplanting shock to a minimum, thus hastening the recovery of the seedlings for faster growth. Once the plants are established, irrigation is no longer needed except when the soil is too dry during hot spells in the summer months. Under such conditions, irrigation is necessary to maintain the moisture content of the soil, to encourage nutrient uptake by the plants or to activate soil incorporated granular insecticides. Some growers irrigate their crops just before harvest to keep them clean and looking fresh for the market. 4.3.1.7. Crop rotation practices. All the vegetable growers interviewed in the Thames Valley practice crop rotation, to avoid depleting certain essential elements and to reduce the accumulation of some disease innoculum by denying them their host plants. However, the rotational cycle and the break crops used, varies tremen dously from one farm to another. Table 10 shows the average number of years over which a break is practiced by the growers interviewed. For brassicas, the break given ranges from 1-3 years, while the break for lettuce is generally longer, ranging from 6 months to 6 years. In the case of leeks and onions, it ranged from 1-6 years. Brassicas have the shortest period averaging between 1.0 to 1.5 years. For 72 peas and beans, and carrots and parsnips, a much longer break can be employed because of the small acreage involved com pared to brassicas, lettuce and leeks. Among the 3 cata- gories of farm size, table 10 suggests that small farms practice the shortest break period in almost all vegetable groups compared to medium and large farms. Table 10. Break period (average number of years) given for various groups of vegetables in large, medium and small farms in the Thames Valley. Break period (av. no. years) for each group Vegetables group Large farm medium farm small farm Brassicas 1.5 1.4 1.0 Lettuce 1.7 1.7 1.25 Leeks and onions 3.4 3.2 1.80 Peas and beans 9.0 5.3 0.70 Carrotsan These findings confirm growers' earlier statements about the tight rotational practices they have been forced to adopt over the past several years. In small farms, crops like lettuce sometimes are repeated in the same plot at least once in six months. Such a situation has led growers to the general belief that current disease problems in brassicas, lettuce and leeks are mainly attributed to their inability to have as long a break as they would like for these crops. 73 4.3.1.8. Marketing outlets. An important reason for vegetable growing in the Thames Valley is proximity to the London wholesale market. Figure 20 shows the major marketing outlets for Thames Valley vege table produce. The fact that almost all growers are selling some produce through the wholesale market reflects this proximity to London. Besides primary wholesale, other major outlets include farm shops, secondary wholesale and super- mar ket s . Supermarket Primary wholesale Farm shop Secondary wholesale Pick-your-own Local retailers Local contract Standing crop Prepackers Processors Percentage of growers selling through each marketing outlets. Figure 20. Marketing outlets used by Thames Valley growers (Note that individual grower can sell through more than one outlet). 74 Table 11 shows the average percentage of grower's vege table produce sold through each marketing outlet. Over 90% of the vegetables produced in Thames Valley are sold through primary wholesale markets, supermarkets, farm shops and secondary wholesale markets. It is interesting to note that for large and medium farms, the primary wholesale market is their main marketing outlet with supermarkets as a secondary outlet. However, in the near future, supermarkets will probably become more important if the current decline of primary wholesalers continue . In contrast, the primary marketing outlet for small farm is their own farm shops. Table 11. Average percentage of grower's vegetable produce sold through various marketing outlets. Av. percentage sales of vegetable produce Marketing outlets Large farm medium farm small farm Supermarkets 27.0 21.0 4.0 Primary wholesale 53.0 67.0 26.5 Farm shops 1.0 3.0 42.0 Secondary wholesale 11.2 6.0 13.5 Pick-your-own 0.0 0.0 7.0 Local retailers 4.4 1.0 2.0 Local contract 3.0 0.0 1.0 Standing crop 0.0 0.0 4.0 Prepackers 0.0 2.0 0.0 Processors 0.4 0.0 0.0 75 The primary wholesale market is only a secondary marketing outlet. 4.3.2. Conclusion. The background information obtained in this survey would f provide a better understanding of what is actually hapening K on vegetable farms in the Thames Valley. Among the important findings of this part of the survey are: (1) most farms in the area are of medium size (101 to 299 acres); (2) majority of the growers are having more than 20 years of vegetable growing experience; (3) majority of the farms, grow between 20-30 different types of vegetables; (4) only some growers adopt module transplanting, mostly on large farms on a trial basis; (5) all growers interviewed irrigated their crops during certain critical stages of the crop period; (6) all growers interviewed practiced crop rotation and that they can only afford to allocate short break for brassica, lettuce and leek crops; and (7) over 90% of the vegetables produced in the area are sold through primary wholesale markets, supermarkets, farm shops and secondary wholesale markets. 76 CHAPTER 5 . INTERVIEW SURVEY OF THAMES VALLEY GROWERS: PESTS AND PEST CONTROL. 5.1. Introduction. As a continuation of the results and discussion of the interview survey introduced in Chapter 4, the following discussion focuses on pest problems on vegetable crops as perceived by the Thames Valley growers, and on what they are doing currently to overcome these problems. The purpose of this part of the interview survey, is to attempt to provide further information for the descriptive analysis of the vegetable pest system. It is hoped that certain key questions could be identified from this descrip tion, towards which, the next stage in research is focused. 5.2. Major Pest Problems of Vegetables in Thames Valley. When asked "what are the major insect, disease and weed problems you have in brassicas, lettuce, and leeks and onions?", growers mentioned 24 pests as problems in brassica crops. Out of these, only 5 pests (cabbage rootfly, Erioischia brassicae (Bouche); cabbage aphids (Brevicoryne brassicae, Myzus persicae and Macrosiphum euphorbiae); clubroot, Plasmodiophora brassicae ; caterpillars; and Alternaria leaf spot, A1ternaria brassicae) were considered common to all growers. Other pests listed under brassicas in Table 12, were comparatively minor and quite specific to 77 Table 12. Major vegetable pest problems in the Thames Valley area. Pests Responses Pests Responses Pests Responses (out of 20) (out of 20) (out of 20) Brassicas. 1. Cabbage rootfly 18 9. Downey mildew b 17. Pennycress 2 2. Cabbage aphids 16 10. Cutworms 3 18. Root aphid 1 3. Clubroot 15 11. Sheperd 1 s purse 3 19. Whitefly 1 b. Caterpillars 13 12. Couch grass 3 20. Pepper spot 1 5. Alternaria leaf spot 9 13. Kew weed 3 21. Barnyard millet 1 6. Flea beetles b 14. Slugs 2 22. Corn marigold 1 7. Croundsel b 15. Wire stem 2 23. Raw ripper 1 8. Pigeons b 16. Ring spot 2 Zb. Knot grass 1 Lettuce. 1 . Aphids 13 7. May weed 5 13. Cutworms 2 2 . Downey vvwt4ew] 11 8. Kew weed 3 lb. Rhizoctonia 2 3 . Internal tip burn 9 9. Sheperd's purse 3 15. Western yellows 1 b . Root aphid 9 10. Groundsel 3 16. Chick weed 1 5 . Butt rot 8 11. Big vein disease 2 17. Wire weed 1 6. Crey mold 6 12. Slugs 2 Leeks and onions. 1. Rust 15 7. Cutworms 5 13. Fusarium 1 2. White tip 12 8. Thrips 3 14. Bean fly 1 3. White rot 12 9. May weed 2 15. Corn marigold 1 b. Leaf blotch 10 10. Red shank weed 1 16. Couch grass 1 5. Onion fly 7 11. Downey mildew 1 17. Knot grass 1 6. Botrytis diseases 5 12. Onion draft 1 Other Vegetables. 1. Carrot fly 8 b. Rumanaria disease 2 7. Pseudomonas 1 (carrot & parsnips ) (spinach ) (fennel ) 2. Cut worms 3 5. Pea moth 1 8. Black bean aphid 1 (general) 3. Frit fly 2 5. Crown death 1 9. Leather jacket 1 (sweetcorn) (rhubarb) (general) 78 one or two individual farms. On lettuce, 17 pests were mentioned of which aphids (Nasonovia ribisnigri and Myzus persicae ), downey mildew (Bremia lactucae), internal tip burn, root aphid (Pemphigus bursarius), and butt rot were quite common to all growers. Grey mould (Botrytis cinerea ), May weed (Anthemis spp. and Matricaria spp.), were less common but could be serious problems in several farms. The rest of the pests listed under lettuce in Table 12 are of minor importance and specific to individual farms only. For leeks and onions, 18 pests were mentioned, of which rust (Uromyces spp . ), white tips (Phytophthora porri), white rot (Sclerotium cepivorurn), leaf blotch (Cladosporium allii- cepae) and onion flies (Hylemyia antigua) were quite common to all growers. Botrytis diseases, Cutworms (Agrotis segetum), thrips (Thrips angusticeps) and May weed were less common and the rest of the pests listed under leeks and onions in Table 12 are limited to one or two farms only. Carrot fly (Psila rosae) was the only pest mentioned quite often by growers as the major pest of the other vegetables. Other pests, such as the Rumanaria disease of spinach, are limited to one or two farms only. The next question growers were asked was "which would you say were the worst 2 pest problems overall?". About 30% of growers named clubroot as one of the 2 worst pest problems on their farm (Table 13). The next most problematical pest is 79 Table 13. The worst pest problems overall in Thames Valley. Pests Responses (out of 40) 1. Clubroot of brassicas 12 • Fungus diseases of leeks and onions 8 3. Cabbage aphids @ 5 4. Cabbage rootfly 4 3. Carrot fly 2 6. Alternaria leaf spot of brassicas 2 7. Caterpillars 1 8. Slugs 1 9. Butt rot disease of lettuce 1 10. Couchgrass 1 11. Groundsel 1 12. Corn marigold 1 13. Rumanaria disease of spinach 1 @ Eight out of 10 growers who did not mention an insect pest as one of their worst 2 problems overall, named cabbage aphids as their worst insect pest. Cabbage aphids as used by growers include all aphids found on cabbage (mainly B. brassicae, M^. persicae and IK. euphorbiae) as opposed to entomologist term, to mean only E3. br assicae. 80 the complex of fungus diseases associated with leeks and onions, especially white rot and white tips. The most problematical insect pest is the aphid complex, as found by Tait (1983) in her survey of brassica growers in Lincolnshire and Bedfordshire. However, cabbage rootfly is still a serious problem on a few farms in the area. Other pests listed in Table 13 were only serious in one or two farms only. Further evidence that growers view cabbage aphids as the number one insect pest problem in the Thames Valley area is obtained from the answers given to the question "which i$ the worst insect pest problem?", which was put to those growers who only mentioned disease pests as their worst two pest problems overall. About 80% of those questioned named cabbage aphids as their worst insect pest problems (Table 13). When growers were asked "Do you foresee any specific pest problems becoming more important in the next 3 years?", 40% of them named cabbage aphids as the pest that is likely to be important in the near future (Table 14). The reasons given by growers for this opinion are also shown in Table 14. 5.3. The Reasons Why Pests Become a Problem. We have already seen that shorter rotations are considered one of the main reasons why diseases, such as clubroot and fungal diseases of leeks and onions, are so serious in the area. What other reasons do growers give for some of the Table 14-. Future pest problems in Thames Valley. Pests Responses The reasons why its going to be important in future. (out of 20) 1. Cabbage aphids 8 Massive immigration from oilseed rape areas, poor control achieved in autumn period, and changing weather pattern favouring aphid abundance. 2. A1ternaria leaf spot 3 Spread from oilseed rape areas. of brassicas 3. Fungus disease of 2 Resistant to only chemical available for white tips, leeks and onions lack chemical control for white rot, tight rotation and giving more susceptible varieties. 4-. Cabbage rootfly 1 Resistant to current chemical control. 5. Caterpillars 1 Resistant to current chemical control. 6. Rumanaria disease 1 No satisfactory chemical control at present 7. Soil bourne diseases 1 Tight rotational practices. 8. Mercury weed 1 No effective chemical control at present. 82 pest problems they have?. Growers were asked "How or why have your worst 2 pests overall, become a problem?". Table 15 lists most of the reasons mentioned by growers for the 6 pests shown. 5.4-. Crop Protection Practices Currently Adopted by Growers. Among the worst pests listed under Table 13, only control practices for clubroot, fungal diseases of leeks and onions, aphids, cabbage rootfly, caterpillars, groundsel and couch grass will be discussed below. 5.4.1. Clubroot of brassicas. Clubroot control is based on 4 methods; liming, soil in corporated chemicals such as manganese, zinc and iron p dithiocarhamate (Trimanzone ), root dipping chemicals, such as p mercurous chloride (Calomel ), and resting the soil for a period of 2-3 years. Most growers with clubroot problems adopt a control strate gy using at least two different methods of control in combination. For example, the most popular strate gy (Table 16) is the combination of liming when soil pH falls below 7.0, a routine soil incorporated manganese, zinc and iron dithiocarbamate, plus the dipping of seedling roots in mercurous chloride solution before transplanting. Some p growers used thiophanatemethy1 (Mildothane ) instead of mercurous chloride as the third treatment, which in future maybe the only option available if mercurous chloride is with drawn from the market for reasons of its high mammalian toxicity • 83 Table 15. Some of the reasons given by growers why major pests have become a problem. g __ Pests____ Reasons why it became a problem Response 1. Clubroot of 1. .Crowing too many b rassicas for a long period of time 9 brassica on the same piece of land with short crop break period. 2. Difficulty in raising soil pH of a naturally acid 5 soil without incurring a high co st. 3. Lack of r e lia b le chemical c o n tro l. 2 k. Pathogen's ability to spread via farm machineries. 2 2. Fungus diseases 1. Tight rotational practice with short crop break 8 of leeks and onions p erio d . 2. Growing more susceptible varieties prefered by consummers. _ S 3. Changing weather pattern lately which seemed to 2 favour pathogens 3. Cabbage aphids 1. Late infestation in autumn when accessibility to the 10 crop by tractors to spray is limited, limited number of spraydays available, poor coverage of spray drop let obtained. 2. Poor percentage kill of cabbage aphids obtained in 8 autumn period by most of the chemicals curren tly used. 3. Cabbage aphids remain in the fie ld eventhough host 3 crops are absent Too in tensive cropping of host plants with very 3 tight rotation. 5. Sandy soil s tru ctu re with poor reten tion of granular 2 aphicide around the root zone area. *t. Cabbage ro o tfly 1. R esistan t to chemicals used cu rren tly such as 3 chlorfenvinphos. 2. Massive spill over from vast acreage of oilseed rape. 1 3. Abundance of cow parsley in the surrounding waste- 1 land which harbour the fly population. 5. Couch grass 1. U n satisfactory control achieved with most h erbicides. 2 2. The requirement of shallow tillage for direct drilling 1 which did l i t t l e damage to the couch grass rhizome. 6. Croundsel 1. Unable to use e ffe c tiv e herbicides because it also 1 retard the crop growth. The response is only for those growers who liste d the above pests as one of their worst pests o v e ra ll: each grower may give more than one reason for each pest. 84- Table 16. Current con tro l programmes adopted for clubroot by growers (descending order of popularity). Method of control Method of Time of Number of Decision application application application procedure 1. a ) Liming Soil incorporation Final cultivation 1/year If soil pH< 7 , ( b) Manganese, and iron Soil incorporation Final cultivation 1 /crop Routine di th i ocarbamate c ) Mercurous chloride Root dips Before planting 1 /crop Routine 2. a ) Liming Soil incorporation Final cultivation 1 /year If soil pH < 6.! b) Manganese, and iron Soil incorporation Final cultivation 1/crop Routine dithiocarbamate c ) Thiophanatemethyl Root dips Before planting 1 /crop Routine 3. a ) Liming Soil incorporation Final cultivation 1/year If soil pH< 7.1 b) Manganese, and iron Soil incorporation Final cultivation 1 /crop Routine dithiocarbamate U. a ) Liming Soil incorporation F inal cultivation 1 /year If soil p H < 7.i b) Mercurous chloride Root dips Before planting 1 /crop Routine 5. a ) Liming Soil incorporation Final cultivation 1 /year If soil pH< 7. b) Resting the soil for - - - If infestation a period of 2-3 years level is high Soil 6. Heavy liming only incorporation Final cultivation 1 /year If soil pH< 7. 7. Avoid planting brassicas on infested soil 85 The decision making problem for clubroot control, which differs from the physical problems caused by the clubroot disease itself, is relatively simple, based mainly on ex perience with the disease. Once clubroot occurs on a farm, the grower has a number of option he can adopt (Figure 21). Once a particular programme has been decided on, the execution of this programme usually becomes routine. 5.4.2. Fungal diseases of leeks and onions. Current control programmes for fungal diseases of leeks and onions adopted by growers arw- shown in Table 17. Captafol (Sanspor ) with only provisional approval for use against white tips and leaf blotch diseases, might be withdrawn in the future for reasons of high toxicity. Ipr odion (Rovral ) seems to be the most popular choice for controlling white rot disease, which is the most serious leek disease in the area, evenjthough growers' opinions were divided with regard to its effectiveness. As seen in Table 17, control programmes for fungal diseases of leeks and onions are mainly based on preventive chemical measures, triggered by the first occurrance of the disease. The decisions that a grower has to make are - the chemical to use, the method of application and the best time of application. Again, once those decisions have been made, the execution of the control package is on a routine basis. 5.4.3. Cabbage aphids. Control programmes for cabbage aphids adopted by growers Rest the infested soil < Liming when pH Mangenese, zinc and fall below 7.0 iron dithiocarbamate \Continue cropping / brassicas on ( Dazomet infested soil \ < Mercurous No liming when chloride pH is above 7. Seedling 1 s Jhiophanate methyl Mercurous No soil chloride treatment Thiobcndazole i + iodophor 00 ON chloride Thiob«ndazole + iodophor Figure 21. Decision tree for clubroot control Table 17. Current control programmes for fungal diseases of leeks and onions adopted by growers. Di seases Method ofcontrol Method of Time of application Number of Decision application application procedures 1. White a) Ipr odion a) Seed Before drilling 1 Routine rot dressing b) Drench 4 weeks after drilling or 2 weeks 1-2 Routine after planting b) Triadimefon Drench 3-4- weeks after drilling or 2 weeks 1-2 Routine after planting 2. Rusts Triadimefon Spray When rust began to appear 2-3 Inspection i 3. White a) Captafol Spray At 2 weeks interval starting 2 weeks 4-8 Routine _ Od after planting or 3 weeks after drilling b) Manganese, Spray At weekly intervals starting from 8-12 Routine , zinc and September to December iron dithio- carbamate 4. Leaf a) Benomy1 Spray At 2 weeks after planting or 3 weeks 1 Routine after drilling blotch b) Ipr odion Spray At 2 weeks after planting or 3 weeks 1 Routine after drilling c ) Captafol Spray At 2 weeks interval starting from 2 4-8 Routine weeks after planting or 3 weeks after drilling 88 differ greatly from one grower to another. There are 5 strategies' that can be identified: (1) a combination of persistent spray with short persistence quick kill spray (strategy no.l in Table 18); (2) a combination of persistent granules with a short persistence quick kill spray (strategy no.2); (3) combined aphid and cabbage rootfly control with a single granular application (strateyi^ no.3); (4) alternating of moderately persistent spray with persistent spray (strategy no.4); and (5) combined aphid and caterpillar control using a single spray (strate^ no.5). A combination of demeton-S-methyl with heptenophos sprays was the most popular choice with growers. Some prefered the complete prophylactic control programmes using demeton-S- methyl at the nursery stage and a routine application of granules at transplanting and heptenophos sprays close to harvest. The use of disulfoton + fonofos granules, to control both cabbage rootflies and aphids, as a subsurface band at planting or drilling time, although claimed to be effective by those growers who used it, was not popular with other growers because of the faster wear and tear caused to farm machinery by the abrassive action of the granules. On the other hand, quite a number of growers have opted combining aphid and caterpillar control by mixing an aphicide, such as demeton-S-methyl, with a larvicide such as tria- zophos, mevinphos, permethrin, f nvalerate or cypermethrin. Decision making in aphid control has never been easy for vegetable growers. A crude decision tree showing the Table 18. Current control programmes for cabbage aphids adopted by Thames Valley growers (descending order of popularity). Control Chemical Method of Time of application Decision No. of appli- strate qy used application procedure cation/crop 1 Demeton-S- Spray At 1 week after planting or 2 weeks Routine 3-4- methyl after drilling then at 3 weeks + intervals. Heptenophos Spray Oust before harvesting. Inspection 1 2 Disulfoton Subsurface At planting or drilling time. Routine 1 granules band Heptenophos Spray Beginning 8 weeks after granular Routine 2-3 application. 4- OD vo Demeton-S- Spray At nursery stage Inspection 1 methy1 3 Disulfoton + Subsurface At planting or drilling. Routine 1 fonofos band granules 4 Pirimicarb Spray As when necessary. Inspection 2-3 rotated with dimethoate 3 Demeton-S- Spray At 2 weeks after planting or 3 weeks Routine and 1-3 methyl + after drilling then as when nece inspection triazophos or ssary . fenvalerate or mevinphos or permethrin 90 sequence of decisions that can be taken by growers is shown in Figure 22. Inspection based treatments, however, do not involve the use of a quantitative threshold but are based simply on direct evidence of aphid's presence or indirect observable "cues", such as the persence of ladybirds, swallows feeding over the crop (on the adult aphids in flight persumably), or the occurrence of favourable weather patterns. 5.4.4- . Cabbage rootfly. Despite what appears to be a wide range of choice, in the Thames Valley area, cabbage rootfly appears to be uni versally controlled by chlorfenvinphos granules applied on a routine basis as a microband at planting or drilling time. Some growers however, prefer using chlorfenvinphos liquid, applied routinely as a soil incorporated drench just before planting, or as drill injection into the soil. Other options for controlling cabbage rootfly, described in Figure 23 are hardly used by the growers interviewed. 5.4- .5. Caterpillars. Caterpillars, such as cabbage white butterflies (Pieris brassicae ), diamond-back moth (Plutella xylostella ), and cabbage moth (Mamestra brassicae), are controlled with one of the following insecticides (in order of popularity); tria- zophos, mevinphos, fcnvalerate, permethrin, and DDT. The decision to apply or not to apply these chemicals are based on inspection or as schedule treatments. Most Direct drilling No treatment at ni . . . seedling stage Plant raising^ a y 'Treated at Routine treatment seedling stage < Crop inspections-^ based treatment's^ Combination of Str a te ^ n o . routine and crop inspection based S t r a t e n o . treatment < Strate<^ no. 3 < Strate ^ no. i Stratecjuj n o . 5^— Demeton-S-methyl vo + triazophos i Demeton-S-methyl + fenvalerate Demeton-S-methyl + permethrin Demeton-S-methyl + mevinphos Figure 22. Decision tree for the control of cabbage aphids 92 Bands of granules over plants^ Treat at 2 ’leaf stage Spot granules Drill Before mid April Soil drench Direct drilling^ No treatment .Drill after No treatment^^ mid April Bow wave band of granules Treat at drilling .Subsurface microband \ of granules \Drilled through band of granules Treat at 2 leaf Spot drench stage or at singling ‘Spot granules Granules band over plants No treatment ------if lifted before May 1st a Soil incorporated b ,Bare root transplanting^ Treat before ^''drench a (seedbed) sowing if lifted^ b after May 1st ^\Soil incorporated c granules c+f Treat at 2 leaf Transplanting ___ Bands of granules a stage if lifted over plant b after May 1st c spot granules d Block treated •«?-- . Peat block —* b transplanting < Block not treated! Module transplanting Module not treated \ Module treated a b c + f d d + f (after St. Light, 1982). 93 growers decide to spray when they observe either a substan tial number of adults flying around or evidence of feeding activity of caterpillars. It is common practice among growers to base caterpillar control on aphid control pro grammes, applying a mixture of aphicide and larvicidc in a single application. 5.4.6. Groundsel and couch grass. Growers were of the opinion that there is no satisfactory herbicide to control groundsel, some growers who use either R R propyzomide (Kerb ) or chlorpropham + fenuron (Herbon Red ) as a routine spray when groundsel is at the 2-3 leaf stage expressed displeasure at the resulting growth retardation effect on the crop. Some growers have opted only for hand weeding when the situation is bad. To control couch grass, a routine application of glyphosate (Roundup ) after the crop is harvested plus spot p treatment with alloxydim-sodium (Clout ) when necessary appears to be the only chemical option available to growers. However, some growers who have severe couch grass problems expressed some dissappoinment with the effectiveness of glyphosate or alloxydim-sodium, despite their high cost. 5.5. The Cost of Current Pest Control Programmes. To gain some idea of how much each grower interviewed spends on pest control programmes, each was asked the question "What percent of your annual turnover did you spend on crop protection on your farm last year (1982)?". On 94 Percent of annual turnover Figure 24. Growers annual expenditure on crop protection programme. average, the growers spent about 9% of their annual turnover on crop protection programmes with the range shown in Figure 24. It is also interesting to note that the average spending by the large farm growers were slightly less (7%) as compared to medium farm (10%) and small farm growers (9%), which may or may not indicate that large farm growers face less pest problems. 5.6. Growers' Crop Inspection Practices. Vegetable growers in the Thames Valley appear to inspect their crop regularly. However, the form of this crop in spection is usually a non systematic assessment of the general condition of the crop, as opposed to pest monitoring, which looks specifically at the size of the pest population. During inspection, the grower walks through the crop looking for certain "cues”, such as the presence of adult butterflies, swallow feeding on flying alatae aphids, flowering cow parsely, predators such as ladybird beetles, and the general absence of pests and pest activities in the crop. 95 On the average, crops are inspected at least once every 10 days (Table 19) for the period January to March. When cropping activities increase, during April to June, crop inspection also increases to one inspection every 3 days, and to daily inspection for some growers during the peak period July to September. From October to December, the frequency drops to about once a week. When growers were asked "Do you think spending more time for more accurate crop inspection would be beneficial to you?", 60% said yes (4-0% said no). The main reason given by those giving positive answers was that it would enable them to spot pest infestation early enough to allow more time to decide on appropriate control measures and at the same time would result in better timing of their treat ments. Those who said no, thought that their current inspection practice was sufficient enough to spot pest problems. 5.7. Problems Faced by Growers in Carrying out their Crop Protection Programmes. To identify some of the problems experienced by growers while carrying out their crop protection programmes, growers were shown a list of possible problems they may encounter and were asked the question "What would you say are the 3 worst problems you face?". Grower's resnonses to this question are shown in Table 20. The worst problem is being unable to spray at the best time, due mainly to unavaila bility of spraydays especially in autumn. On the other Table 19. Crop inspection frequencies adopted by growers. Number of growers adopting each frequency Average Period of the year daily 3/7 2/7 1/7 1/10 1/14 1/30 days frequencies 3anuary to March 0 0 0 9 2 5 4 1/10 days April to CJune 2 3 5 9 0 1 0 1/3 days Duly to September 7 1 5 6 0 1 0 1/2 days October to December 0 1 0 11 3 3 1 1/8 days 97 hand, the overwhelming majority of growers have no problem in recognising pests. But, when the growers were asked "Are there any other particular insects, diseases, and weeds that you find difficult to identify or are likely to confuse with other pest?", 45% of the growers said yes and cited recognising early disease symptoms as their main difficulty. Table 20. Major problems faced by growers in carrying out their crop protection programmes. Problems Response ______(out of 2 0) 1. Not being able to spray at the best time. 17 2. Delivering chemicals to the right target- 15 3. Learning and choosing the best control strate - 11 ies and the best available chemicals. 4. Allocating time to inspect the crops as 11 frequently as they would want to. 5. Achieving the desired level of control. 4 he. 6. Resistance to chemicals that proved to^effective 1 in the past. 7. Buying chemicals that they want. 1 8. Recognising the pests. 0 98 5.8. Grower's Opinions on How their Present Crop Protection Programmes Could be Improved. Opinions on how present pest control programmes could be improved varies from grower to grower. The most popular view was that improved spraying techniques showed most promise. In particular, there is room for improving coverage of spray droplets on the plants, performance in adverse spraying conditions, and reducing the volume of water used. Some growers suggest that more resistant varieties should be made available, especially for those diseases that cause serious problems at the moment. Although growers appreciate the difficulty involved in developing such varieties, they felt the emphasis on such work by existing research bodies was generally lacking, compared to varietal improvement for productivity. Another important opinion mentioned by some growers was finding an effective chemical for clubroot of brassicas, white rot of leeks, and butt rot of lettuce, for which, at the moment, there is no effective control. Other opinions on how current crop protection programmes might be improved are listed in Table 21. 5.9. Sources of Advice and Information Used by Growers. There are 6 major problems on which growers often seek advice or information; pest identification, method of control, choice of chemicals, spraying equipment, soil analysis, and choice of vegetable varieties. When given a card showing the Table 21. Grower’s opinions on how vegetable crop protection programmes could be improved. Opinions Response ______(out of 20) 1. Improved spraying techniques. 9 2. More resistant varieties made available to 4 growers• 3. Provide effective chemical control for club- 4 root, white rot, and butt rot. 4. More effective chemicals made available to 3 growers. 5. Employ one full time worker to concentrate on 2 pest management. 6. Specialize in a few profitable crops. 2 7. Longer rotation with cereals. 1 8. Increase the residual effect of granules. 1 9. Provide a working pest control manual to 1 growers. 10. Spending more time on crop inspection. 1 11. Urging chemical manufacturers to produce 1 chemicals specifically tailored for minor crop use. 12. More effective stickers made available 1 to growers. 13. Provide a technique for incoporating 3 to 4 1 chemicals together in a peat block. 100 various sources of information and asked "From which of these sources do you get information or advice for each of the following problems?", it was found that, for pest identifi cation and method of control, the majority of growers seek the advice of ADAS and an independent crop consultant in the area (Table 22). Some growers also referred to farm journals and magazines, the NVRS, chemical company repre sentatives, and some agricultural colleges. When it came to the choice of chemicals, almost all growers soLujM-the advice of an independent crop consultant. The other popular source is ADAS, which most growers regard as an independent source of advice. With regard to spraying equipment, the most popular source of advice is provided by agricultural machinery dealers and, to some extent, by farm journals and magazines, an independent crop consultant, and ADAS. Most growers send their soil samples for analysis to ADAS, although some growers sometimes seek the help of their fertilizer suppliers. For the choice of vegetable varieties, all were advised by seed companies. Sometimes, the growers obtained information from farm journals and magazines, ADAS and TVVG varietal trial grounds, NVRS, NIAB, or even from their fellow growers. When growers were asked whether they had any problems in obtaining or using information on specific aspects of crop protection, only 30% of them said yes (70% said no). This would indicate that the current information on crop pro tection provided by sources listed in Table 22 are more than 101 Table 22. Sources of advice and information used by growers. Problems Sources of advice and Information Response (out of 20) 1. Pest identification a) ADAS 16 b) Independent crop consultant 16 c) Farm journals and magazines h d) NVRS 2 e ) Chemical company representatives 1 f) Agricultural colleges 1 2. Method of control a) Independent crop consultant 19 b) ADAS 11 c) Farm journals and magazines k d) NVRS 2 e) Chemicals company representatives 1 3. Choice of chemicals a) Independent crop consultant 19 b) ADAS 6 c) Chemical company representatives 1 4. Spraying equipment a) Agricultural machinary dealers 19 b) Farm journals and magazines 7 c) Independent crop consultant 5 d) ADAS 5 e) NIAE 1 f) NVRS 1 9 ) Agricultural colleges 1 5. Soil analysis a) ADAS 19 b) Fertilizer suppliers 7 c) Independent crop consultant 5 d) Chemicals company representatives 2 6. Vegetable varieties a) Seed companies 20 b) Farm journals and magazines 9 c) ADAS and TVVG varietal trial grounds 8 d) NVRS k e) NIAB 2. f) Fellow growers 1 102 adequate for the majority of the growers. However, of those who said they had problems, half cited problems in keeping up to date with the latest available chemicals because they did not have time to digest most of the information they received. As a result, many rely entirely on their indepen dent crop consultant for advice. Other minor problems in volve translating scientific terms used in disease in formation booklets and pamphlets, and in understanding weed control recommendations. 5.10. The Problems of Cabbage Aphids. With the aim of providing a better understanding of what is actually happening on vegetable farms in the Thames Valley, a survey of 20 growers has been carried out. One important finding from the survey was the importance of cabbage aphids as a major pest in the Thames Valley area. According to growers, there are several reasons why cabbage aphids are such a problem in the area. The most important, is the occurrence of late aphid infestations especially in autumn t where the crops are mature, resulting in the following control problems: (1) limited opportunity for spraying without damaging the crop; (2) coverage of spray droplets on the plants; (3) unsatisfactory percentage kill of cabbage aphids achieved by most chemicals employed; and (4) limited availability of spraydays during this period of the year. The next step is to investigate some of these aspects. In particular, it would be useful to have information from farmers fields on changes in aphid population during the 103 cropping season together with their parasites and predators, to assess the percentage kill of aphids achieved during the autumn period, to assess the spray coverage achieved by growers at heading stage, and to check the calibration of grower's sprayers. SECTION III INVESTIGATION OF FACTORS DETERMINING THE PROBLEM OF CABBAGE APHIDS ON COMMERCIAL FARMS IN THAMES VALLEY 105 CHAPTER 6 POPULATION STUDIES OF THE CABBAGE APHIDS, THEIR PARASITES AND PREDATORS IN COMMERCIAL CABBAGE FIELDS. 6.1. Introduction. Cabbage aphids (J3. brassicae, M. persicae, and N[. euphorbiae) are one of the worst pest problems in vegetable crops in the Thames Valley, as has already been discussed in the previous section. Based on grower’s opinions (see Table 15, page?3), the following were thought to be the main reasons why cabbage aphids are a problem: (1) severe aphid infestation occurs in late autumn when cabbage crops are at the heading stage; (2) the percentage kill of aphids achieved with most chemical sprays is not high enough in that period; (3) the coverage of spray droplets on the cabbage plants is poor; and (4) poor timing of chemical sprays and ineffici encies of grower's spraying equipment. Other reasons men tioned by growers are less specific, for example, massive migration from oilseed rape, shorter rotation and growing too much brassicas. This section is devoted to the investigation of some of these factors. To obtain information, the cabbage aphids, their parasites and predators were monitored on commercial cabbage crops, the percentage kill of cabbage aphids under commercial operations was assessed, the spray droplets coverage on cabbage plants obtained commercially was deter mined, and the calibration of growers' sprayers was checked. 106 The aims, material and methods, results and discussion of these observations are presented in this section which is «2S divided into two chapters. Chapter 6 discuss^the biology of aphids and population studies of the cabbage aphids, their parasites and predators in commercial cabbage fields, as and Chapter 7, discuss^the rest of the studies conducted. 6.2. Biology of the Aphids. Aphids (Order Homoptera: Family Aphididae) form one of the most important families of insects economically, especially in temperate regions where they are the chief vectors of plant viruses (Tones and Tones, 1964). On brassicas only three species of aphids are of importance (Ahmad, 1970). These are the cabbage aphid (Brevicoryne brassicae), the peach-potato aphid (Myzus persicae ) and the potato aphids (Macrosiphum euphorbiae). Among the three, per sicae is outstanding in distribution, in host range, and as a pest which causes not only direct damage but is able to transmit over 100 virus diseases of plants in about 30 different families, including many major crops such as beans, sugarbeet, brassicas, potatoes, tobacco and citrus (van Emden ej: j^, 1969). On brassicas, however, brassicae is more important, because it is an aphid with a relatively narrow host range, spending all its life on various species of brassicas and other Cruciferae without any woody winter host (Tones and Tones, 1964). The general biology of aphids are discussed in detail by Kennedy and Stroyan (1939), and Tones and Tones (1964). 107 6.2.1. The cabbage aphid. The biology of the cabbage aphid, B. brassicae. has been described in detail by many authors (Herrick and Hungate, 1911; Bonnemaison, 1951; Hafez, 1961; Hughes, 1963; Jones and Jones, 1964; Raworth, 1984). According to ADAS (1979), cabbage aphid is a common and destructive pest which in some seasons causes serious losses to growers of broccoli (Brassica oleracea var. botrytis), oilseed rape (Brassica napus var. oleifera), and swede (Brassica napus var. napobrassica). A heavy attack on young plants may check growth beyond recovery. Infestation on mature crops spoils the crop by contaminating them with cast skins and honeydew, providing a good medium for growth of fungi (Jones and Jones, 1964). Moreover, cabbage aphids are able to penetrate into the hearts of cabbage, Brussels sprouts and cauliflower, thus rendering the produce unsaleable. Beside this direct damage, cabbage aphid is also a vector of two viruses, cauliflower mosaic and cabbage black ringspot (Broadbent, 1957, 1960; Day and Venables, 1961). As in many other aphid species, exploitation of the resources of short-lived host plants w achieved by a complex system of polymorphism, each form having a different function (Hughes, 1963). The predominant form is the degenerate, wingless, parthenogenetic, viviparous female (aptera), which exploits the food supply to the limit and produces young as fast as possible. The apterae are bluish-grey, covered with fine white mealy powder and are usually found in colonies on the underside of the leaves. On its abdomen, black spots or 108 bars are present, and the legs are dark. Its cauda is short and the cornicles are about the same length but swollen in the middle with the mid-frontal of the head curved outward, shown in Figure 25 (Cones and Jones, 1964). The nymphs produced by the apterae undergo 4 nymphal instars before becoming adults. A variable proportion of these adults develop wings (alata). These are also parthenogentic, vivi parous females but their progeny are characteristically wingless (Hughes, 1963). Thus the alatae, which have a black head and antennae, dark thorax, green abdomen with black markings, brown wings and veinations, and short, dark cornicles and cauda, are well adapted to disperse the species and to colonize the available host plants in an area. The life cycle of cabbage aphid is outlined in Figure 26. The overwintering eggs begin to hatch in late February and by the end of April all hatchings are completed (ADAS, 1979). The nymphs hatched from the overwintering eggs (fundatrices) first feed on the leaves and shoots, but by May, they are mostly feeding on the flower heads of brassica crops either grown specifically for seed or neglected crops that have been left to flower (Jones and Jones, 1964). In mid-May alatae appear and migrate to the newly planted host crops. They are at first inconspicuous on the new crop, but in favourable weather rapid embryonic and nymphal development results in the telescoping of generations which is the main factor in speeding numerical increase. In summer months several generations are produced, including the summer migrant populations which emigrate to seek and infest other newly 109 Figure 25. Diagram showing the head (a), cauda (b) and cornicles (c) of Myzus persicae (A), Brevicoryne brassicae (B), and Macrosiphum euphorbiae (C). 110 ® ECG winter host (spindle tree, sterile guilder, rose, and standing brassicas) November-February OVERWINTER OVIPARAE & MALES FUNDATRICES (sexual reproduction) winter host Winter host March September-October t AUTUMN COLONIES ADULTS & NYMPHS SPRING COLONIES return migrant during mild winters or (apterae) (gynoparae) in warmer part of England winter host brassicas, lettuce, November-February April potato and etc. OVERWINTER August T SUMMER MIGRANT SUMMER COLONIES SPRING MIGRANT brassicas, lettuce, (apterae) <— winter host potato and etc. brassicas, lettuce, May Duly potato and etc. June ® EGG stalks of brassicas November-January OVERWINTER OVIPARAE & MALES FUNDATRICES (sexual reproduction) brassicas Overwintering late February-March brassicas October t AUTUMN COLONIES ADULTS & NYMPHS SPRING COLONIES (qynoparae) during mild winters brassicas brassicas or warmer part of April September Eg 1 and October-March OVERWINTER SUMMER MIGRANT SPRINC HICRANT Drassicas brassicas July-August April SUMMER COLONIES brassicas 3une Figure 26. Life cycle of peach-potato aphid(A) and cabbage aphid (B). Ill planted crops. In cold climates such as in most parts of Britain, the true sexual forms of the aphid (sexuales) are produced towards the middle of autumn. After mating, the true sexual females (oviparae) lay their eggs, which are small, elongate, and shiny black in colour on the stems and leaves of the host-plant. The cabbage aphid overwinters in the egg stage. However, in mild districts or mild winters, small colonies of asexuals (apterae, alatae and nymphs) live on well into the winter months. In the south and south-west of England few eggs seem to be laid in most years (ADAS, 1979) and it is believed that the aphids generallv overwinter there as active stages in the winter sheltered parts of brassica plants, including various weeds such as charlock (Sinapis arvensis), shepherd's purse (Capsella bursa pastoris) and wild radish (Raphanus raphanistrurn). Intensive studies of aphid population dynamics have already been conducted for the cabbage aphid (Hughes, 1963; and Gilbert, 1968; Raworth ejt 1984; R^ujcrfe,,). According A to Raworth e_t a_l (1984), the development time of cabbage aphid from birth to adult was 126 degree-days (based on maximum & minimum temperatures) above a threshold of 6.7°C ( °D 6.7), the fecundity averaged 40.7 nymphs/female which did not vary with temperature or frequency of nymphs removal. In terms of longevity, 100% of the aphids survived at least upto the reproductive peak (73°D 6.7) but some survived upto 600°D 6.7. However, in the field in British Columbia, Canada, the development time, fecundity and longevity of cabbage aphid were 1.33, 0.66 and 1.2 times the respective laboratory 112 value above. 6 .2.2. The peach-potato aphid The peach-potato aphid is cosmopolitan in distribution and in host plant range (Bodenheimer and Swirski, 1957; Bunzli and Buttiker, 1959; Cottier, 1953; Eastop, 1948; Ossiannil sson, 1959; Palmer, 1952 ; Smith £t aj., 1958). It rarely occurs in populations sufficiently dense to cause direct injury by its feeding but is able to transmit over 100 virus diseases on 30 different plant families including cauliflower mosaic and cabbage black ringspot (Kennedy £t al, 1962). The aoterae of peach-potato aphid are medium-sized, pale green or pink with a pair of well marked frontal tubercles which project inwards (Figure 25). The cornicles are long and cylindrical and maybe slightly swollen. The cauda is prominant and half or less than half the length of the cornicles (Jones and Jones, 1964). The biology of peach-potato aphid has been summarised by Broadbent (1953) and van Emden e_t a_l (1969) dealt with its ecology in great detail. Figure 26 outlines the life cycle of peach-potato aphid. In temperate regions with cold winters survival may depend primarily on production of sexuales, requiring the seemingly hazardous autumn flight to primary hosts, and the production of eggs late in the year when adverse weather could be harmful. Bonnemaison (1951) showed, in France, that the production of gynoparae (winged forms 113 which migrate to winter hosts and produce the egg laying female, or sometimes referred to as autumn migrant) is in fluenced by daylength, the critical photoperiod being 12.5 to 14- hours. The winter host range is limited to certain Prunu s spp, such as Prunus persica, the peach. In. regions of mild winters many M^. persicae overwinter as active stages on a wide range of herbaceous and brassica crops, weeds (shepherd's purse, fat hen, charlock and black nightshade), chitting potatoes, mangels and beet in store (ADAS, 1983). Here pathenogenesis is the sole form of reproduction which is enhanced by agricultural and horticul tural practices that provide nutritious crops throughout the year (van Emden et^ £1^ 1969). The critical low temperature for survival of apterae of M_. persicae is below 2°C, the level depending on developed cold hardiness (Adam, 1962). Heie and Petersen (1961) suggest that M^. persicae apterae can survive when the mean temperature remains above 4°C during the three coldest months. However, temperatures above 28°C prevent f4. persicae development (Bald and Norris, 194-3; Barlow, 1962; Bodenheimer and Swirski, 1957: Fenjves, 1945; Heinze and Profft, 1940; van der Plank, 1944). Injfact, its population almost disappears when the mean daily maximum temperature reaches 32°C (van der Plank, 1944). 6.2.3. The potato aphid. The potato aphid occurs mainly on potato, roses, tomato, chrysanthemum and lettuce (3ones and Oones, 1964). 114 Occassionally, it is found on brassicas (Ahmad, 1970), usually in the early part of the crop period. Air alata is a relatively large, green or pink aphid with a long body and a dark stripe along the middle of the thorax. The cornicles are long, green, with dark apices (Figure 25). The cauda is knob-like and about half the length of the cornicles. The pink alata has a yellowish head and a yellow- pink thorax. An aptera is similarly green or pink, and is longer (4.0-4.1 mm) than an alata. The nymphs are also yellowish green or pink in colour and are normally covered with a powdery coating of wax (Jones and Jones, 1964). This species may overwinter as eggs but usually hiber nates as apterae on herbaceous hosts or chitting potatoes; e s it migratesto potato^ in May and June. In years of heavy infestation there is often a dispersed migration in late July, but there is rarely a noticeable autumn migration. On potato, M_. euphorbiae infestations are widespread but only reach substantial numbers in occassional years. Unlike the other leaf aphids, there is a marked tendency for this species to multiply on the flowers and near the tips of the shoots, where, if very numerous, they can cause "false top roll" (ADAS. 1983). 6.3. Population Studies of the Cabbage Aphids, their Para sites and Predators in Commercial Cabbage Fields. The aim of this series of observations was to investigate the development of cabbage aphids, and their parasite and predator populations in cabbage fields with and without 115 chemical control. The outcome of this study would either confirm or dispel growers' claims that severe aphid infesta tions in late autumn are one of the reasons why aphids were such a problem in the area. At the same time, it would reveal the effects of parasites, predators, and chemical control on the aphid population in relation to growth stages of cabbage crops. 6.3.1. Material and methods. The observations were conducted on 5 sites. Four of these were located on commercial vegetable farms in the Thames Valley (site 2, 3, 4, 5) and another site (site 1) was at Silwood Park (Pound Hill). The size of the plots at sites 2, 3, 4 and 5 was 0.016 hectares each and site 1 was 0.014 ha. Site 2 and 3 were planted with cabbage variety "Market prize" on June 25 and Duly 4, 1983, respectively. Site 3 was planted with a mixture of cabbage varieties, "Stonehead" + "Market Prize" + "Vela" on July 3, 1983. Site 4 was planted with cabbage variety "January King" on June 26, 1983. At site 1, the plot was planted with cabbage variety "Original" at 0.61 x 0.61 metre spacing. Aphid populations in plots located at the 4 commercial farms (site 2, 3, 4, 5) were chemically controlled by the growers themselves. The plot at Silwood Park (site 1) was left untreated. Direct counts of live aphids, aphid mummies, parasites and predators on 20 plants per plot were carried out at weekly intervals begining one week after transplanting. The sampled plant is obtained at every 10 paces while walking 116 down the rows until 20 plants, in total, from among all the rows were sampled. The live aphids were divided into alatae (winged female), apterae (wingless female) and nymphs (the immature stages - instars 1 to 4). To count fresh aphid mummies, these had to be distinguished from old mummies by their milky white colour, compared to brown old mummies. Mummies were removed immediately after they had been counted. In the case of parasites, only Diaretiella rapae (McIntosh) was counted. The predators counted include adults and larvae of coccinellid beetles, larvae of syrphid flies, spiders, adults and nymphs of anthocorid bugs. The decision to use direct counts on whole plants, even- though very time consum ing, was because it gave by far the most accurate assessment, compared to the "three leaf" method (Church and Strickland, 1954-( or the "six leaf" method (Chua, 1976). Chua (1976) compared the "six leaf" method with "whole plant" method and concluded that the "six leaf" estimate was far from being accurate. As a result he aban doned the "six leaf" method infavour of the "whole plant" method. Besides direct counts, yellow water pan traps were used to estimate the aerial population of alatae and adult para site and predators. Five yellow water pan traps of 17 cm in diameter (one each at the four corners, 10 feet from both edges of the plot, and one at the centre of the plot) were fixed to wooden posts at a height of 20 centimetres from the ground. Each of these traps was filled with a weak detergent 117 solution and replaced weekly after all of the insects trapped during the previous week were collected and brought to the laboratory for identification and counting. Only 3 species; B. brassicae, M. persicae and f4. euphorbiae were identified and counted separately. The rest of the aphid species were lumped together and counted as other species. For parasites only adult j). rapae was identified and counted. In the case of predators, coccinellid beetles, syrphid flies, spiders, anthocorid bugs and lacewings were identified and counted. 6.3.2. Results and discussion. 6.3.2.1. General trends in the development of live aphid and mummy population in a cabbage field without chemical control. Aphid populations in the field were calculated as the dri'tCu/ueftc mean from the 20 plants sampled. In site 1 at Silwood Park where cabbages were left untreated, the aphid populations (defined here as all live aphid whether para sitized or not) reached their highest peak of 334.50 aphids per plant (average of 20 plants) in October, i.e 14 weeks after transplanting (WAT) as shown in Figure 27. The aphid population trend, generally followed a bimodal curve as observed by Hafez (1961), Heralky and El Ezz (1970), Akinlosotu (1973) and Chua (1976). The migratory alatae population started to come in as soon as the cabbage seedlings were planted into the field at the end of dune. As a result, the initial peak was recorded MEAN NUMBER OF LIVE APHIDS AND MUMMIES/PLANT (N=20) iue 7 Ppl rn o lv ahd ( aphid live of trend n o i t Popula — 27. Figure JUL— JUN“*1«— um (P-O i a utetd cabbage untreated an in ) O - P ( mummy vrey a st 1 Slod . ) k r a P (Silwood 1 site at ) " l a n i g i r O " (variety ----- G U A ------118 ► ► ---- P E S ------>!◄ ---- ♦ " " # OCT C O ) and ) ------►! 119 as early as 2 WAT. This is due to the high aerial popula tion (see Figure 38, page 141) of migratory alate (in June and July) from the crops planted in April or May in the surroun ding areas. Aphid number recorded at the initial peak was 103.30 aphids per plant. After the initial peak, the aphid population then de clined to less than 10 aphids per plant during the period 4 to 6 WAT (late July to early August). The decline in the aphid population could be due to several reasons. One of the reasons is probably due to the death or mummification of some of the migratory alatae, after they have settled on the crop, as opposed to the emigration of these newly arrived alatae out of the crop again, or the shift to alatae in reduced reproduction at this early stage of establishing a colony. From then on, the aphid population increased slightly to what appeared to be a second peak at 8 WAT (late August) before a slight decline at 9 to 10 WAT. By this time, cabbages were already heading. This cessation in the rapid rise of the aphid population could be due to the effects of predation by syrphid larvae which are abundant in late August and early September (see Figure 36 & 37, page 137 and 138). Begining at 10 WAT, the aphid population again increased K rapidly to its highest peak at 14 WAT (early October). The aphid number recorded at this peak was 554.50 aphids per plant. This rapid population increase maybe attributed to 120 high numbers of apterae, giving birth to high numbers of nymphs, and also the decline in the J). rapae population (see Figure 34, page 133) and in population of some predators (see Figure 36, page 137). Following 13 and 16 WAT, the population declined very rapidly from 554.50 aphids per plant to less than 10 aphids per plant. This was probably due to the emigration of alatae generated within the crop to other crops nearby, death of apterae (old age), shift to alatae in reduced reproduction, and also the rotting of leaves, especially the wrapper leaves which previously supported large aphid colonies. The cabbages were harvested at 17 WAT. The trend in the number of mummies closely followed that of the live aphids, with 2 main peaks. The earlier, and usually a smaller one, was equivalent to the immigrant alate peak as some of these alate were no doubt already being parasitized prior to take off for migratory flight (Chua, 1976). The number of mummies recorded at this peak was 21.25 mummies per plant (about 20% of the live aphid numbers) The later peak was equivalent to the main aphid peak where the number of mummies recorded was 33.20 mummies per plant (about 6% of the live aphid numbers). It was obvious that mummy population peaks occurred at the same time or a week later than the live aphid's population peaks. 121 6.3.2.2, Trends in the number of live aphids and mummies in commercial cabbage fields with chemical control. Aphid populations at sites 2, 4 and 5 (Figure 28, 30 and 31 respectively) reached their highest peaks of 366.25, 1,620.00 and 530.25 aphids per plant respectively in Septem ber or October. Site 3 was exceptional in having a low aphid population and a very short crop period (Figure 29). From these results it is clear that the level of infestation and rate of population growth varies tremendously from one site to another due to the control strate gies adopted, cabbage varieties grown, and the different levels of immi grant parasite and predators present. However, the popula tion trends in all of the four sites still followed the bimodal curve. As in the untreated situation, migratory alates were found as soon as the cabbages were transplanted into the field at the end of June or early 3uly. Some of the growers (site 3, 4- and 5) began spraying as early as 2 or 3 WAT, i.e as soon as they saw aphids settling on the cabbage plants. As a result the decline in the aphid populations, which occurred even without chemical sprays, was much faster and more severe on treated sites compared with the untreated site. However, this decline was only sustained by repeated spraying in the following week or two (site 2 and 3). Without the successive sprays, aphid numbers started to climb up again in the following week (site 4- and 5). It may be that early sprays for aphids at this level of infestation are unprofitable. MEAN NUMBER OF LIVE APHIDS AND MUMMIES/PLANT (N=20) JUN-*H iue 8 Pplto ted f ie pi ( ) and ) # ( aphid live of trend Population 28. Figure ------JUL ----- farm) mummy mummy vrey fakt rz" t ie (commercial 2 site at Prize" ,fMarket (variety ------0 * 0 ( G U A ) n nhmcly rae cabbage treated anchemically in ) ----- 122 ------SEP E S ----- ►i^ --- C—------H OCT— 123 i 1 l 200 150 100 50 0- igure 29 Population trend of live aphid ( #~# ) and mummy ( 0 - 0 ) in a chemically treated cabbage (variety "Market Prize") at site 3 (commercial farm). MEAN NUMBER OF LIVE APHIDS AND MUMMIES/PLANT (N=20) iue 0 Pplto ted f ie pi ( ) and ( aphid live of trend Population 30. Figure mummy ( 0*0 ) in a chemically treated cabbage cabbage treated ( chemically a ) in 0*0 mummy t ie (omril farm) (commercial 4 site at (variety "Stonehead" + "Market Prize” + "Vela" "Vela" + 1 Prize” "Market + "Stonehead" (variety - 124- 125 3 0 0 I II II 2 5 0 200 150 100 50 0 . 6L 8L 10L 13L 16L 18L HG H1 H2 H3 IN-M^------JUL------■ ■ AUG ■■ ■ ------■ ► ■■ !«------SEPOCT------M igure 31• Population trend of live aphid ( ) and mummy ( 0-0 ) in a chemically treated cabbage (variety "January King11) at site 5 (commercial farm). 126 Most growers appeared to be quite successful in sup pressing the populations in the middle of the crop periods i.e, 5-9 WAT (August). However, following 9 WAT the aphid population increased very rapidly to the highest peak in September or October, apparently regardless of the extent of spraying (Figures 28 and 31). The time of occurence of this peak however, varies from farm to farm and year to year probably due to the different cabbage varieties planted, dates of planting and for other unknown reasons. For a short maturity like "Market Prize" (Figure 29), the peak occurred 11 to 12 WAT (middle of September). On the other hand, the long maturity varieties such as "Oanuary King" (Figure 31), the peak occurred at 14 to 15 WAT (middle of October). From the standpoint of aphid control strate gies, gran ular aoplication appears to be superior to spraying injterms of its effectiveness in reducing aphid numbers under autumn weather conditions. This is clearly shown by the results for site 3 (Figure 29) compared with sites 2, 4 and 5 (Figure 28, 30 and 31 respectively). By putting in granules at 3 WAT, the aphid numbers at site 3 were maintained below 100 per plant through to harvest. However, this may or may not be the case until further comparisons are made. At site 2, 4 and 5, the aphid numbers rose to more than 500 per plant before the last harvest despite spraying being carried out even after the 10 WAT. The implications of these findings are that aphid in festations appear to be most critical from 9 WAT onwards, 127 in terms of aphid control, because of the high aphid popu lation and the potential crop lost at that stage coincided with the month of September to October. During this part of the crop period, aphid populations increased very rapidly, probably due to the large numbers of newly developed apterae giving birth to large numbers of nymphs and to the reduced pressure imposed on this population by the declining parasite and predator numbers. On the other hand, growers' aphid control programmes appear to be directed mainly at the first half of the crop period (1 to 8 WAT). The lack of control from 9 WAT onwards i.e, where it appears to be most needed, could be attributed to several reasons, for example the un availability of spraydays (which appears to be much less compared to summer months), the risk of mechanical damage to crops, growers too occupied with other spray job$ or un- a awreness of the increasing aphid build up in the crops. A. The trend of mummy numbers under treated conditions was similar to the untreated, with two main peaks. However, after the first peak, the number of mummies were greatly suppressed, i.e more suppressed than live aphid populations by the chemical control activities. However, on the whole, the number of mummies still followed closely that of live aphid numbers, with the second peak usually much higher than the first (Figures 30 and 31) in response to several fold increases in live aphid numbers. 128 6.3.2,3. Changes in the percentage composition of alatae, apterae and nymphs in relation to crop stages and chemical control activities. The percentage composition of alata, aptera and nymph population varies according to crop stage and insecticide treatment. Without treatment (site 1), nymphs were pre dominant throughout the crop period (Figure 32). For the earlier half of the crop period (1-8 WAT), nymphs consti tuted more than 70% of the aphid population but gradually declined to below 30% in the latter half of the crop period (9-16 WAT). The opposite was true for apterae, which began to appear only at 3 WAT and gradually increased to about 40% of the aphid population towards the end of the crop period. On the other hand, alatae constituted about 30% YL of the aphid population at the begining and towards the A end of the crop period, remaining at below 5% level in the middle of the crop period. Immediately after transDlanting, the migrant alatae started to come in and settled on the plants, giving birth to nymphs. Within one week, every adult that had landed and settled had a colony of at least 14 nymphs. As more alatae settled, the number of nymphs increased and began to develop into the apterous form. As nymphs matured to the apterous form, they inturn gave birth to more nymphs, thus enlarging i the aphid colony. Towards the middle of the crop period, the settling of migratory alatae began to decline, so that the majority of newborn nymphs were generated by the apterae of the established colony. Towards the end of the crop 129 JUN-frH------JUL------EM<1------AUG ------SEP ------>l<------OCT ------ Figure 32* Percentage composition of alatae ( 0—0 ), apterae ( ) and nymphs (OP) in an untreated cabbage at site 1 (Silwood Park). 130 period, most of the established colonies were large, re sulting in overcrowding and a decrease in nutritive value of the leaves where these colonies were sited. Probably for this and for other reasons, nymphs began to develop into the alatae form, as a result, the proportion of nymphs in a population began to decline. At the end of the crop period, the percentage of nymphs again increased due to the emigration of alatae and also the decline of the apterous f or m. In sites 2, 3, 4- and 3 (Figure 33) where spraying was carried out, the percentage composition of alatae and nymphs varied not only according to crop stage but also in response to the effects of sprayings. At the early stage of the crop, whenever cabbages were sprayed, the number of nymphs declined sharply in the following weeks. It appeared that the alata population was not affected by the spray evenjthough alatae are as susceptible to the chemical spray as the nymphs. This is due to the settling in of new immigrant alatae in the following week after the spray. However, towards the middle of the crop period when alata?. immigration had de clined, there was an upsurge in the percentage composition of apterae whenever spraying was carried out which might suggest that nymphs are more susceptible than the apterae. 6.3.2.4-. Field populations of Diaretiella rapae and their rate of parasitism. Field counts of J). rapae were generally low at sites with or without chemical control. Since the parasites are very MEAN % OF ALATAE, APTERAE, AND NYMPHS/PLANT iue 33. Figure Percentage composition of alatae, O—O ; apterae,A-&; and nymphs, nymphs, and apterae,A-&; ; O—O alatae, of composition Percentage o a hmcly rae cbae t ie 2 3 4, n 3. 4-, 3, and 2, sites at cabbage treated chemically a □ on , 131 132 mobile, many may not be accounted for in the direct counts. Nevertheless, direct counts do provide a good indication of J). rapae1 s presence in the field at various crop stages. In the untreated plot (site 1), £. rapae started to immigrate into the plot together with the aphids and reached a peak of 3.10 per plant at 2 WAT (Figure 34). Then the population began to decline probably due to the declining aphid numbers, to zero level 5 to 6 WAT (early August), u However, begining 7 WAT [). rapae population increased again k gradually to a second peak of 1.03 per plant at 12 WAT (mid-September) which was much lower than the earlier peak. Towards the end of the crop period, the population declined again to less than 0.5 j). rapae per plant one week before the final harvest of the crop. Similar patterns of J). rapae population development were observed in the chemically treated plots at sites 2, 3, 4 and 5 (Figure 33). However, the populations in these treated plots were much more variable than that of the untreated plot due to the effects of chemical treatments. The percentage of mummies out of the total number of aphids counted per plant indicates roughly the rate of para sitism by JD. rapae in the field as only those parasites in the larval or pupal stages were included. Evenlthough it may have underestimated the actual percentage parasitism which include the egg stage of the parasite, it was thought to be quite sufficient for our purposes. MEAN NUMBER OF D IA R E T IE L L A RAPAE/PLANT (N=20) ~n H* CQ C c d U) ■p * T) H* r t "0 c d 3 pr O *-1 CD ■ a 7? CD c 3 3 i—• CD C D c CD r t 3 P H» r t O •1 "O 3 f D CD CD >1 r t r t 0 0) C D CD Cl =3 3 r t a . O C D 133 CD tQ o cr CD -+> cr 0) -o o IQ 01 H* CD at CD•I CD 01 CD r t H» r t r t H* 01 H* CD H* 0) r t 3 CD 0 CD •1 CD CD "O cn ■a CD H* 3 * CD M * a . O o I Q. 1 CD 3 CL MEAN NO. OF D. RAPAE/PLANT (N=20) iue 5 Pplto ted f . aa ( ad t rts f aphid of rates its ) and ( rapae D. of trend Population 35. Figure parasitism parasitism ) + — * ( n abg a st 2 3 4-, 3, 2, 3. site and at cabbage on MEAN PERCENTAGE PARASITISM/PLANT (N=20) 135 Generally, the percentage parasitism of aphids was high Yl at the begining and towards the end of the crop period with K lower values in between (Figures 34 and 35). The first peak was recorded in early 3uly, about 3 to 4 WAT, while the second peak occurred around mid-October, i.e 1 to 2 weeks before the final harvest. The earlier peak was much higher (20-40%) than the later peak (5-15%) which in some chemically controlled plots seemed to be absent (Figure 35, sites 2 and 5). The much higher earlier peak can be attri buted to higher numbers of £. rapae in the field against much lower aphid numbers (see Figure 34 and 35), and also to the fact that some immigrant alatae are already being parasitized before settling in the plots. The second peak which was absent in some of the treated plots, is due to \ decreasing ratio of mummies to live aphid numbers evenithough the actual number of mummies per plant was highest at that time (see Figures 28, 30 and 31). Those results seem to indicate that the role of aphid parasitism by JD. rapae appeared to be quite substantial in suppressing the aphid population in the early part of the crop period (i.e in Duly). However, in the second half of the crop period (especially in September and October), aphid parasitism by _D. rapae is negligible. The percentage field parasitism of El. brassicae has been studied by George (1957), Hafez (1961), van Emden (1963), Daiber (1971a and b) and Chua (1976). Hafez (1961) found parasitism in the first generation of immigrant alatae could be as high as 80 to 90%, but this was the only instance of 136 such a high rate of parasitism being observed. Generally, a he recorded.much lower rate of parasitism and concluded that K only a small fraction of the aphid's motality was caused by parasites and therefore that this is not the main factor causing changes in aphid populations. George (1957), van Emden (1963) and Chua (1976) also recorded low field parasitism, the respective maximum levels were 14-%, 30% and 56%. They and others (Petherbridge and Mellor, 1936; Otake, 1961; Oatman and Platner, 1973) concluded that the primary parasite could not control the aphid population. The unanimous reason given (George, 1957; Hafez, 1961; Sedlag, 1964; Paetzold and Vater, 1967; Oatman and Platner, 1973) was that the effectiveness of the primary parasites was reduced drastically by secondary parasites such as Alloxysta brassicae (Ashmead), Asaphes vulgaris Walker, A. suspensus (Nees), and many others. 6.3.2.5. Field populations of predators. Five groups of predators were included in the weekly direct counts of 20 cabbage plants; namely, adult and larvae of coccinellid beetles, larvae of syrphid flies, spiders, larvae of lacewings, and adults and nymphs of anthocorid bugs. Their populations are plotted in Figure 36 and 37 as the geometric mean of 20 plants sampled at weekly inter vals. At site 1 (Figure 36), the untreated plot, predator populations were generally higher than those of sites 2, 3. 4 and 5 (Figure 37) which were chemically treated. Syrphid MEAN NO. OF COCCINELLIDS, SYRPHIDS AND SPIDERS/PLANT (N=20) iue 6 Fed ouain rn o coccinellids of trend population Field 36. Figure n n nrae cbae il a st 1 as 1 site at field cabbage untreated an in eemnd y h drc cut method. count direct the by determined ) srhd ( , n sies ) ( spiders ), and ( syrphids ), ( 137 iue 7 Fed ouain rn o peaos ccield, srhd, - ; and B-M ; syrphids, ( coccinellids, predators of trend population Field 37. Figure MEAN NO. OF PREDATORS/PLANT (N=20) s eemnd y h drc cut method. count direct the by determined as pdr,AA i a hmcly rae cbae ils t ie 2 3 4- 3, 2, 5 sites and at fields cabbage treated chemically a in spiders, A—A) 138 139 larvae and spiders were predominant throughout the crop period, whereas the adults and larvae of coccinellids are present in low numbers at the begining of the crop period and then disappear by the middle of the crop period. The numbers of lacewing larvae and anthocorid bugs were very low and could be considered negligible. As a result they are omitted from Figures 36 and 3 7. The syrphid larvae population was high in the first 3 weeks and in the second half of the crop period, with very low numbers in the middle of the crop period. This low syrphid larval population coincided with the low aphid population (refer to Figures 2 7, 2 8, 29, 30 and 31) in the middle of the crop period (August). Petherbridge and Mellor (1936) found that syrphid larvae were the most effective natural enemies of aphids and, could certainly control aphid populations in the absence of its hyperparasites. On the commercial farms (sites 2, 3, 4 and 3) where chemical control was carried out regularly, the predator populations, especially syrphid larvae, were greatly suppressed (Figure 37). However, when the interval between each chemical application was more than 3 weeks, the syrphid larvat population showed some sign of resurgence and in creased to high population levels before the next application was applied (Figure 37, sites 3, 4 and 5). Spiders, however, seemed to be less affected by the aphicide applications than other predators. 140 6.3.2.6. The aerial population of alatae and the adult parasites and predators of aphids in cabbage fields. In all five sites, the immigrant alatae were caught in greatest numbers in the second week (i.e Ouly) after the cabbages were transplanted (Figure 38 and 39). The number of alates caught by the yellow water pan traps closely followed that of direct counts on the plants for the first 6 weeks. Since the alatae found on the plants at the be- VI/ gining of the crop period were mostly immigrants, this simi- k larity is to be expected. The immigration of alatae began to decrease as the season progressed to less than 10 per trap between 9 to 12 WAT (September). Then, at 13 WAT, the number of alatae caught again increased to a second peak (14 WAT), which was much smaller than the first peak, before decreasing again to less than 10 per trap by the time of the final harvest. As might be expected, at sites 2 and 3 (Figure 39), where the final harvest occurred 3 or 4 weeks earlier than the rest of the sites, the second peak was absent. In contrast to earlier work by Chua (1976), the presence of the second peak suggests that many of the alatae developing from the nymphs within the field itself were emigrating. The species composition of the alatae caught, varied according to the stages of the crop. In the first half of the crop period (upto 9 WAT) J3. brassicae, fl. persicae and M^. euphorbiae made up 13 to 30% of the total weekly catches of alatae. The other 70 to 83% of the catches consisted of bean aphids (Aphis fabae), the lettuce aphids (Nasanovia MEAN NUMBER OF ALATAE CAUGHT/TRAP/WEEK (5 TRAPS/SITE) iue 38* Figure . uhrle, ad te species, other and , , euphorblae M. Yellow water pan trap catches of alatae alatae of catches trap pan water Yellow (B. brassicae, □ , M. per per M.sicae , □ , , A—A brassicae, (B. - i cbae il a st 1. site at field cabbage in) Q-Q 141 MEAN NO. OF ALATAE/TRAP/WEEK (N=5) iue 9 Ylo wtr a ta cths f lte B basce ; . persicae, ; M. brassicae, B, ( alatae of catches trap pan water Yellow 39. Figure —A; . uhrie ~ i cbae il a sts , , , n 5. 5 and 4, 3, 2, sites at field cabbage in )#~# euphorbiae, ; M. A A— 142 143 ribisnigri and Pemphigus bursarius), the buckthorn- potato aphid (Aphis vvasturtii ), the pea aphid (Acyrthosiphon pisum), the carrot-willow aphid (Cavariella aegopodii), the cereal aphids (Macrosiphum averae, M. grenarium, f4. fragariae Rhopalosiphum padi) and various tree aphids (such as Myzocalis, Apis, Adelges and Phylapis species). At the YL begining of the crop period the ratio between the numbers \ of JB. brassiace, M_. per sicae, and f4. euphorbiae caught was 3:2:1 respectively. However, as the crop period progressed, the total number of alatae caught declined and so did the numbers of jl. per sicae and M_. euphorbiae until they dis appeared completely by 6 to 8 WAT (middle of August). Towards the end of the crop period (13 to 14 WAT) the total catch of alatae increased again to form a second peak and more than 90% of these alatae were j3. brassicae. This implies that at 10 WAT, almost all of the aphids caught in the yellow water pan traps were alatae generated within the field itself because in the direct counts on the plants during that time, only JB. brassicae was present and that the composition of alatae in field population was high (see page 128). The yellow water pan trap catches of D>. rapae adults followed closely the direct counts on the plants and also the field aphid population, i.e the bimodal curve (Figure 40) In the untreated field (site 1), the much higher earlier peak occurred at 2 WAT (early July) and the latter peak at 12 WAT (end of September), but the latter peak may or may not occur in the chemically treated commercial sites (sites MEAN NUMBER OF ADULT DIARETIELLA RAPAE/TRAP/WEEK (5 TRAPS/SITE) SITE 1 SITE 3 SITE 4 SITES Figure 4-0. Yellow water pan trap catches of adult adult of catches trap pan water Yellow 4-0. Figure irtel rpe n abg fed at' fields cabbage in rapae Diaretiella ie 1 2 3 4 ad 5. and 4, 3, 2, 1, sites W 1 145 2, 3, 4 and 5). This variability in the occurrence of the latter peak is probably due to the effects of spraying . The occurrence of the earlier peak suggests that £. rapae might be immigrating along with the alatae aphid populations. However, since [). rapae adults can emerge from overwintering mummies left in the soil (Way e_t jal, 1969) this may not be the case. In comparison with alatae aphids caught, adult JD. rapae catches were very low, varying from 0 to 30 per week per trap. However, these low catches of adult J). rapae may or may not reflect the actual field population due to the trapping bias (such as the variable degree of attractiveness of the yellow colour of the traps to different species of insect), eventhough it complements the earlier low population i obtained in the direct counts on the plants. Among the predators that were caught in the yellow water pan traps, syrphid flies, coccinellid beetles, spiders and anthocorid bugs were the most common. Whereas lacewings, robber flies and wasps were only caught occassionally. Among the coccinellids, the most commonly caught in cabbage fields were Coccinella 7-punctata, Propylea 14-punctata, and Adalia bipunctata. In the case of syrphid flies the following species were identified: Syrphus balteatus, Metasyrphus corollae, Sphaerophoria scripta/merthastri, and Syritta pipiens. The yellow water pan trap catches of predators were even 146 lower than J). rapae, but in general, the catches were compli mentary to the direct counts on the plants. Most of the predators species commonly caught in the yellow water pan traps were at the highest peak of their populations during the early half of the crop period (Figure 41). Only syrphids and spiders were present throughout the crop period. In chemically treated sites (sites 2, 3, 4 and 3). the yellow water pan trap catches of predators were much more variable and lower than the untreated site (site 1). On the whole, aerial predator populations seemed to be appearing at the same time as [). rapae and the alatae aphid population and certainly peaked in the same period (2 WAT). This early predator and J}. rapae peak probably contributed to the de cline of the aphid population in the first half of the crop period (i.e the earlier peak of the aphid population). This hypothesis could be tested by studying the aphid population trends free of natural enemies. 6.3.2.7. Changes in the percentage of the cabbage plants infested with aphids in relation to crop stages and chemical control. At Silwood Park (Pound Hill) (Figure 42, site 1), where cabbages were not sprayed, all of the plants sampled in the first 2 WAT were found to be infested by at least one aphid per plant. However, at 4 and 3 WAT the percentage infestation of cabbage plants dropped from 100% to about 60% before increasing again and settling around 75% in the middle of the crop period. This decline could be due to the fact that some of the alatae landing on the plants were already MEAN NO. OF COCCINELLIDS, SYRPHIDS. SPIDERS, AND ANTHOCORIDS/TRAP/WEEK iue 1 Ylo wtr a ta cths of catches trap pan water Yellow 41. Figure SITE1 SITE 3 SITE 4 SITE 5 ocnlis ) Srhd ( ), Syrphids ( Coccinellids pdr ( ad nhcrd ( ) ( 4, 3, 2, 1, Anthocorids ) and sites at fields cabbage in ( Spiders n 5. and 147 D*~D ), 148 JUN— I-----JUL------I------AUG------I-----SEP------I ----OCT' S I T E 2 4 | I l °5L 6L 9L 12L 14L 16L HG H1 H2 J U N — I------J U L ------1------A U G ------1 ------SEP ------I------O C T SITE 3 SITE 4 | 1 | SITE 5 | I I Figure 42. Percentage of cabbage plants infested with at least at least one aphid through out the crop period at sites 1, 2, 3, 4, and 5. 149 parasitized and so failed to start a colony. In the middle of the crop period the percentage infesta tion of cabbage plants varied between 70 to 85%. Subsequent reductions in the number of infested plants war* probably due to colonies being destroyed by the predators especially syrphid larvae (see page 137, Figure 36). However, as more alatae enter the crop, some plants may be reinfested. In this unsprayed plot, towards the end of the crop period, the percentage of the cabbage plants infested with aphids dropped to less than 50%. This reduction could be explained by the fact that at this period, the immigration of aphids into the plot had almost ceased, resulting in no new infestation. At the same time, deinfestation continued due to the destruction of aphid colonies by syrphid larvae as well as the emigration of alatae produced in the crop, and the rotting of leaves where large aphid colonies were sited. With insecticide application, the reduction in the number of aphid infested plants was much greater early in the crop period (Figure 42, sites 2, 3, 4 and 5). Where the crop was sprayed, the number of infested plants had increased again to almost the same level as that in the first 2 weeks, a week after spraying. However, in the case of site 3, where granules were applied, the proportion of infested plants were reduced from 100% to 0% 3 weeks after application, before it climbed back up again. 150 In contrast, during the second half of the crop period (9 WAT onward), the percentage of plants infested kept on increasing evenjthough spraying was carried out at 10 or 11 WAT (Figure 4-2, sites 2 and 5). The most likely e x p l a n a tion was that the percentage kill achieved by the sprays at that period was greatly reduced due to poor coverage of the spray droplets on the plants (see page 155), resulting in the survival of aphid colonies eventhough many of the individual aphids in the colony were killed. Towards one or two weeks before the final harvest, the percentage infested plants increased further due to the removal of most of the unin fested plants during the first or second rounds of harvesting. 6.4-. Conclusion. By monitoring aphid populations both in commercial vege table farms and at Silwood Park, we have established that for the autumn cabbages planted in late June or early Ouly an aphid population trend following a bimodal curve with 2 peaks occurred. The earlier peak occurs at 2-3 WAT, and the latter peak at 12-14- WAT. Such a population trend was already observed by other earlier works (Hafez, 1961; Heralky and El Ezz, 1970; Akinlosotu, 1973; and Chua, 1976). In contrast, summer cabbages planted from mid-April to early May (not studied in this work) were shown by Chua (1976) to have a unimodal curve with a single peak occurring towards r the last quarter of the crop period (end of June or early July). It is assumed that at the peak of the aphid popula tion of the summer cabbages, large numbers of alatae are being produced and these alatae then migrate and infest the 151 autumn cabbages as soon as the crops are transplanted into the fields. On the other hand, the D. rapae and predator population trends evenjthough showed a bimodal curve, the latter peak is much smaller than the earlier peak which for some predators such as coccinellids, lacewings and anthocorids were absent. This decline in D. rapae and predator populations at the u begining of September coincided with the rapid increase in K aphid populations. Under chemical control regimes, this decline is accelerated, resulting in a much depressed latter peak with minimum effectiveness in suppressing the late up surge in aphid numbers in autumn. The results given above seem to be in agreement with what the growers have been claiming, that aphid infestations in the second half of the autumn crop period were a serious problem. Insecticide spraying is reasonably effective in the first half of the crop period but not in the second half of the crop period. However, it seems that the earlier insecticide spray might be unnecessary and, infact, maybe detrimental to the parasite and predator population which seem to be effectively reducing aphids at this stage of the cr op. A very high aphid population in some commercial farms in the second half of the crop period could be attributed to the following reasons: (i) inappropriate emphasis on early aphid control which did most damage to parasite and 152 predator populations rather than preventing the late aphid resurgence in the third quaters of the crop period; (ii) slackened control activities during the second half of the crop period due to many reasons, allowing aohids to multiply rapidly; (iii) a general decline of the parasite arid predator populations in the second half of the crop period to such a low level or even totally absent, such as in the case of coccinellids, anthocorids and lacewings, their impact in reducing the increasing aphid population is negligible; and (iv) the effect of insecticide sprays in the second half of the crop period in reducing aphid numbers and the per centage of infested plants seemed to be substantially reduced. 153 CHAPTER 7. INVESTIGATIONS INTO GROWERS' CONTROL STRATEGIES. 7.1. Introduction. As discussed in chapter 6, we have seen that the occurrence of a severe aphid infestations in late autumn is probably one of the main reasons why cabbage aphids are a problem in the commercial vegetable farms in Thames Valley. To continue the investigations into factors causing growers' failure to control this late aphid infestations in autumn cabbages, this chapter is focussed on obtaining further information on the current grower's aphid control practices. It is hoped that with this information, we would be able to comprehend better, any further avenues for improvements in the current aphid control programmes. 7.2. Assessment of Field Percentage Kill of Aphids, Para sites and Predators Achieved by Commercial Vegetable Growers. We have already seen how late aphid infestation is causing aphid control problems in autumn cabbage. The second factor thought by growers to cause serious aphid problems was the low percentage kill of aphids achieved during the late aphid infestation. The objective of this study was simply to investigate the percentage kill of aphids in farmers' fields at different stages of the cabbage crops. 154 7.2.1. Method of study. The series of observations to assess the percentage kill of aphids, parasite j). rapae and predators were con ducted on the same four commercial farms used for the population studies. An assessment was made at 1-4, 5-9. and 10-17 WAT. Those three crop periods represent three growth stages: 7 to 10 leaf stage (1-4 WAT), 15 leaf to head initiation stage (5-8 WAT), and finally head maturing stage (9-17 WAT). Two hours prior to the spray operation, 20 cabbage plants infested with at least one aphid colony were tagged (number 1 to 20) and the number of aphids, J}. rapae, and predators on each plant were counted. The spraying operation was carried out by the grower, using his own choice of chemicals. Two hours after spray, live aphids (i.e those which moved when disturbed), £. rapae and predators were again counted on the same tagged plants. Another count was made 48 hours after spraying (2 days). Among the predators counted were coccinellid adults and larvae, syrphid larvae, spiders, anthocorid adults and nymphs, and lacewing larvae. The percentage kill of aphids was calculated for each plant by dividing the difference in aphid numbers before and after spraying by the aphid numbers before the spray, multiplied by 100. For J). rapae and predators, average numbers over 20 plants were used, instead of numbers on individual plants, because some plants have zero number of J). rapae or predators before spraying due to the selection 155 of sample plants were based only on the presence of aphids. Therefore, the analysis of variance and means separation were performed on data for aphids only and not ]). rapae or predators. 7.2.2. Results and discussion. Site 2, 4 and 5 were sprayed at least once during the three crop periods (1-4, 5-8, and 9-17 WAT) but at site 3, spraying occurred only in the second week, and then granules were applied in the third week. Consequently, data is not available on this farm for 5-8 and 9-17 WAT (Table 22). At site 5, demeton-S-methyl plus triazophos were used for all the three crop periods (1-4, 5-8, and 9-17 WAT), site 4 used demeton-S-methyl plus fcnvalerate at 1-4 and 5-8 WAT then switched to mevinphos at 9-17 WAT, whereas at site 2, demeton-S-methyl only was used at 1-4 and 9-17 WAT but at 5-8 WAT demeton-S-methyl plus triazophos was used. Table 22 shows the mean percentage kill of aphids at site 2, 4 and 5 is significantly reduced as the crop period progresses from 1-4 WAT to 5-8 WAT to 9-17 WAT. On average, the mean percentage kill of aphids is reduced from 96% 1-4 WAT, to 87% in 5-8 WAT, to 74% 9-17 WAT. This result seems to substantiate grower's claims that unsatisfactory control of aphids late in the crop period is one of the contributing factors to the seriousness of aphid problems in the area. These results also seem to indicate that over 9-17 WAT, a higher percentage kill of aphids is achieved by using mevin phos (site 4) or a combination of demeton-S-methyl plus 156 Table 22. Field percentage kill of aphids, D, rapae and predators achieved during the various crop stages by Thames Valley growers. Mean percentage killsa^ of Aphids, D. rapae and Predators at 2 hours (In bracket) and 48 hours after spray,______Commercial Aphids 0. rapae Predators farms 1-4 WATb/ 5-8 WAT 9-17 WAT 1-4 WAT 5-8 WAT 9-17 WAT l-<* WAT 5-8 WAT 1-17 WAT i 94.073^ 86.78b2/ 69.08c1^ 87.501/ 78.002/ 71.43^^ CO 80.002 ^ 60.001/ (79.68) (80.73) (63.70) (82.35) (75.00) (65.00) (74.07) (80,00) (40.00) 2 89.57 ** 86.671/ * # 76.921/ * * (75.83) - - (76.67) - - (69.23) - - 3 97.31a3 ^ 88.51b3 ^ 75.98c^ 94.443/ 80.003/ 74.44^ 84.213/ 77.273/ 73.68^ (96.21) (86.56) (73.42) (88.89) (80.00) (72.22) (84.21) (72.73) (73.68) 4 96.05a2/ 87.60b 2 ^ 76.87c2/ 96.152/ 85.712/ 79.352/ 82.352/ 78.002/ 72.542 / (94.50) (85.76) (75.82) (92.31) (85.71) (79.35) (76.47) (78,00) (68.62) a/ Means followed by the different letters (a, b, c) are significantly different (P < 0.05) as determined by the Duncan Multiple Range Test. The percentages were transformed to arc sine for the analysis of variance. b/ WAT = weeks after transplanting. 1/ Demeton-S-methyl 2/ Demeton-S-methyl + triazophos 3/ Demeton-S-methyl + fenvalerate 4/ Mevinphos * The grower did not $pray, instead Disyston granules were applied at 3 weeks after transplanting. 157 triazophos (site 5) compared to demeton-S-methy1 alone (site 2). However, there are some problems, with such a inter site comparison because of the variation in the cabbage varieties used, the different level of aphid population, actual stage of the crop between each sites at the time when the assessments were made, and also the different chemical and volume of spray used, A similar reduction in the percentage kill of £. rapae and predators was found as the crop stage progressed. The percentage kill of ID. rapae appears to be close to that of aphids, while the percentage kill of predators seems to be less than that of aphids. Although this cannot be tested for reasons given earlier, such an effect seems reasonable since predators are generally larger in size than aphids or J). rapae and therefore they were less susceptible to the spray. One of the reasons why the percentage kill of aphids, parasites and predators declined as the crop progressed is the increased surface area and overlapping of the cabbage foliage, thus reducing spray droplet coverage on the plant surface and the distribution of the insecticide in the different parts of the plant. Another reason could be the colonising behaviour of the alatae, because, when cabbages were at the heading stage, most E3. brassicae were found under the wrapper leaves, or on the lower surface of the bottom leaves in the case of f4. persicae and M. euphorbiae, where they are less likely to be covered by spray droplets 158 (see page 165). There are several other possible reasons for the reduced percentage kill or aphids, such as (i) spray drift, and the washing down of dilution of the spray droplets due to wet and gusty weather that prevails in the autumn period, (ii) changes in the composition of alatae, apterae and nymphs as the crop progresses which may have variable susceptibility to spray concentration, and (iii) the slowing down of nutrient uptake as the crop matures, affecting the distribu tion of the chemicals within the plant. The importance of these other factors remain unknown until further research is conducted. 7.2.3. Conclusion. The findings indicate that there is a significant reduction in the percentage kill of aphids as the crop progresses, contributing to the seriousness of aphid problems in autumn cabbage. This substantiated the belief of the growers as revealed in the interview survey of vegetable growers in Thames Valley area conducted earlier. The most likely reason for this reduction in effectiveness is the increase in surface area of cabbage foliage, reducing the coverage of spray droplets on the plants. 7.3. Aphid Distribution Within a Cabbage Plant at Various Stages of the Crops. In order to determine just where specifically the aphid colonies are on the individual cabbage plant at different 159 stages of the crop growth, the percentage of aphids found on each part of the cabbage plant (shoot/head, upper leaves, middle leaves, bottom leaves, leaf axils, and stems) was assessed during the direct counts of aphids, parasites and predators on the plants. The results show that when the alatae started to immi grate into cabbages at site 1 (Silwood Park), about 50 to 80% of them settled in the shoots of the plants, although some were also found in the upper, middle and bottom leaves, on the axils and on the stems (Figure 4-3). It was very noticeable when the plants were heavily infested, resulting in the curling of the shoots, and when the leaves grew larger the leaves were left with white scars (discoloration). One reason for the shoot being preferred at the early crop stage was that it provides more of a refuge for the alatae against rain drops, wind, sunlight, parasites and predators compared to the more exposed middle and bottom leaves. Between 4- to 12 WAT (from 10 leaf stage to 5 weeks after heading), the majority of the aphids (70-90%) were concentrated on the middle and bottom leaves of the cabbage plants, especially on the lower leaf surface. This change may not have anything to do with aphid preference but because of the fact that originally the majority of the aphid colonies were started in the shoots, and naturally when these shoots grew larger to become the upper leaves, then the middle leaves, and finally bottom leaves, they carry with them the same aphid colonies. 160 T 6 7 8 10 15 18 HG H1 H2 J U N — I ------J U L ------I ------AUG ------I ------S E P ------l ----- OCT ------1 111 ll Figure 4-3. Aphid distribution on different parts of cabbage plant (shoot, 0—0; upper leaves,#-#; middle leaves, D~~D ; bottom leaves, ; wrapper leave and head, A—A ) throughout the crop stages at untreated site 1 (A) an dtreated sit e 3 (B ). 161 At the end of the crop period (13 to 17 WAT), the number of aphids found on the upper leaves and the head again had increased from 20 to 93%, and declined on middle and bottom leaves. This time most of the aphids were found on the head, especially underneath the wrapper leaves. Although the number of aphids found on different parts of the plant varied with crop stage, preference for certain parts of the plants was observed to vary with aphid species. E3. brassicae seemed to prefer younger parts of the plants such as the shoots, wrapper leaves and head, upper leaves and middle leaves. Very seldom was j3. br assicae found on the bottom leaves. On the other hand, M. persicae and Nl. euphorbiae preferred older bottom leaves. When chemicals were applied to cabbage plants, aphid distribution within a plant seemed to be unaffected (compared with untreated) except that the percentage of aphids found on the wrapper leaves and cabbage heads towards the end of the crop period was increased (as examplified by site 5 in Figure 43). 7.4. Assessment of Spray Coverage on Cabbage Plants in Commercial Vegetable Farms. As in Section IV. 4, one of the reasonswhy the percentage kill of aphids was substantially reduced during the second half of the crop period, was the poor coverage of the spray droplets on the cabbage plants, to the increased surface area of the cabbage foliage. The objective of these observations was to investigate the coverage of the spray 162 droplets on mature cabbage foliage achieved by growers using their conventional tractor mounted sprayers. 7.4.1. Materials and methods. The observations were conducted on the same commercial farms in the earlier studies. The assessment of spray coverage was carried out in each of the four commercial vegetable farms during the heading stage (10-12 WAT) which was in September. Before spraying was carried out, 5 plants were chosen at random. A strip (26mm wide) of water- sensitive paper (developed for field use by CIBA-GEIGY) was attached to both the upper and lower leaf of one top, middle and bottom leaf of each plant (Figure 44A and B). The strip was attached by staples to the midrib, from the tip of the leaf to the leaf petiole. Before the water-sensitive paper strips were attached, each leaf was first wiped to remove any moisture, preventing the water-sensitive paper being stained prior to the spray operation. The crop was then sprayed with water, using the growers tractor mounted medium volume sprayer calibrated according to normal usage by the growers. About 20 minutes after the spray, the water sensitive papers were removed and wrapped individually in tissue paper and brought back to the laboratory for analysis. Each strip was divided into 3 sections corresponding to the outer leaf, mid-leaf and inner leaf (Figure 44C). The coverage of the spray droplets on each section of the leaf was then ranked according to the criteria below (see Figure 43): 163 Wrapper leaves Top leaves Middle leaves Bottom leaves Soil line Upper leaf surface Lower leaf surface leaves on the plant (A), upper and lower leaf surface (B) and the relative section of the leaf (C). 164 Figure 45. The coverage of the spray droplets on upper (A) and lower (B) leaf surface of the top (I), middle (II) and bottom (III) leaves of a cabbage plant recorded by the water-sensitive paper strips. Rank criteria 1. (No coverage) 0 droplets/Cm 2 2. (Poor coverage) 1-15 droplets/Cm 2 3. (Moderate coverage) 16-30 droplets/Cm 2 4. (Good coverage) 31-60 droplets/Cm c 5. (Excessive coverage) Each droplet coalesced another to form irregular patches. 7.4.2. Results and discussion. Figure 45 shows the distribution of spray droplets (blue dots) on the water-sensitive paper (yellow), while Figure 46 shows the average rank of the coverage. Eventhough statistical tests could not be carried out because of rV arbitrary differences between rank 1 or 2, 3, 4 and 5, the results consistently show differences in the coverage of the spray droplets within and between leaves. The upper leaf surface of the top leaves were excessively covered by the spray droplets in all of the trials (the rank range from 4.2 to 5.0). However, on the lower leaf surface only moderate coverage was obtained (rank of 2.8 to 4.6). There was no difference in coverage on the different sections of these top leaves (Figure 46). For the middle leaves, the coverage on the upper leaf surface was excessive on the outer leaf section with good coverage on mid-leaf section and moderate coverage on the inner leaf section. However, on the lower leaf surface, all sections of the leaf received poor coverage (rank of 1.4 to TOP LEAVES MIDDLE LEAVES BOTTOM LEAVES ULS LLS ULS LLS ULS LLS S I T E 1 S I T E 2 166 S I T E 3 S I T E 4 Figure 46. The distribution of spray droplets on upper (ULS) and lower (LLS) leaf surface of top , middle,and bottom leaves of cabbage plants as recorded by the water- sensitive paper strips at commercial vegetable farms 1, 2, 3, and 4. 167 2.8). This uneven distribution of spray droplets on the different sections of the leaf was due to overlapping of the leaves, i.e top leaves covering some parts of the middle and bottom leaves. In the case of bottom leaves, the top section of the upper leaf surface still received excessive coverage of the spray droplets with the middle section receiving between moderate to good coverage and poor to moderate coverage on the inner section. On the lower leaf surface, all sections of the leaf received poor coverage of the spray droplets. This poor coverage is probably due to the absence of the minute spray droplets penetrating into the space trapped between the top, middle and bottom tiers of the foliage. Thus, only the lower leaf surface of the top leaves received a moderate coverage of the spray droplets, unlike the lower leaf surface of the middle and bottom leaves. On the whole the distribution of spray droplets on cabbages was consistent at all the 4 sites. Although it appeared that at site 2 and 4 the coverage was less excessive on the upper and middle leaves, compared to those of site 1 and 3. This excessive coverage could very well be due to faulty calibration or inefficiencies of the spraying equipment itself. 7.4.3. Conclusion. It is evident from the results of assessing spray drop let coverage that with the conventional tractor mounted 168 medium sprayers (Volume median diameter of 200-4-00 microns), growers are getting good coverage on the exposed parts of the plants only i.e on the lower leaf surface of the first and second wrapper leaves, and the upper leaf surface of the top and middle leaves (see Figure 4-7). There is poor coverage of the upper leaf surface of the first and second wrapper leaves, and the lower surface of the middle and bottom leaves. It has already been established that in autumn most of the aphid colonies are found underneath the first wrapper leaves (on the upper leaf surace) and also on the lower leaf surface of the middle and bottom leaves. This coincides well with those areas of the plant that receive least con centration of aphicide, suggesting that poor spray droplet coverage on those parts of the plants where aphids are -the. found is likely to be^major factor causing a reduction in the percentage kill of aphids in the autumn period. 7.5. Calibration of Conventional Tractor Mounted Medium Volume Sprayers Used by Commercial Vegetable Growers. To investigate further the likely reasons for poor aphid kill in^autumn period, the calibration of sprayers used by growers at commercial farms 1, 2, 3 and 4- were checked and recalibrated. Obviously, the general condition of their sprayers and their calibration has a considerable effect on the actual rate of insecticides applied, the size of the spray droplets, coverage of the droplets on the foliage, and the percentage kill of the target organisms. 169 Figure 4-7. Diagram showing the distribution of the spray droplets (dots) on upper and lower leaf surface of wrapper leaves (a), top leaves (b), middle leaves (c) and bottom leaves (d) of a cabbage plant. The letter x indicate the location of the majority of the aphid colonies in autumn. 170 7,5.1. Calibration method. Conventional tractor mounted medium sprayers used on the commercial farms described in the previous section were calibrated in September 1983. The following calibration procedures were followed: (1) The number of nozzles per sprayer boom and the distance (in centimetres) between each nozzle on the sprayer boom was recorded. (2) Using a one gallon bucket, the volume of water discharge by one nozzle (at normal pressure used by the growers in spraying operation) in 20 seconds was taken and measured (in cubic centimetres) with a measuring cylinder. The procedure was repeated 5 times, each time a different nozzle on the sprayer boom was used. (3) Using stopwatch, the time taken (in seconds) by the tractor to travel a distance of 50 metres during the actual spraying operation was recorded. The same procedure was repeated 5 times. (4) The speed (metres/second), volume discharged (litres/ second), swath (metres) and volume sprayed per hectare (litres/hectare) were calculated, from the data obtained from 1, 2 and 3 above, using the following formulas: (i) Speed (metres/second) = ______50 metres______ Av. time taken by the tractor to travel 50m (3) Ui} Volume discharged (litres/second) No. of = Av. volume discharged/nozzle(2) v nozzles/ * boom (1) 20 seconds (iii) Swath (metres) No. of = Distance between each nozzle (1) X nozzles/ boom (1) (iv) Actual volume sprayed/hectare (litres/hectare) = 10,000 metres x Volume discharged (ii) Speed (i) x Swath (iii) Before the actual calibration was conducted, the following information was gathered from the operator and through in spection of the sprayers: the type and age of the nozzles used, height of the nozzle tips from the ground, and the intended volume to be sprayed per hectare. Once the actual volume and the intended volume of spray was known, th Actual, volume sprayed - intended volume of spray x 100 Actual Volume sprayed 7.5.2. Results, discussion and conclusion. All of the sprayers used in the study were patented under the trade name of Evers and Wall. Details on types and age of nozzle used and the results of the calibration are given in Table 23. 7.5.2.1. Types and age of nozzles. The age of nozzles used on these farms varies from less than 1 year to 5 years old. The indication was that most of the farms (Farms 2, 3 and 4) did change their nozzle set at least once in 3 years. However, in Farm 1, 8 out of 12 Table 23. Results of the calibration of conventional tractor mounted sprayers used by four vegetable growers in Thames Valley area. Commercial Types & age No. of nozzles/ Swath Height of nozzles Speed Pressure Actual volume Operators intended % Farms of nozzles Boom in m (ft) in m (f t) from the ground in kph (mph)k^ in ksc (psi)c^ sprayed in volume in error in m (ft) lph (qpa)d^ lph (gpa) 1 HARDY 411030 12 5.49 0.74 7.32 4.08 703.26 600 + 17.21 2-5 years (18) (2.42) (4.55) (58.0) (62.64) (55) 2 Tee Set 9008 12 5.49 0.76 6.86 3.62 607.10 670 - 9.19 less than (18) (2.50) (4.26) (46.4) (54.07) (60) 2 years 3 HARDY 411030 20 10.16 0.66 5.49 4.50 635.50 600 + 5.92 172 less than (33.33) (2.17) (3.41) (64.0) (56.61) (55) 2 years 4 Tee 3et 11008 12 5.49 0. 76 7.27 2.95 560.44 560 - 0.08 less than (18) (2.50) (4.52) (42.0) (49.92) (50) 1 year a/ m = metre (ft = feet) b/ kph = kilometres per hour (mph + miles per hour) c/ ksc = kilogrammes per square centimetre (psi = pounds per square inch) d/ Iph - litres per hectare (gpa = gallons per acre) 173 nozzles on the boom were more than 4- years old showing quite severe wear and tear. Such old nozzles would increase the volume median diameter of the spray droplets, thus giving an excessive spray coverage on the exposed parts of the cabbage foliage. 7.5.2.2. Number of nozzles per boom and swath. Three of the calibrated sprayers (Farms 1, 2, and 3) were fitted with 12 nozzles to the spray booms having a swath of 5.4-9 metres, and one sprayer (Farm 3) with 20 nozzles on a boom having 10.16 metres swath. The wider the swath, the greater was the area covered in a given time but, would require a larger turning radius at the headland. On the other hand, the more nozzles fitted to the boom, the greater was the pressure required to achieve the same volume median diameter of the spray droplets. This was clearly shown in Farm 3 where 20 nozzles were used, the pressure was increased to 4-.50 kilogrammes per square centimetre (64-.0 pounds per square inch). However, the same effects could also have resulted from the use of worn nozzles instead of new nozzles as in the case of Farm 1. 7.5.2.3. Height of nozzles. There was quite considerable variation also between farms in terms of the adjusted height of the tips of nozzles from the ground (range from 0.66 to 0.76 metres). If the height of the nozzle tip from the ground is too high then the over lapping area of the spray droplet coverage would increase . This may result in increased drift of the smaller droplets 174 and gives a very uneven coverage of the spray droplets on the foliage. On the other hand, if the height was too low, there would be no overlapping coverage at all. This would results in taller than average plants not being covered by the spray droplets and at the same time gave an excessive coverage of the spray droplets on the exposed parts of the plant to the point of run-off. However, these effects would result only in the extreme cases. Such an effect is minimal at the height of nozzles used in all of the 4 farms studied. 7.5.2.4. Actual volume sprayed and the percentage error made. On the whole, the actual volume sprayed usually exceeded the intended volume of spray with the exception of Farm 2, where the actual volume sprayed fell short of the intended volume by 9.39%. At the other extreme was Farm 1 where the actual volume sprayed exceeded the intended volume by 17.21%. Such a high percentage error could be attributed to the improper calibration of the sprayer and severe wear and tear of the nozzles used. Similar findings were obtained by ADAS (1976) who have conducted a survey of the perfomance of field crop sprayers on 91 farms throughout England and Wales. In this survey, they found wide variations in the output of in dividual nozzles and the actual application rate existed among the farms. In the case of the actual application rate, they found that almost half of the machines calibrated had a percentage error of over 10%. Such an oversight on^grower's part could have serious implications on the efficiencies of their sprayers in 175 achieving the desired rate of application and the coverage of the spray droplets on the plants. Consequently these shortcoming might reduce the expected percentage kill of the target organisms. However, it should be reminded here that the unsatisfactory control of aphids obtained by growers in&e autumn period could not rest solely on the inefficiencies of the sprayer and its operations, but also on a multitude of other factors. This is clearly evident in the case of Farm 4, where the percentage error of spraying was almost negligible (0.08%) but, still, the control of aphids obtained in autumn is unsatisfactory. In conclusion, it could be said that eventhough consider able variation occurs in the type and age of nozzles used, tractor's speed of travel adopted, intended volume of spray to be used, and the actual volume of spray achieved among the four farms studied, many other factors are involved in growers failure to control the increasing aphid populations in autumn. Those other factors include inappropriate timing of the sprays, inappropriate chemical control strate^i^ , and a general decline in the biological mortality factors such as parasites and predators in autumn. 7.6. Conclusion. Further investigations into factors causing grower's failure to control the late aphid upsurge in autumn revealed that the percentage kill of aphids obtained by the growers participating in the studies were significantly reduced (by upto 30% of that achieved during the first 4 WAT). The main 176 reason thought to cause such a reduction was the poor cover age of the spray droplets on cabbage foliage due to the increase in the surface area of the cabbage foliage as the cabbages matured. When the coverage of the spray droplets was assessed in autumn, it was shown that the lower leaf surface of the middle and bottom leaves received poor coverage of the spray droplets. Also it was observed that the upper leaf surface of the first and second wrapper leaves received poor coverage of the spray droplets too, but was not tested because of the difficulty in sticking the water-sensitive paper strips due to these two leaves being closely attached to the head. It was also observed that those parts of the plant which received poor coverage of the spray droplets were also where most of the aphid colonies were located. This seemed to suggest that poor coverage of the spray droplets could be the main cause to the reduction in the percentage kill of aphids in autumn. Further evidence of poor coverage was provided by cali brating the grower's sprayers. The study revealed that con siderable variation in the type and age of the nozzles used, tractor speed and pump pressure adopted, intended volume of spray and the actual volume sprayed per hectare existed among the growers. The results showed that 3 out of 4- sprayers calibrated were having more than 3% error (tendency to over spray). Such a variability could very well tie up with the poor coverage of the spray droplets obtained by these growers 177 on parts of the plants where most of the aphid colonies were located. Evenjthough there is a tendency to overspray, the extra volume does not improve the spray coverage on the less exposed parts of the plants, instead the extra volume are being deposited excessively on the exposed plant parts. However, all of those factors discussed above appeared to be only part of the reason why the growers failed to control an increasing aphid population in autumn. Figure 28, 29, 30 and 31 (page 122) showed that the growers concen trate their control efforts only in the first half of the crop period. fudging from the very high aphid numbers in most of these farms during the last quater of the crop period (3 to 14- fold), emphasis on early aphid control seemed to provide little benefit to growers in maintaining low aphid numbers in the second half of the crop period. For autumn cabbage, such an emphasis appeared to be unnecessary, because eventhough migratory alatae population was high, JD. rapae and predator populations were also at their highest peak and, infact, it was observed that a high percentage of these migratory alatae had already been parasitised prior to a immigrating into^new cabbage field. This resulted in the early aphid population peak being short-lived which would not require any insecticide treatment. Infact, those early treatments were doing more damage to the £. rapae and pre dator population rather than to aphid population which showed some resurgence effects in the absence of those natural enemies. This evidence has lead us to a conclusion that the growers were placing a wrong emphasis of early 178 aphid control. In the second half of the crop period, the growers seemed to space their spray intervals too long apart (3-4- weeks) to allow the aphid populations to recover and increase several fold before the next spray. It was also evident that granular insecticide application at the end of the first quater of the crop period (3-4 WAT, Figure 29) N seemed to give better control of aphid populations in autumn compared to sprays (Figure 28, 30 and 31). This is probably due to the wet conditions of the autumn months resulting in good uptake of the granular insecticide by the roots for a period of 6-8 weeks. Furthermore, granular application only caused minimal damage to the natural enemy population because they were not exposed to the granules which are incorporated into the soil as a subsurface band. In light of these findings, if the current state of unsatisfactory control of aphids in autumn were to be im proved, the growers should be advised to adopt a revised, new control strategy. This revised strategy should take into consideration some, if not all of the factors as to why current aphid control in autumn crop? w&ra. unsatisfactory as discussed above. Besides improvement on the control strategy, the growers also need to increase the efficiency of their sprayers and spraying operations by having a regular nozzle change, better nozzle design, and more efficient nozzle arrangement on the boom so as to give better penetra tion and coverage of spray droplets on all parts of the 179 cabbage plants. Details on some of these suggested improve ments are discussed in the following section. 180 SECTION IV CONCLUSION 181 CHAPTER 8 RECOMMENDATION FOR FUTURE APHID CONTROL. 8.1. Introduction. The purpose of this thesis has been to investigate pest management problems of vegetable crops in the Thames V alle y area. To do this, a decision-analysis approach has been adopted, including a descriptive analysis of the vegetable pest system, as presented in section I . In this process, se ve ral key q u e stio n s were ra ise d and i t was in attem pting to provide answers to some of these key questions, especially those p e rta in in g to the management aspects of vegetable pest problems, that an interview survey of vegetable growers in the Thames V alle y was undertaken (Section I I ). This in turn revealed the problem of late aphid infestation in autumn cabbage to be of great concern. The p roje ct then proceeds with an investigation into some of the most im portant factors, thought to be causing the aphid problem (Se ctio n III) with the hope that this would give a better idea of where future improvements in aphid control might be sought. In this concluding section, there are 3 aims: to discuss recommendations for future aphid control programmes which is discussed in this chapter; to discuss recommendations on further research work arising from the findings of this thesis; and to appraise the value of the decision-making approach to pest management which is discussed in Chapter 9. 182 Three areas for improving the grower's current aphid control practices are presented, concerning improvements in: (1) The timing of chemical application; (2) the methods of chemical application; (3) the coverage of the spray droplets on the plants; and (4-) the choice of chemicals. 8.2. The Timing of Chemical Application for Aphid Control. There are four main cabbage crops grown an n u ally: summer cabbages, planted in late April or early May and harvested in August; autumn cabbages, planted in la te June or e arly July and harvested in October or November; and winter/spring cabbages, planted in August and harvested from February to May. According to the findings of the interview survey (Section II ) growers appear satisfied with their current aphid control programmes for controlling populations in summer and spring cabbages but are far from satisfied with aphid co n tro l in autumn and w inter cabbages. The philosophy behind current aphid control programmes, as commonly p ractic e d by growers in the Thames V alle y area, is early aphid control i.e, spray as soon as aphids appear on the cabbages. One argument for early aphid control is that once the pest becomes established, it is difficult to control especially when the soil is dry since this restricts the translocation of systemic aphicide (ADAS, 1984). This co n tro l philosophy i s applied to both, summer and autumn cabbages. As a result, most growers begin to spray as early as 1 WAT, and then schedule spray 2 or 3 more times at 2 week intervals. This normally lasts to the middle of the crop period, leaving the second half of the crop period unpro- 183 tected in the case of autumn cabbages. Figure 4-8 (based on results of section IV), shows the general trends in field populations of aphids, parasites, and predators in summer and autumn cabbages. In summer cabbages, the aphid populations begins to appear at the middle of May and peaks in the middle of duly (Figure 48a). In autumn cabbages, aphid p op u latio n s appear as soon as the crop is transplanted and then have 2 peaks; one in early Duly and one in late September (Figure 48b). The latter peak is several fold higher than the earlier peak and is much more serious in'terms of potential crop loss (see page 106) as observed in all of the commercial fields involved in the study where about 10-30% of the cabbages were not harvested largely because of aphid infestations. The populations of J). rapae (Figure 48c), coccinellids and syrphids (Figure 48d) follow the aphid population trends, there being two peaks of natural enemy activity. The much higher, earlier peak coincides with the single peak of the aphid population in summer cabbages and the earlier peak of aphid p opulation in autumn cabbages. However, the r e latively smaller, lat Based on th is evidence, it would seem that the current practices of early aphid control is appropriate for summer cabbages, since early control should suppress the gradual Relative Numbers/Plant iue 8 Te eea ted o field pplto o aphids of population d l e i f of trends general The 48. Figure on summer cabbages cabbages summer on abgs hogot h ya o cmeca farms commercial on year the throughout cabbages n h Tae Valey. y lle a V Thames the in (b), ela rpe c ad rdtr () on (d) predators and (c) rapae lla ie t e r a i D 184 (a), n o atm cabbages autumn on and 185 build up of an aphid population that is subject to populations of J). rapae, coccinellids and syrphids that reach their peak in that period. However, early aphid control in autumn cabbages may not benefit growers (as shown in Figure 28, 30 and 31 page 122) because of heavy reinfestation by aerial populations of alatae which over the last 16 years, appear to have peaked in the month of July (Figure 49). Neverthe less, this early peak of aphids on the crop declines for at le a s t 3 to 4 weeks of August (F igu re 48b) even w ithout chemical control, mainly due to^high rate of parasitism and p redation. Therefore, i t appears that for autumn cabbages, n the later half of the crop period, begining in early Sept- K ember, would be the most appropriate time for growers to concentrate their effort in controlling the aphid populations, provided the early peak itself is not damaging. Yet as evident in Figures 28, 30 and 31 (page 122), few growers spray autumn cabbages in early September, fo r a number of reasons (see page 127). Even if they do, the interval between sprays seems toolong (2-3 weeks), allowing the surviving aphids to reproduce and surge back to the popula tion level before spraying (see Figures 28, 30, and 32 on page 122). In the light of this, three factors need to be consi dered in making recommendations concerning the best time to apply chemicals to control aphids in autumn cabbages: (1) Potential damage caused by the early aphid population to newly transplanted seedlings (the first 4 WAT) which are very susceptible to aphid infestations. NUMBER OF ALATES (Log (n+1)) CAUGHT/SUCTION TRAP/MONTH Figure Figure 49 Ted o aeri pplto fr h ps 1 years 16 past the for population l ia r e a of Trends . t iwo Pr: . asiae e ssica ra b B. Park: Silwood at ta I ec Sre Ahd l n). ) in t lle u B Aphid Survey ct se In stead ) ad . uhrie ( euphorbiae M. and ); ( 186 ...... ); — ( Suc: Rotham- (Source: ) . scae a rsic e p M. 187 (2) The high rate of parasitism and predation during the early crop period. (3) The high aerial population of alatae in the early crop period which provide a continuous immigration of alatae into the crop during CJuly-August. E arly in the crop period, the p o te n tia l damage on the seedlings is likely to be quite substantial due to their small size and the stress of the transplanting shock. However, the p o t e n tia l damage would depend very much on the number of a la ta e and nymphs present on the p la n ts in the first 2-3 WAT after which their population will decline to a low le v e l (as shown in F igu re s 28, 29, 30 and 31). Therefore the timing of chemical sprays in this period seems best served by the economic threshold (defined earlier, see page 38) i.e spray as soon as the number of aphids per plant reached the economic threshold value. si The high rate of paratism and predation during the K early crop period seems to suggest that they are playing an effective role in reducing the early aphid population. However, if sprayed unnessarily in this period, the effect of parasites and predators in reducing aphid numbers will be greatly reduced. This, affirm?the need to spray only when necessary, by using an economic threshold early in the crop period. The high aerial population of alatae in the early crop period, provides a continuous immigration of alatae into the 188 crop during the Cluly-August period. This means that the aphid population will be maintained at the same level as that of before spraying, if not higher in the following one or two weeks after spraying. Infact there seems to be a tendency for aphid resurgence to occur due to the destruction of the parasite and predator populations by the spray. Therefore, it seems that chemical applications are best T directed towards the third quater of the crop period i.e, in A September for the control of aphids in autumn cabbages. This could be achieved in many ways depending on the methods of chemical application preferred: either spray or granular application. For spray applications, there are 2 critical periods on which spraying programmes should be concentrated (Figure 4-8b, A and B). For the critical period (A) an economic threshold could be used to avoid the unnessary spray, since the aphid population will decline naturally due to the high rate of parasitism and predation within the field itself as well as the mummification of immigrant alatae which have been parasitised prior to immigrating into the new cabbage fields. Research to establish this economic threshold is already undertaken by 0. Fakalata which is described later. In practice, the use of economic threshold is likely to be constrained by the inability of the growers to spend more time on monitoring the aphid population (see Table 20, page 97). However, this could be overcome by 189 using a fast and accurate means of assessing the aphid population in the cabbage field, such as the sequential sampling method. At the most critical period (B) where the aphid popula tion reaches the highest peak (i.e from end of August to early October), several sprays may be necessary, depending on infestation levels. Due to the rapid rate of reproduction by the apterae and also the reduced percentage kill of aphids achieved by the spray in this period, it is suggested that each spray is spaced at one week interval. Ideally, where granules are applied, they should be timed to cover both of the critical periods (A) and (B). However, this is unlikely to be possible with just one application, even with soil incorporated granules because it has to be applied at the time of drilling or transplanting which will be effective upto 10-15 weeks only under normal weather conditions (Suett and Padburtj , 1977; and Ahmad, 1970), but will be shortened by 3-4- weeks under very wet or dry conditions. Therefore, to cover critical period (B), this treatment will have to be augmented with at least one application of pumice formulated granules, one to two weeks prior to period (B). This additional application of pumice granules is appropriate at that stage (8-9 WAT) because the crop has attained maximum growth of foliage which will catch most of the granules applied and trap the insecticide vapour released by the fumigant effect of the pumice granules within 190 the cabbage canopy. This additional treatment could be also be achieved by spraying, but is likely to be less effective as described above. 8.3. The Coverage of Spray Droplets on Cabbage Foliage. It has already been shown that with conventional tractor mounted medium sprayers (Volume Median Diameter of 200-4-00 microns), growers only get good spray droplet coverage on the upper, exposed parts of the crop but not the upper leaf surface of the first and second wrapper leaves or the lower surface of the middle and bottom leaves (see page 155 to 158). It is on these surfaces that are not getting good spray coverage especially the head, the first and second wrapper leaves where most of the aphids are found during the late infestation period i.e, 2-3 weeks after the cabbage head is initiated (September). Clearly, this is likely to be the main reason why the percentage kill of aphids are upto 30% lower during the third r quater of the crop period compared to the percentage kill in the first quarter of the crop period of autumn cabbages (see page 155). To correct this shortcoming, it is suggested that growers should add one or two extra sprays and reduce the v spray interval to one week during the third qua.ter of the crop period. Another alternative is to improve the spray coverage efficiency of the conventional tractor mounted sprayers currently used by the Thames Valley growers. One of the most 191 practical suggestions is to increase the volume of spray from the current rate of 500-700 litres/hectare to not less than 1,000 litres/hectare, and at the same time use an extra wetting agent (ADAS. 1984). Such a change could easily be adopted by the growers provided water availability does not cause too many problems and increase the spraying time. As noted from the result of the survey, some growers expressed their desire to use less volume of water (see page 98). Another suggestion is to redesign the position and angle of the nozzle on the spray boom and, by replacing the existing tee-jet nozzle with a fan-jet nozzle, to increase the pene tration of spray droplets inside the wrapper leaves and the overlapping cabbage foliage. A similar approach has already been adopted in Brussels sprout with the use of ''pendant lances " attached to the boom (ADAS, 1984). A more revolutionary suggestion would be to replace current, conventional sprayers with new generation sprayers, such as the ulva mast (already available) or the electro static sprayer, which is still in the process of development. However, there are likely to be problems with these new sprayers too. For example, electrostatic spraying of cab bages has been tried and found to distribute the spray parti cles on the top parts only. This does not look promising t for cabbages (Dr. Mathews- personal communication). Even if they show promise however, this new technology would only be available to growers in the near future and in the mean time could only rely on ulva mast. Other alternatives still have to be explored. 192 8.k . The Methods of Chemical Application (Granules versus Spray). A majority of growers interviewed prefer to spray rather than apply granules (see page 88). The reason for this preference is not because sprays are more effective than granules but, it would appear, because the sprays are cheaper than granules and give flexibility interms of time of appli cation and in mixing of chemicals to control 2 or 3 pests with one application. It is customary for growers to mix aphicide with larvicide in one spray operation even if only one pest is the main concern. We have already seen, however, that spray poses several problems: (1) the percentage kill is reduced due to poor coverage of spray droplets on mature cabbage foliage; (2) there is rapid degradation of the active ingredient, especially in hot and wet weather; (3) reinfestation occurs throughout the month of July and August, which requires repeated sprays at short intervals; and (4) there is a limited availability of spraydays in the autumn period. Eventhough faced with these problems, 4 out of 3 control strategies adopted by growers interviewed in the survey (see Table 18, page 87) are based on sprays. By contrast, granular application should remain effec tive for at least 10 weeks: indeed, it is claimed to be effective for upto 5 months (Thompson and Percivall, 1975) under normal conditions. Clearly, this is superior to sprays interms of their field effectiveness in controlling aphids. 193 With early maturing cabbages (harvested at 9-12 WAT), a single application of soil incorporated granules at planting should be adequate in keeping the crop relatively free from damaging aphid infestation right upto harvesting time (see Figure 29, page 123), However, for late maturing cabbages, that are not harvested before 12 WAT, an augmentary pumice granular application might be necessary (Suett and Padburij , 1976) to protect the crop through the critical period (B) (Figure 48b, page 184), which is often refered to as late aphid infestation. The main problem with granular application is during drought conditions; since the plants are not actively growing at this time, the active ingredient is not readily taken up by the roots (both pumice and Fuller's earth granular formu lations) for translocation to various parts of the plants. However, most vegetable growers in the Thames Valley irrigate their crops during dry periods (see page 70), so this problem should not arise. Also, as the control of cabbage rootfly is necessary on most farms in the area, treatments against rootfly could be combined with aphid treatment by using dual purpose granules such as chloropyrifos plus disulfoton R R (Twinspan ), fonofos plus disulfoton (Double Down ) or p quinalphos plus disulfoton (Knave ) at drilling or transplan ting time, after mid-May. However, such combined treatment does not appear to protect crops adequately against late aphid infestation (Thompson and Percivall, 1978), so an augmentary application of pumice granules may be neessary. 194 8.5. The Choice of Chemicals. The chemicals recommended for aphid control by ADAS in 1984 is listed in Table 24. The choice of which chemical to use rests with the growers themselves or their advisers because practical problems of controlling aphids differ in detail from grower to grower and crop to crop, as well as from season to season. Obviously, information on the capa bility of each individual insecticide, i.e, what it can or cannot do, and its cost, would help growers to choose the right insecticide. For example, when M. persicae is the main problem. I would suggest that pirimicarb should be used because some populations of fl. per sicae are highly resistant to certain organophosphorus insecticides (Dunn and Kempton, 1966; Ahmad, 1970). If growers are concerned with wildlife safety, demeton-S-methy1 or pirimicarb would be the best choice for spray application because they are least harmful (ADAS, 1984). Injterms of the choice between granular or spray formula tions,. growers must weight the limitations of both formula tions, although it would appear that granules are likely to be better in many respects (Table 25). When chemical cost is considered, 2 granular applications of fonofos + disul- R foton (Double Down ) and disulfoton on pumice for example, will cost £79.92/hectare, compared with 4 spray applications of pirimicarb and one spray with heptenophos,costing £63.07/ hectare. Both formulations would more or less have the same cost when the cost of applying the sprays were added to the £63.07. Therefore, based on these arguments I would recommend 195 Table 24. Lists of chemicals recommended by ADAS (1984) for the control of cabbage aphid (Brevlcoryne brassicae). Chemical Rate at 61 cm Minimum Restriction Manufacturers (24 in) spacing interval to Recommended price Name kg/ha (lbs/acre) harvest (Days) (E/ha) excluding VAT A. Soil incorported qranules . 1. Chlorpyrifos (4$) + 29.0 (26) 42 Part II 47.27 disulfoton (6%) (TwinSpan**) 2. Disulfoton (10%) 14.0 (12.5) 42 Part II 14.56-17.78 on Fullers earth 3. Phorate (10%) 28.0 (25) 42 Part II 26.88-28.42 4. Fonofos (4%) + 29.0 (26) 42 Part II 58.00 disulfoton (6%) D (Double Down ) 5. Quinalphos (2.5%) + 29.0 (26) 42 Part II 45.53 disulfoton (5%) (Knave**) B. Foliar qranules 1. Disulfoton (10%) 14.0 (12.5) 42 Part II 17.92-19.04 C. Sprays 1/ha (fl oz/ac) 1. Chlorpyrifos 48% EC 1.50 (22.00) 21 Not included 14.85 2. Demeton-S-methyl 0.56 ( 8.0) 21 Part III 6.72-8.12 55% EC 3. Dimethoate 40% EC 1.00 (15.00) 7 Not included 3.36-6.65 4. Heptenophos 50% EC 0.85 (12.00) 1 Not included 12.71 5. Malathion 60% ECa/ 2.10 (30.00) 4 Not included 4.76 6. Mevinphos 24% ECb/ 0.58-0.86 (8-12) 3 Part II 7.86-11.65 7. Pirimicarb 50% Dsp. 0.42 kg (6 lb) 3 Not Included 12.59 Grain 8. Thiometon 25% EC 1.26 (18.00) 21 Part III 6.37 a/ Not Brussels sprouts b/ High rate - cool weather application. Table 25. Advantages and disadvantages of using granules or spray formulations for the control of aphids on cabbages. Granular formulation Spray formulations A. Advantages: A• Advantages: 1. Long persistency (upto 10-12 weeks) 1. Can be applied at any time during the crop period. 2. Less harmful to the natural enemies 2. Can mix 2 or 3 chemicals for different pests and and other wild life populations. apply just in one spray operation. 3. Less hazard to the applicators. 3. Cost of application is cheaper. 4. No problem with chemical drift. T B. Disadvantages; B. Disadvantages: 96 i 1. May not be effective under drought 1. Short persistency (3 to 21 days) condition without irrigation. 2. Can only be applied at certain stage 2. Relatively more harmful to the natural enemies. of the crop period. 3. Cost per application is expensive. 3. Pose more hazard to the applicators. 4. May have problem with spray drift. 5. Application in autumn period is limited by the availability tp spraydays. 197 the use of granules such as chlorpyrifos + disulfoton or quinalphos + disulfoton applied as subsurface band at drilling or transplanting for the autumn cabbage, and then augmented with one application of disulfoton on pumice at 8-9 WAT. 19 8 CHAPTER 9 RECOMMENDATION FOR FURTHER RESEARCH. 9.1. Introduction. This thesis has attempted to analyse the pest problems of vegetable crops in the Thames Valley area using a decision- analysis approach. Some of the major problems experienced by growers were the focus of the study. Subsequent investi gation has led to suggestions concerning short term improve ments in current aphid control programmes with special reference to autumn cabbages. However, this does not mean that long term solutions to aphid problems in vegetable crops are ignored. For instance, Wheatley and Thompson (1979) suggested the possibilities of ultilizing traditional (farm hygiene, crop rotation and insecticides) and new methods (partially resistant cultivars, chemosterilants, lure and biocontrol agents) in an integrated system as a long term solution to both cabbage rootfly and cabbage aphid problems. However, these authors pointed out the organizational pro blems as well as the technical difficulties that need to be overcome before such procedures can be inaugurated in the principal vegetable growing areas of Britain. One aim of this thesis was to provides background infor mation on the practical problems experienced by commercial growers that will enable further research, appropriate to the real need of growers, to be identified. In this con cluding chapters the scope for further research to comple 199 ment the work of this thesis is examined and the approach employed in the thesis is appraised. 9.2. Recommendation on Further Complementary Research. To complement the work of this thesis, it is suggested that further research should focus on the following topics: (1) to determine the economic threshold level for cabbage aphid on newly transplanted cabbage in Duly, and to help determine whether sprays are necessary at the early critical period (A in Figure 48b, page 184)for autumn cabbage. This work is already underway at Silwood Park (summer 1984) as part of an M. Sc. project by 0. Fakalata. (2) to conduct field trials in the Thames Valley on the comparative per formance of granular control programmes and spray control programmes. (3) to determine an accurate but rapid method of assessing aphid numbers in a cabbage field for mss. by the growers in conjuction with the action economic threshold. (4) to determine a methodology for forecasting aphid infesta tion in summer and autumn cabbage which would be of valuable assistance to growers in deciding on granular control pro grammes in high aphid infestation years or spray control programmes based on economic threshold irr low aphid infestation years. However, there appears to be some diffi culties in establishing such a forecasting methodology for j3. brassicae (Way & Cammell, 1974). 9.3. Appraisal of the Approach. With regard to the third objective of the thesis i.e, to develop and test the decision-analysis approach, this 200 thesis has shown how the approach can be used in tackling pest management problems. The problem has been approached in k stages: (1) A descriptive analysis, involving literature reviews, preliminary record examination, a detailed analysis of a few selected farms in the area, various discussions with organizations and prominant people (including growers) involved in the sector, and an interview survey of growers in the area studied. (2) The identification of key questions, where the problem of late aphid infestation in autumn cabbage and the factors thought to cause it were raised. (3) Research, involving detailed field investigations to assess why aphid infestation in autumn cabbages is such a problem to growers. (4-) Recommending improvements to current control practices to reduce these problems. The process involved in the descriptive analysis of this thesis involved several methods of obtaining background information. The interview survey as carried out in this thesis appears to be an absolute necessity, especially when the required information is not available and prevents further analysis of the problem. However, with experience, the time spent on the interview survey could be shortened especially the time spent preparing the questionnaires and on the length of each interview, depending on how much in formation is required. 201 As already observed in this thesis, the qualitative analysis phase as suggested in the original approach by Norton (1982a) did not seem appropriate for the key questions involved in this thesis. Such a situation is likely to happen with many other problems where most of the key ques tions involved lead straight to the research phase. There fore there is some reservation as to the usefulness of the qualitative analysis phase as a separate stage. It is perhaps better regarded as an integral part of the research phase. Besides the work of this thesis, the decision-analysis approach to pest management has been usefully applied to problems of orchard pest management Barlow jrt a_l, (1979) and rice pest management in Malaysia (Norton, 1982b). On the whole, the decision-analysis approach is systematic and practical. It guides the researcher towards an understanding of the system within which pest problems are being diagnosed and subsequently should increase the chances of appropriate and practical solutions to the problem being found. 202 ACKNOWLEDGEMENTS I owe the greatest thanks to my supervisor, Dr. G.A. Norton, for his considerable interest, guidance, critism, and encouragement throughout t h is study. My thanks also go to Dr. 3.D. Mumford and Prof. M.3. Way for their advice and encouraging discussions, and to members of the Environm ental Unit at Silwood Park fo r th e ir varied assistance. 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ZANDSTRA, H.G., PRICE, E.C., LITSINGER, 3.A., MORRIS, R.A. (1981). A Methodology for On-Farm Cropping System Research. IRRI, Los Banos, Philippines. 147 pp. ZANDSTRA, H.G., SWANBERG, K.G., ZULBERTI, C.A., NESTEL, B.L. (1979). Experiences in rural development, the Cacqueza Project. International Development Re search Centre, Ottawa, Canada. 218 APPENDIX 1 Interview Survey Questionnaire: THAMES VALLEY VEGETABLE PEST PROBLEM SURVEY CONFIDENTIAL Grid Reference: Farm: ______ County: ______ Date: (A) BACKGROUND INFORMATION (1) How many years have you been growing vegetables on this farm? ______years (2) What is the predominant soil type on your farm? ______ (3) What is your total vegetable acreage? ____ ac/ha (4) Do you grow crops other than vegetables? (Yes/No, if yes) What other crops do you grow and what is their acreage? Crops Acreage (i) acres/hectares (ii) acres/hectares (iii) acres/hectares (5) Which of these vegetable crops do you grow (CARD)? BrusseJs sprout Calabrese Broad beans Courgette Spring greens Watercress Runner beans Marrow Early Sum. cabb. Kale French beans Asparagus Sum. & Aut. cabb. Radish Green peas Fennel Winter cabbage Turnip Dry peas ______219 Savoy Swede Beetroot White cabb. Cos lettuce Carrot Red cabb. Butt, lettuce Parsnips Chinese cabb. Crisp lettuce Celery Sum. cauliflower Leeks Spinach Aut. cauliflower Salad onion Sweetcorn Win. cauliflower Dry bulb onion Rhubarb What percentage of your vegetable acreage do the following groups of vegetables occupy (CARD)? Brassicas % Peas and beans % Lettuce % Carrot and parsnips % Leeks & onion % Other vegetables % How long is your rotational cycle (or break) for the following groups of vegetables (CARD)? Brassicas once in every years Lettuce once in every years Leeks & onions once in every years Peas & beans once in every years Carrot & parsnip once in every years Have you stopped growing certain types of vegetables the last 5 years ? (Yes/No, if ye s) Which vegetable have you stopped growing and why? Which vegetables? Why (Probe)? (i) (ii) ( i i i ) 220 (iv ) ______ (v) ______ (9) Have you started growing any new vegetables in the last 5 years? (Yes/No, if yes) Which new vegetables and why? Which new vegetables? Why? (i) ______ (ii) ______ (iii) ______ (10) Do you think the number of crops you will be growing in 5 years time will be more or less than now? (More/ Less/Same). Why? (II) This card shows the planting methods used for diffe rent types of vegetables (CARD). a. Which method or methods do you use for brassicas/ lettuce/Leeks? b. What percentage of the crop is planted using each method? c. Which method do you think you are likely to use more in future? 221 Vegetable Bare root, 1 peg ' Peat block Direct Module groups or pulled plant trans drilling transp- transplanting planting planting C/ used % more used % more used rO more used % more Brassicas Lettuce Leeks (12) Do you irrigate your crops? (Yes/No) (13) What proportion of your sales are: a. Supermarket direct? ______ b. Wholesale market? ______ c. Farm shop ______ d. Others (state)? ______ SO FAR, I HAVE ASKED GENERAL QUESTIONS ABOUT VEGETABLE PRODUCTION. NOW, I WOULD LIKE TO ASK MORE SPECIFIC QUESTIONS ON CROP PROTECTION. (B) MAJOR INSECT, DISEASE AND WEED PROBLEMS, CURRENT CONTROL METHODS AND DECISION MAKING. (14) a. What are the major insect, disease and weed problems you have in brassicas, lettuce, and leeks and onions? Brassicas Lettuce Leeks <5 222 Onions Other ______ serious ______ pests ______ b. Which would you say were the worst 2 problems overall? c. (If no insect pest) Which is the worst insect pest problem? d. Could we now look at each of these 2 or 3 in more detail? (Refer to Table I). (15) Here are some of the worst problems that growers face in carrying out crop protection programmes (CARD). What would you say are the 3 worst problems you face? 1. Recognising the pests. 2. Learning and choosing the best available chemicals for treatment. 3. Allocating time to inspect the crop as frequently as you want to. 4. Not being able to spray at the best time. 5. Delivering chemical to the right target. 6. Other problems (state) ______ (16) Do you have any opinions on how your present crop protection programmes could be improved? ______Table I Question Problem I. Problem 2. Problem 3. I. How or why has it become a problem? 2. What method of control did you use last year? 3, How do you apply the chemicals? When do you apply 223 the chemicals? 5. What was the number of appli cations per crop last year? 6. How do you decide when to apply to these chemicals? 224 (17) Do you forsee any specific pest problems becoming more important in the next 5 years? (Yes/No, if yes) Which and why? Which? Why? (i) (ii) (iii) (18) What percent of your annual turnover did you spend on crop protection on your farm last year? ______% (19) Do you inspect your crop to check an insect pests, diseases, and weeds? (Yes/No, if yes) How often do you or others inspect your crop during the following periods of the year? Oan-Mar ______3ul-Sep ______ Apr-3un ______Oct-Dec ______ (20) Do you think spending more time for more accurate crop inspection would be beneficial to you? (Yes/No) Why? ______ (21) Are there any other particular insects, diseases, and weeds that you find difficulty to identify or are likely to confuse with other pest? (Yes/No) What are they? ______225 (C) SOURCES OF ADVICE AMD INFORMATION USED TO MAKE DECISIONS. (22) Here is a card showing the various sources of infor mation (CARD). From which of these sources do you get information or advice for each of the following problems? Problems I 2 3 4 5 6 7 8 Others(State) Pest identification Method of control Choice of chemical Spraying equipment Soil analysis Choice of varieties I = ADAS, 2 = Agriculture college, 3 = Independent crop consultant, 4 = Chemical company representative,^ = Seed company, 6 = Farm, journals and magazines, 7 -= ■ Thames Valley vegetable growers association, 8-•= National Vegetable Research Station. P = personal L = literatures (23) Do you have problems in getting or using information on specific aspects of crop protection? (Yes/No, if yes) On what specific aspect? ______