Journal of Nematology 20(2):204-213. 1988. © The Society of Nematologists 1988. Theory and Practice of the Cropping Systems Approach to Reducing Nematode Problems in the 1

JAMES P. NOE 2

Abstract: Plant-parasitic nematodes are major constraints to the productivity of tropical farming operations. Intensive land use and climatic conditions favorable to nematode development contribute to increased losses due to these pests. Many farmers in developing tropical countries have limited resources and management options. Cropping systems research is a relatively low-cost, low- input method of optimizing existing agricultural practices with respect to limiting losses due to plant-parasitic nematodes. Specific tropical farming practices are discussed along with problems they pose for research in quantitative hematology. Comprehensive, systematic research methods for delineating and using nematode-host relationships are described, and new ways of dealing with complex multicropping systems are suggested. Key words: cropping system, tropical , nematode-host relationship, muhicropping, intercropping.

The term "cropping systems" has been pests, economics, and sociology also must used to describe various concepts, and two be considered in the systems analytic phase. extremes of generality have emerged for A comprehensive approach considers all its use. In a more general sense, cropping available management techniques simul- systems have been confused with farming taneously; however, is em- systems. Farming systems incorporate all phasized as a method of limiting losses due aspects of farm operations, which in ad- to plant-parasitic nematodes, especially dition to cropping systems also include live- within developing tropical countries. Crop stock management and cultural and eco- rotation is one of the oldest and most ef- nomic studies. At the other extreme, fective means of limiting nematode losses however, cropping systems also have been (19). This technique requires few high-cost used as synonyms for crop rotations, which or high-technology inputs and is histori- are very specific elements of the farm op- cally a part of many subsistence-level trop- eration. A workable definition for the term ical farming operations. Optimizing exist- probably lies somewhere between these two ing crop sequences in a rotation should be extremes. a natural application for systems analytical For the purpose of this paper, cropping techniques. systems research is defined to include Although the study of tropical farming quantitative analyses of the relationships systems may emphasize crop rotation, oth- amOng , pests, farming practices, and er nematode-management options, such as all management techniques deployed or nematicides, biological control agents, and deployable in the target system. In nema- crop resistance, must be evaluated if they tology, this area of research is further lim- are available. These alternatives, however, ited to consideration primarily of nema- often are of limited use to tropical farmers. tode-host relationships, although other Nematicides are increasing in cost, and there are increased restrictions on their availability. Biological control agents are Received for publication 7 July 1987. important emerging alternatives, but they Symposium paper presented at the annual meeting of the have been of limited commercial effective- Society of Nematologists, 19-22 July 1987, Honolulu, Ha- waii. This work was supported by the United States Agency ness. The control afforded by currently for International Development, Crop Nematode Research available biocontrol agents is best used in and Development Project, under cooperative agreement No. DAN-4149-A-00-4054-00 Project No. 936-4149, and by state conjunction with other cultural methods, and Hatch funds allocated to the Georgia Agricultural Ex- such as crop rotation. Resistance is a pre- periment Stations. 2 Assistant Professor, Department of Plant Pathology, Uni- ferred method of limiting losses due to versity of Georgia, Athens, GA 30602. plant-parasitic nematodes, but it is avail-

204 Tropical Cropping Systems Research: Noe 205 able in only a few important crops and ceived extensive study by agronomists, crop against certain nematode species. Even if scientists, and economists (18,21,27). Anal- new sources of resistance are eventually ysis of the nematode components of these developed through emerging technolo- farming practices would be a productive gies, they will need to be deployed as new extension of existing information. Deri- cultivars in a comprehensive cropping sys- vation of quantitative host-parasite rela- tem. Appropriate deployment of any new tionships in tropical systems is complicated source of resistance will be necessary to by various unique spatial and temporal pat- maintain its effectiveness. terns of planting crops. Common planting All of these limitations emphasize the patterns include intercropping, in which necessity of comprehensive on-site studies two or more crops are planted at the same of existing tropical agricultural systems, so time in regular geometric patterns; relay that improved crop sequencing and pro- cropping, in which a second crop is planted duction practices can be developed. Opti- between the rows of a previous crop before mization of existing practices is required the first crop is harvested; and mixed crop- to increase production efficiency immedi- ping, in which various crops are planted in ately, and time must be allowed for the an area with little regard for time of plant- deployment and evaluation of new man- ing or spatial pattern. Mixed cropping usu- agement technologies. Thus, this discus- ally occurs in more primitive areas of shift- sion of cropping systems research in trop- ing cultivation, but there are variations of ical agriculture will focus on methods for all three systems and indistinct boundaries analysis of quantitative nematode-host re- among the definitions. Perennial and an- lationships, leading to optimization of ex- nual crops may be combined in any of the isting management practices. cropping patterns. Shifting cultivation: A shifting cultivation TROPICAL FARMING PRACTICES pattern begins with slash and burn clear- Unique systems of agriculture have ing, followed by varying periods of culti- evolved in the tropics for a number of cli- vation and long-term reversion (forest re- matic and cultural reasons. Each of these generation), During reversion, the farming systems presents a different set of chal- community typically moves to a new loca- lenges for the design and implementation tion. Shifting cultivation patterns are wide- of research in quantitative nematology. spread in the tropics and may account for Major divisions in tropical cropping prac- 45% of the use (27). Quanti- tices are according to rainfall patterns, i.e., tative studies of nematode communities in arid, semiarid, and humid tropics. The pri- these patterns would be useful for deter- mary delineation of growing in the mining the optimum lengths of cropping tropics is the occurrence of a dry , versus reversion periods, as as for de- in contrast to delineation by a winter in termining the best cropping sequences temperate regions. The effects of a dry sea- within the cultivation period. Novel ap- son on nematode population dynamics may proaches to determining nematode-host be quite different from the effects of an relationships will be required, since crops overwintering period. Where a definite dry are seldom planted in rows and mixed season does not occur, or where cropping is common. Shifting cultivation is available, crops may be grown continu- may have a negative impact on populations ously, resulting in another series of nema- of plant-parasitic nematodes (20); how- tode dynamics. ever, improved systems of mixed cropping Additionally, unique land-use patterns and sequences of crops for limiting in- in the tropics will significantly alter the creases in populations of plant-parasitic methods for analysis of nematode-host re- nematodes would improve production dur- lationships. Several categories of land-use ing the crop phase and could shorten the patterns in the tropics already have re- required reversion period. 206 Journal of Nematology, Volume 20, No. 2, April 1988

Reversion cultivation: A second tropical son delineation may pose special problems pattern, reversion systems, is closely relat- and opportunities for quantitative studies ed to shifting cultivation. In this pattern, in nematode population dynamics, since the land is cleared and cropped for 3-10 nematodes may survive in anhydrobiotic and then left idle for a similar period stages. The reliance on perennial crops of time. The length of cropping and idle makes determination of crop yield-nema- periods and the crops grown in the systems tode density relationships more involved. vary widely in different agricultural areas. Large-scale farmers usually have more re- The primary difference between this pat- sources and management options available tern and shifting cultivation is that the to them, such as nematicides and resistant farming communities are landholders with cultivars, and may be reluctant to consider relatively large holdings consisting of rotating to less valuable crops. All of these smaller segments which are continuously factors may make these intensive rainfed rotated in and out of production. There is tropical systems less attractive targets for more opportunity to implement improved cropping systems analysis, and the possible cropping systems in this pattern, since the rewards are limited at this time. However, farmers are geographically stable and more as fewer management options remain interested in the long-term productivity of available to these growers, particularly the land. Keeping population densities of through the reduced availability of effec- plant-parasitic nematodes below economic tive nematicides, there will be increased thresholds would be important to the land- interest in optimizing production through holding community. Quantitative studies a comprehensive analytical approach. would again focus on optimizing the lengths Subsistence rainfed systems: Subsistence of cropping and idle periods and on choos- rainfed agriculture is perhaps the most ing appropriate crop sequences during the promising target for increasing production production phase. Determination of nema- through cropping systems analysis. Rela- tode-host relationships in these systems is tively large numbers of farmers depend on greatly complicated by the common prac- these systems, and a large number of an- tice of spatial intercropping, often includ- nual crops typically are grown, since the ing annual and perennial crops interplant- farmer is growing everything included in ed in the same system. the family's diet. Available management Intensive rainfed systems: Intensive rainfed options are quite limited, and monetary in- cropping systems are similar to their tem- puts are low. This production type is ideal perate counterparts in many ways. Farm- for optimization through analytical meth- ing occurs primarily on clearly defined in- ods, especially when the natural crop di- dividual landholdings, and cropping is versity is considered. Crop rotations usu- continuous to the extent that seasons and ally have evolved over many years, and rainfallwill allow. Within this pattern are much can be learned from a careful study landholders of distinct economic strata, in- of existing subsistence systems. The small eluding a large group of subsistence-level size of landholdings and intensive cultiva- farmers, usually with less than 2 ha of ar- tion of vegetable crops suggests the occur- able land~, and a smaller group of capital- rence of severe nematode infestations, al- intensive farmers with large land holdings. though there have been few studies of The large-scale farmers operate in the plant-parasitic nematodes in tropical sub- same manner as their temperate sistence-level farming operations (7,26). A counterparts, except for the delineation of comprehensive monograph has been pub- seasons by dry periods and a greater pos- lished on nematode problems on sibility of -round crop production. (13), an important crop to many subsis- There is also a greater reliance on peren- tence tropical farmers. This review can nial crops, such as , cacao, , serve as a good starting point for further etc., as high cash-value crops. The dry sea- research in these cropping systems. Tropical Cropping Systems Research: Noe 207

Determining quantitative nematode- rigated crop. Although the primary source host relationships in subsistence systems is of irrigation water may be rainfall, the complicated by the complex spatial and water still is managed artificially by chan- temporal arrangements of crops during the nels, terraces, and dikes. The cropping sys- long growing seasons. Interplanting, in- tems are centered on production as cluding relay cropping, is common, and the most important food source in many many crops are not planted in rows or oth- tropical areas. The number of rice crops er orderly patterns. Planting patterns usu- per year ranges from one to three in var- ally follow topography and the natural dis- ious regions, depending on temperatures tribution of micro-sites within the and rainfall, and other crops are usually arable area. Topographic complications are grown with the rice through interplanting, common, with crops and management relay cropping, and sequential rotations. practices changing rapidly over small dis- Extremely intensive rice-based systems of tances as altitude increases. up to eight crops per year (including in- Results of cropping systems analyses from terplanted vegetables) are common in tropical subsistence-level farms will prob- southern Taiwan and Southeast Asia. ably be of very limited, local use. Local These will be challenging systems for re- scientists and crop protection specialists search in quantitative nematology. Period- must identify important nematode pests for ic flooding of the wetland rice induces each area. Experiments should be repeated unique shifts in the nematode communities with diverse crop combinations and in dif- (23) but also should be useful in limiting ferent topographical-climatic settings. All populations of some plant-parasitic nema- of these factors, however, underscore the todes. The interplanting and relay plant- need for an efficient, standardized ap- ing of vegetables will provide further chal- proach to the implementation of cropping lenges in the manipulation of nematode systems research in nematology and in oth- communities. Increasing food production, er plant protection disciplines. As the la- however, even by slight increments, is a borious task of collecting data beings, re- necessity in the Asian rice-based systems, searchers must collect their data by and growers should be responsive to any systematic, standardized methods to en- recommendations which may increase sure their usefulness in future studies as yields. With land so intensively cropped, the crop-pest database is expanded. Ac- plant-parasitic nematodes can increase to curate quantitative models will be useful to levels that become limiting factors in pro- scientists in other regions and may be ex- duction. The complexity and diversity of tendable to modeling efforts in larger com- crops and planting patterns offer much op- mercial farming systems. portunity for further optimization through Intensive irrigated systems: Irrigated farm- systems analysis, after problems in deter- ing systems, especially those built around mining multiple-species quantitative host- rice production, constitute the most inten- parasite relationships in mixed plantings sive and high-yielding cropping operations are overcome. in the tropics (25). Temperatures are suit- able for plant growth year round, and crops CROPPING SYSTEMS RESEARCH are produced continuously, with some areas In order to simulate, optimize, and make producing 6-8 crops per year. Irrigated predictions about systems of crops, pests, farming may include intensive vegetable and management techniques, mathemati- production, usually for export to winter cal models must be derived for essential markets in temperate areas, various high- relationships in the system. As indicated in cash crops such as and pine- the discussion of tropical cropping prac- apples, or intensive grain-based produc- tices, there will be many unique and inter- tion of food for local consumption. esting challenges in the formulation and Wetland rice usually is considered an ir- derivation of these models. The results of 208 Journal of Nematology, Volume 20, No. 2, April 1988 optimization and predictions will be no bet- ductive curves. The use of nematicides, re- ter than the mathematical models on which sistant cultivars, and biological control they are based, and the models will be no agents all can be expressed through their better than the data used to derive them. effects on these fundamental relationships. Sound approaches to experimental design, The mathematical functions also serve as data collection, and analysis are needed to a basis for modeling crop sequences. Der- make progress in understanding the enor- ivation of damage functions and reproduc- mous complexities of tropical systems. Also, tive curves that accurately represent the some degree of uniformity in approaches situation in farming operations can be done will facilitate sharing of information and using small-plot methods in naturally in- results among different regions with simi- fested fields. lar problems. A proposed delineation of The emphasis on understanding the me- methods for use in cropping systems re- chanics underlying crop performance and search in hematology (15) has been pub- nematode population dynamics within each lished and distributed by the Crop Nema- growing cycle separates cropping systems tode Research and Control Project, a research from traditional crop rotation USAID-funded project administered at work. In traditional approaches, specific North Carolina State University. One of crop sequences were selected and planted the goals of that project was to encourage in replicated designs, extending over as and implement cropping systems research many growing seasons as were required to among nematologists in developing tropi- study the selected crop rotations. Data col- cal countries, and distribution of the guide lection was oriented toward determining was a part of that effort. the final results of each rotation in terms Methods for cropping systems research of target-crop yields, changes in nematode are built on the efforts of many researchers population levels, or other final-state in- in quantitative nematology. Considerable dicator variables. These methods consti- work already has been reported on the de- tuted a black-box approach to understand- termination and formulation of nematode ing the internal dynamics of the system and damage functions (1,5,22), reproductive were useful primarily for selecting a rec- curves (10), and in crop rotation develop- ommended rotation from those tested. ment (7,8,12). Further studies have shown In contrast, the cropping systems ap- the utility of small-plot methods in deter- proach implements a research design for mining nematode-host relationships determining mathematical relationships (2,9,12). Nematode spatial patterns in in- which in turn represent system compo- fested fields have been described by var- nents. These components then are assem- ious mathematical approaches (6,14). Re- bled into simulations of crop sequences and cent research has provided new insights on management practices. Functions repre- how to determine the effects of multiple senting individual crop-nematode rela- infestations (more than one nematode tionships can be assembled in different ar- species on a single host) (3,24) and how to rangements to simulate any possible derive multiple-point damage functions for cropping sequence or combination of man- describing long-term nematode effects on agement practices. By understanding sys- perennial crops (17). tem relationships, researchers are not lim- All of these areas of research provide ited to post hoc evaluations of management tools necessary for a tropical cropping sys- sequences which were implemented indi- tems research program, but the methods vidually in trial plots. must be assembled into an appropriate and There is a price to pay, however, in the orderly approach to delineating system increased data collection necessary to es- components. Most important are the timate parameters of the required func- nematode-host relationships, including tions for all crops and nematodes in a com- damage functions and nematode repro- plex farming operation. Additional Tropical Cropping Systems Research: Noe 209 problems in experimental design arise analyses of nematode-host relationships where attempts are made to include other should be done in replicated designs across pests in the systems analyses. Work can be fields for each crop and nematode species minimized, however, by using efficient re- included in the system. This research is search-plot designs in naturally infested labor intensive, but it does not require high fields, using grower-managed fields where technology equipment or resources from possible, and taking full advantage of cer- institutions within developing countries. tain aspects of the spatial ecology of plant- Costs are further restrained by doing the parasitic nematodes. research in growers' fields. In developing Field-plot research: Plots arranged in rect- tropical countries, labor is often the most angular grids in naturally infested com- available resource to scientific researchers. mercial fields can be used to collect data Technical assistance in the data analysis necessary for estimation of the parameters phases may be required, but this can come in mathematical representations of nema- after the field phase, and expertise in sta- tode-host functions. Deploying a grid sys- tistics often is available locally. tem of relatively small plots (20-30 m 2) Derivation of models, simulations: Because takes advantage of the naturally clustered of the long growing seasons in the tropics, horizontal spatial pattern of plant-parasitic mathematical models required for systems nematodes (6,14). Large differences in optimization can be derived in a relatively nematode densities among plots within the short time for a large number of crops. grid are typical because of the natural vari- Studies should be done over more than one ation in counts. This large range in counts growing season to assess the effects of cli- allows the determination of density-depen- matic variation, but the systems approach dent effects on crop yield and nematode will not take as long as studies where each reproduction. The use of small plots in nat- rotation or management sequence has to urally infested fields will minimize the re- be implemented in the field and conclu- sources required from developing country sions cannot be made until the last rotation institutions for implementing this re- is terminated. Although the initial data col- search, although follow-up and auxiliary lection phase of cropping systems research experiments in the greenhouse and micro- may entail more effort and more tedious plots would be useful. sampling than traditional crop rotation After the establishment of field plots, all studies, the resulting models will provide that is necessary in a relatively simple data- rapid evaluations of promising crop se- collection scheme is assessment of nema- quences and other management practices. tode densities at planting and at The systems analyses will add to our un- and measurements of yields in each plot. derstanding of crop-nematode interac- Yields then are expressed as a function of tions and indicate areas for further re- nematode densities with an appropriate search. Models that can be built upon and mathematical model, and final nematode expanded to other pests are essential for densities are expressed as a function of ini- long-term studies of complex tropical ag- tial densities. Elaborations on this design ricultural systems. can include assessment of midseason nema- The final phase of a cropping systems tode counts, or counts at regular intervals analysis consists of assembling the models for perennial crops, which then are used for yield relationships and nematode pop- to estimate functions representing time or ulation dynamics for all crops and nema- degree-day dependent reproduction. Soil todes in the system into a simulation mod- parameters may also be measured in the el. This model then can be used to assess plots and then used to estimate the effects the effects of sequences of crops and the of edaphic characteristics on nematode- impact of various management options. At host relationships (16). this step, economics, sociology, and cul- Implementation of grids of plots and tural factors can be brought into the anal- 210 Journal of Nematology, Volume 20, No. 2, April 1988 ysis. Weighted averages of commodity volumes of soil occupied by host zones prices can be used to translate yield infor- and the preferential feeding behavior mation into income and profitability stud- among the nematode species. Relative haz- ies. Economic thresholds can be calculated ard indices can be developed for each crop where data on the costs of various man- and nematode species and for each crop agement practices are available. Crops can and nematode combination to be consid- be weighted in importance according to ered by using greenhouse or microplot their cultural desirability and local mar- studies (4). This information then can be ketability. All of this information then can used to formulate models that explain net be assembled into decision models and used outputs in the multiple plantings, based on to derive optimal cropping sequences. relative nematode densities. Results of the cropping systems analyses, A multiple crop-multiple nematode in the form of recommended cropping species damage function could be formu- practices for specific nematode infesta- lated as: tions, will have to be validated in follow- up field studies. Variability in the data will Y = alFl(nl, n2, n3 ..... n,) + Ii(F2, nl, n2, n~ ..... nn) typically be high, and very broad confi- dence intervals must be applied to the rec- + a2F~(n,, n2, n3 .... , nn) + I2(F1, nl, n2, n3 ..... n,). (1) ommendations. Grouping recommenda- tions into general hazard categories and Where Y represents the total yield output letting farmers choose desirable sequences of the intercrop system, F~ and F2 are may be a more realistic approach. Regard- nematode damage functions of the two less of the problems in precision and ac- crops, relating yields of each crop to vari- curacy, however, the recommended man- ables representing all plant-parasitic nema- agement sequences will be based on the todes n~ ..... nn in the cropping area, al best data available, as opposed to best and as are scale factors representing the guesses, and can be refined with additional proportion of total area planted to each experience. crop (a~ + as = 1), and I~ and Is are func- Quantitative host-parasite relationships in tions representing the intercrop compo- multiple cropping systems: Research designs nent of productivity for each crop, in terms for delineating nematode-host relation- of the performance of the other crop, F1 ships in multiple cropping systems, as are or F~, and nematode densities, n~ ..... nn. common in the tropics, require unique F1 and F 2 can be any nematode damage methods. Techniques have been proposed function which accurately represents the for deriving nematode damage functions observed relationship in experimental data. for concomitant infestations of more than Curves for individual crops can be derived one nematode species (3,24) and for nema- by traditional microplot or small field-plot tode dynamics on perennial crops (17), but research. In the simplest situation, where no methods have been proposed for delin- only one parasitic species is present, the eating effects of mixed populations of Seinhorst formula (22) can be used. Where nematodes in mixed plantings of host crops. there are many nematode species in the Although formidable, the task of sepa- systems, more elaborate models relating rating nematode-crop dynamics in multi- crop yield to multiple infestations will be ple cropping is not impossible. Approaches needed. Yields should be expressed in com- need to be developed which combine in- mon production-oriented units across formation from the mixed plantings with crops, such as kg/ha, food calories/ha, or information from simpler research designs monetary units, so that the summation to based on single crop-nematode relation- total yield Y will be meaningful. ships. Key elements in partitioning nema- The actual number of nematodes in- tode density-based crop-loss models among cluded in the damage function for each several susceptible crops will be the relative crop will depend on the host range of each Tropical Cropping Systems Research: Noe 211 nematode species. Obviously, nematode be observed simultaneously. Appropriate species will be excluded from models for mathematical models for these functions nonhost crops, but a more complicated sit- will include multiplicative (interaction) and uation arises from preferential feeding be- simple additive terms. havior among nematodes presented with Studies of nematode population dynam- more than one host. In a spatial intercrop- ics in intercropping systems may be even ping, parasitic species may feed preferen- more complicated than studies of yield- tially on one of the crops, all but excluding loss relationships. In fact, the most work- the less desirable host. In this situation, the able approach may be to analyze beginning estimates of nematode densities, nt, used and final state variables, where input (Pi) in Ft and F2, can be scaled by a series of and output (Pf) are measured for each crop constants bii , where bi~ represents the pro- combination and nematode species in the portion of nematode ni preferentially at- system. The dynamics subsequently can be tacking the host represented by F t, and bi~ described by an empirically determined represents the proportion remaining on mathematical model. This approach is less host F2 (bit + b~2 = 1). Estimates of these than desirable, since it adds little to our proportions can be obtained in greenhouse understanding of the within-system dy- or microplot experiments. namics, but it will serve to move the re- The intercropping productivity func- search forward. Even in this simple ap- tions, I1 and 12, represent the differences proach, however, simultaneous analyses of between yields of the crops grown sepa- the dynamics of all species are required. rately and the yields of the intercrop. Usu- Reproduction of one species impairs to ally, if two compatible crops are planted some extent the availability of resources together, there is more total yield than if for the reproduction of other parasitic an equivalent area were planted in each of species, due to competition for feeding sites the crops. Increases are as much as 60% in and plant energy. The dynamics of each some combinations (27). Intercropping species must be followed carefully, even for systems have evolved in the tropics empir- species unimportant to the current crops, ically, as a method of allowing more in- to allow analysis of the impact of those tensive use of land and water resources. species on the performance of future crops, Additional yields are the result of compen- which will have different susceptibilities. sation among the crops and more efficient A system of linked equations can be used utilization of solar energy and other re- for simultaneous analysis of the population sources through noncompetitive spatial ar- dynamics of multiple infestations of plant- rangements of specific crop combinations. parasitic nematodes. Each equation in such These systems have been slow to evolve in a linked system represents changes in the more developed systems of agriculture be- population of a single species, in terms of cause they are not well suited to mecha- the initial population densities and dynam- nized cultivation and harvesting. ics of the other species: The interplanting function for each crop, dNt = Rt(PNli, dN2, dN3 ..... dNn) It and I2, is expressed in terms of the per- dN2 = R2(PN21, dNt, dN3, . .., dNn) formance of the other crop, F1 or F 2, and dN3 = R3(PN~i, dN1, dN2 ..... dN~) nematode densities, nl ..... n,, in recog- nition that the yield of one crop depends to some extent on the performance of the other crop, and both yields and the ability dNn = Rn(PN.i, dNa, dN2, •.., dNn-,) (2) to compensate depend on the severity of nematode infestations. Experimental der- Where dNt, dN2 ..... dNn represent the ivation of the intercrop functions should changes in population densities of nema- be done in field-plot studies, where all com- tode species Nt, N~ ..... N n under the in- ponents of the crops and nematodes can tercropping, Rt, R2 ..... Rn represent 212 Journal of Nematology, Volume 20, No. 2, April 1988 nematode population dynamic functions, measurable effects on the population dy- expressed in terms of initial population namics of the system because of the tre- densities PNll, PN~i..... PNnl of the partic- mendous variability inherent in nematode ular species, and population changes of all assays. For the purposes of cropping sys- other species dN1, dN2 ..... dNn in the tems analysis, all that will be required is a system. Extended time series functions can reasonable prediction of residual densities be used to represent dynamics in perennial of plant-parasitic nematodes representing crops, or in annual crops over a number a risk to future crops. of growing seasons. Species that do not in- fest any of the currently planted hosts can CONCLUSIONS be excluded from the models and repre- Crop losses due to plant-parasitic nema- sented by a simple function showing the todes in tropical agricultural systems could decrease under a nonhost. be reduced by improved deployment of Although interactions among popula- crop rotations and other existing manage- tion changes of the various nematode ment practices. Optimizing these agricul- species can be tracked by these systems of tural practices would not require any high- equations, it will be much more difficult to technology, high-cost inputs and could be separate the components of reproduction accomplished in a relatively short time. A occurring on individual crops in the spa- comprehensive approach to the analysis of tially mixed plantings. First, the spatial ar- cropping systems, incorporating previ- rangement of the will have to be con- ously published analytical methods and new sidered. If the root zones of crops planted techniques, can be used to provide the in- in alternate rows remain separated for most formation necessary to begin simulation of the season, then we may be able to con- modeling and optimization. Scientists and sider the intercropping as two spatially sep- crop protection specialists in tropical coun- arate systems, with little interaction. In this tries already have begun the task of iden- case, however, placement of future rows of tifying important nematode pests, and some crops will have to be studied carefully to have begun evaluating crop rotations. take advantage of the different levels of These scientists have the capabilities and infestation remaining in alternate rows. facilities to continue in the delineation of If the root zones of the interplanted crops nematode-host relationships as part of a are mixed, then many types of interactions cropping systems research design. Coor- are possible. The entire area could be con- dination, communication, and assistance in sidered as planted in a single crop that has the implementation of standardized re- a proportional combination of the host search methods are necessary to promote ranges and reproductive potentials of the information sharing and to avoid duplica- two crops. The constants of proportion- tion of efforts. Considerable research will ality should represent relative volumes of be required to increase our understanding soil occupied by the individual root sys- of nematode dynamics in the many com- tems, which could be determined experi- plex cropping patterns found in the Trop- mentally. These proportions then can be ics. applied to the linked systems of equations representing multiple-species population LITERATURE CITED dynamics on each host crop. 1. Barker, K. R., and T. H. A. Olthof. 1976. Re- Other factors, such as preferential lationships between nematode population densities and crop responses. Annual Review of Phytopathology nematode feeding behavior where both 14:327-353. crops are hosts, can cause further compli- 2. Cooke, D. A., and I. J. Thomason. 1978. The cations. Separating these effects will be te- relationship between population density of Heterodera dious and will require elaborate experi- schachtii Schmidt, soil temperature, and the yield of sugar beet. 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