Wind, crop pests and agroforest design

Agricultural Systems 26 (1988) 99 110

Wind, Crop Pests and Agroforest Design J. S. O. Epila Forestry Department, Faculty of Agriculture and Forestry, PO Box 7062, Kampala, Uganda [Received 9 June 1986: revised version received 6 July 1987: accepted 6 July 19871

SUMMA R Y The pert'asive it~[tuence (?/"wind on insect .flight per/ormanee, di.spersal and resource exploitation is briefly reviewed. Wind essential@ reinJorces the surviral instincts o/'a mqjority (~/ insects through the conveyance 0/" cital chemical messages emanating /?ore the resources and b)' passively tran,v~orting responding individuals to the target habitats. Because./tying insects are generally poor aeronauts, once airborne, the mq/ority of them reh" on natural windbreaks to qffi)ct physical contacts with perceived target resources. These natural harriers obstruct air)qow and subsequently reduce wind speeds, thus creating wind-shadows which enable insects to land on, or near, the resources. The qffec'ts g/windbreaks o[" woody plant on primary insect in/estation patterns on crop plant components (?/ agr(?/orestry are discus,red in relation to a,grg/brest design.

INTRODUCTION Many factors motivate insects to make trivial movements within their own population territories or to migrate away from such territories to new, distant habitats (Southwood, 1962). Invariably, the goals for these movements are connected with the search for food, a mate or shelter. In particular, chemical messengers are critically important in the accomplishment of these important biological tasks. Odours or scents of coded messages are periodically or continuously puffed into the atmosphere and wind conveys them to respondent individual insects (Vinson, 1976). And although, at take-offs, many responding individual insects may rely on their flight muscles (Kennedy & Thomas, 1974: Moran et al., 1982), weak flying 99

A~ricultural Systems 0308-521X/88/$03'50 ~, Elsevier Applied Science Publishers l.td, England, 1988. Printed in Great Britain

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insects (e.g. aphids, thrips, whiteflies, etc.) are passively transported by wind in the later part of their journeys to their target resources (Johnson, 1954; Taylor, 1958; Bull & Reynolds, 1968). The evolution of air-transportation ability in the majority of insects implies improved aeronautical techniques and yet, to the contrary, most flying insects are not particularly good aeronauts (Kennedy, 1956; Taylor, 1958, 1960; Johnson, 1960). Thus, once airborne, the flight directions, landings and distributions of most insects within the target habitats depend almost exclusively on the horizontal speeds and directions of the wind (Hagen, 1962; Lewis & Stephenson, 1966: Lewis, 1967; Dean & Luuring 1970; Lewis & Dibley, 1970: Kennedy & Thomas, 1974; Willard, 1974~ Moran et al., 1982). In essence, two distinct wind speeds are requisite for the accomplishment of the search for the resources amongst such insects. Wind speeds higher than their respective flight speeds to effect passive transportation and the slower wind speeds to allow insects to land on, or near, the resources in question. Therefore, physical barriers which obstruct airflow become important variables in the processes of discovering resources. These barriers, acting as brakes, create shelters of relative calmness and thus enhance the landings and accumulations of insects (Lewis, 1965h, 1969; Lewis & Stephenson, 1966; Lewis & Dibley, 1970). The effectiveness and distribution of the shelters produced depend on the height and permeability of the physical barriers themselves (Lewis, 1965a,b; Lewis & Dibley, 1970; Schwab et al., 1971; Leyton, 1983). Open (porous) barriers provide relatively poor windbreaks compared to the solid ones (Lewis & Stephenson, 1966; Lewis & Dibley, 1970). In practice, however, it is the strength of the incident wind which ultimately determines the patterns of insect accumulations behind the physical barriers. For example, Lewis (1969) reported distinct insect accumulations in strong, (3-12kph) rather than in weak (2"5kph), winds behind similar barriers. Thus windy areas such as mountain ridges, valleys and shoulders (Hutte, 1968) with multi-layered vegetation canopies can be enhancive of concentrated primary insect infestations. This undoubtedly is an important element to consider in the potential insect pest problems of cropping systems. A conscious design of crop canopy architectures for manipulating wind velocity which, in turn, will influence pest infestation behaviours within the cropping systems is therefore an important basic requisite step in combating weak flying insects. Of course, designing crop canopies becomes more complex and bewildering in a polycultural situation. This problem is further compounded in systems where herbaceous plants are grown in mixtures with woody perennial plants such as in agroforestry systems. This paper reviews the documented effects of crop canopy architecture and wind on the primary insect pest infestations under monocultural

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agroecosystems and examines the probabilities for the occurrence of similar effects under traditional agriculture and modern agroforestry systems. A remedial design approach for the reduction of insect pest problems in agroforestry systems is also suggested as a contribution towards better management of modern agroforests.

W I N D A N D INSECT I N F E S T A T I O N S

Within agroecosystems Agroecosystems vary widely in stability, complexity and the area they occupy. Similarly, pest species numbers and population levels occurring in these agroecosystems also vary greatly. Typically, low species numbers and high populations are characteristic of insect pests which predominantly invade ephemeral monocultural cropping systems while, in contrast, greater species diversity and low populations per species are generally true of mixed cropping systems (van Emden & Williams, 1974). Many biophysical reasons are invoked to explain insect population incidences within agroecosystems. Basically, however, it is true that most crop fields escape insect pest invasions simply because factors that facilitate insect infestations are either absent or grossly interfered with. Of particular interest to us, at least for this paper, are factors directly associated with insect movements. Indeed, the management of pests is critically dependent upon the probability of movement to a host habitat from a colonizer source. These sources can be replicate areas of similar cropping systems or distinct areas that support alternate hosts or contain refuge sites for the pest species. But such factors are rarely given sufficient attention, e.g. the pervasive influence of wind on the insects" dispersal (Hogg, 1965; Leyton, 1983) and their distributions within a crop field (Dean & Luuring, 1970: Willard, 1974) which has never been firmly invoked to explain both the occurrence and infestation patterns of insect pests in agroecosystems. And yet, for example, small differences in crop heights may cause atmospheric turbulence for weak flying insect pests (Bull & Reynolds, 1968) which may greatly influence their infestation distributions in crop fields. Indeed, the tripartite effect of crop canopy, topographical barrier and wind on the distribution of aphid infestations in temperate monocultures of turnip, lettuce, wheat, oat, barley and hop have been conclusively demonstrated {Taylor & Johnson, 1954: Lewis, 1965& Dean & Luuring, 1970; Campbell, 1977). Thus, the so-called edge effects so prevalent in most cropping systems (Dean & Luuring, 1970: Carne el al., 1974; Trumble, 1982) are, in the main, attributable to the influences of wind on the primary infestations of crop fields by insects.

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Undoubtedly, the multi-layered crop canopy typical of tropical mixed cropping systems enhances the occurrence of such wind-shadows more than in temperate monocultures because, in addition to windbreaks of multiple agricultural crops, other natural windbreaks are also present within the traditional agricultural systems (e.g. remnant trees, young tree coppices, dead standing trees, bushy termite mounds etc.). And the windbreaking efficiencies of these barriers vary greatly. Dead trees are poor windbreaks. Termite mounds can provide excellent windbreaks. Similarly, it is quite conceivable to speculate that both crop and tree coppice canopies have the same effects on the speed of winds (see Bull & Reynolds, 1968). However, windbreaking efficiency of remnant trees is variable, depending on speciesspecific traits such as crownbreak height, crown size and porosity as well as inter-spatial relation of the trees. In consort, all these factors may influence wind speed-profiles (Bull & Reynolds, 1968; Gloyne, 1968) and consequently the insect infestation patterns on the food crops beneath such remnant trees. Although as yet there are no empirical data on the distribution patterns of shelters of remnant trees, it is reasonable to suggest that the distribution of such shelters is mainly random. Two reasons support the view of randomness. First, it is almost impossible to select woody plants of uniform sizes from a natural stand of mixed tree species such that their inherent differential influences on the windspeed profiles are minimized. Indeed, the populations and sizes of remnant trees in traditional agroecosystems are determined by a number of constraints and/or considerations, e.g. the influences of such trees on crop health; fertility of the soil under the individual tree canopy; utility values of such trees; taboos and superstitious beliefs associated with some tree species, etc. Secondly, even if it was possible to achieve such spatial arrangement in an approximate term, the majority of such trees will have been heavily selected for small, poor crowns to promote and maintain crop health beneath them. Thus, depending on the strength of the wind, branches of these weak trees may be blown together or apart, thereby affecting the permeability of the crowns (Bull & Reynolds, 1968), making the distribution of their shelters erratic and random. Therefore, it remains true that infestation patterns in mixed cropping systems are complex and difficult to predict compared to those occurring in most agroforestry systems, as briefly described below.

Within agroforestry systems Insect infestation patterns in one-year old agroforests are predictably similar to those in monocultures. However, from year two onwards, more pronounced, temporal differences become discernible depending, of course, on the species of woody plants used insofar as these markedly determine the

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types of windshadow produced, the cropping protocols and successive kinds of crops grown. Thus, while windbreaks of trees will influence insect infestation patterns on the one hand, marked differences in the infestation patterns are accentuated by the canopy control on the cropping regimes and the types of crops grown, which indirectly affect the temporal variation of pest types. For example, since pest guilds attacking shade-intolerant crops (beans, peas, simsim, cereals, etc.} are radically different in respect of sizes, flight boundary layers and behaviours compared to the guilds that attack shade-tolerant crops (root crops}, wind influence on their infestation will similarly be different. Nevertheless, the c o m m o n determinant in both cases is the direction of the incident wind in relation to the lines of woody plant component. Winds blowing parallel to the lines of trees have minimal effects on infestation patterns; such winds only manage to create a series of small, ineffective shelters within the lines where some pests may accumulate while the rest get blown away by faster inter-line winds. Crosswinds, on the other hand, do have decisive effects on insect infestation patterns. In practice, most airborne insects should get "deposited' on the windward of agroforests as Dean & Luuring (1970) found in their study of several species of aphids infesting cereal crop fields sheltered on the windward by hedgetree barriers. Furthermore, this pattern may vary with the local topographical features as each feature influences the behaviour of surface winds differently (see H utte. 19681. In which case, therefore, insect infestation patterns could be resultants of combined effects of windbreaks of local topographical features and trees. This speculation forms one of the important stages of an agroforestry design synthesis that seeks to ameliorate insect pest problems such as described below. SYNTHESIS OF R E M E D I A L DESIGN FOR A G R O F O R E S T PEST PROBLEMS Basic factors

Designing an agroforest is clearly a multi-faceted, iterative process (Steppler & Raintree, 1983). Many basic questions have to be answered before agroforests can be established. Nutritional, economic, social and managerial consequences of agroforests at local level (Buck, 1981: Raintree, 1983: Bently, 1986) plus concern for biotechnical aspects of transforming generated agroforest technologies into production of goods and services are some of the typical questions to which designers must address themselves and answer satisfactorily. The degree of sophistication of a design strategy will vary with the types of agroforest envisaged. However, in traditional design processes, there are basic biophysical factors which can never be

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compromised, e.g. optimal plant populations (Cannell, 1983); complementarity of component crops (Huxley, 1983); edaphic factors, particularly fertility and 'root floor' (Leyton, 1983, Oldeman, 1983) and macro-climatic regimes, especially rainfall of the locality (Leyton, 1983). These factors are collectively very crucial in agroforestry design synthesis.

Complementary factors Nevertheless, there are other elements (abiotic or biotic) whose contributions to the success of agroforest designs are compelling and yet are continuously omitted from the outset. For example, a number of biophysical factors which largely determine disease incidences and insect pest population behaviours under agroforestry systems are usually either given cosmetic consideration or none at all. And indeed in conformity with current idiosyncratic attitudes of governments, NGOs and major donor agencies, design emphases are more often hurriedly placed on how best discovered sets of social factors can be blended together with basic biophysical factors to realise satisfactory achievements of the management goals ofagroforests. Thus the complementarity of such biophysical factors is traded off for the social factors. Amongst the relegated environmental factors, wind is probably the most important because, whether in primitive or civilized society, wind has influenced--and continues to influence--the exploitation of various resources. For example, notwithstanding technical advances in material technology, the influences of wind on the sitings of man's handiworks (buildings, bridges, highways, airports, etc.) still continue to dominate the designer's mind and thought. Unfortunately, however, man has rarely consciously attempted to harness the architectural differences and/or qualities of his domesticated plants to remedy some of the detrimental tendencies (e.g. windthrow of trees, lodging of crops, spread of plant diseases and pests, etc.) of wind. And yet, as reviewed above, evidence from monocultures firmly shows the undisputed influential role of wind on the pattern and behaviour of insect pest infestations on crop plants. Similarly, although no such empirical data are yet available for agroforestry, the geometrical patterning of woody plants promotes orderly primary insect infestations that can be exploited in the remedial design synthesis for agroforest pest management.

Integrating wind effects However, designing agroforests mindful of wind influences on the bioecology of insect pest infestation of crop plant is complicated, as

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acknowledged earlier in this paper. The reasons are many but, more importantly, it is because we have virtually no control over the underlying causes of the prevailing wind regime of a locality. Fortunately, however, the aerodynamic behaviour of wind within agroforests can be influenced through spatial arrangement of woody plant components. Thus, provided we have sufficient knowledge about the various components of the local winds (e.g. structure, speed and direction) and the migration and infestation bioecology of the potential pest, one can synthesize a design strategy that will consciously anticipate, and most likely predict, insect infestation patterns on crop plants of agroforests: the design that nevertheless must consider and evaluate the intricate interrelationships existing amongst the wlriables shown in Fig. 1. For example, in the envisaged design, it would no longer be enough to evaluate the crown forms of tree components in re{'erence to (i) cropping regimes, (ii) crop types, (iii) their abilities to predispose crop plant components to pest attacks through the subtleties of micro-climatic changes; (iv) the qualities of the products and services (e.g. t\~dder, firewood, timber, shade etc.) but perhaps more importantly also to evaluate tree crowns as physical barriers that would determine insecl infestation patterns in agroforestry systems. Nor would it suffice to evaluate local topographical features in relation to (i) general management strategies, (ill harvesting techniques, (iii) soil erosion, (iv) potential windthrows, etc., but also as important factors in insect pest problems of a cropping system. Similar scenarios can be articulated for wtrious tree ti~ctors" (age, height, canopy, spacing, etc.) influential roles on the behaviour of winds (Bull & Reynolds, 1968) which, in turn, affect primary insect pest infestation behaviours. Nevertheless, some potential agroforest trees do not need such critical evaluation because, phenologically, their crowns do not act as windbreaks when crop plants would be most vulnerable to pest infestations (e.g. Felker, 19781. Basic bioecological data on the insect pests such as age {physiological and chronological) at the time of invasion, longevity, fecundity, sex-ratio, refuge, food habits and migration seasons are all important bases that would immensely enhance the synthesis of a sound design strategy. Other considerations that would further facilitate the process of integrating thc effects of wind into the remedial design for agroforest pests are the implications related to the knowledge that windspeed increases with height (Bull & Reynolds, 1968) and that the boundary layers for most flying insects lie between 5 and 10m (Haskell, 1966) above the ground. Thus, for example, we can more accurately predict that even in the humid tropics where plant growth could be astronomically fast, a period of 3 5 years, depending on the tree species, must elapse before their crowns can pierce through" the boundary layers for most insects. In other words, the crowns of most

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potential agroforest trees do remain effective physical barriers for weak flying insects for several years, after which indeed their canopy closures force the farmers to abandon the cultivation of most seed-crops. Nevertheless, the crowns of such trees do not necessarily act as windbreaks for insects whose flight boundary layers are higher. Of course, when boundary layers for the powerful fliers are outgrown by the tree crowns, insect landings and distributions within the target habitats are also largely determined by windbreaks of trees. This assertion is partly supported by the work of Whelan & Main (1979) and Free & Williams (1979). While assessing the influence of herbivores on plant succession after burns, Whelan & Main (1979) found that grasshoppers (powerful fliers) colonized from the edges and seedling damage was much reduced in the centre of large burn areas. Similarly, Free & Williams (1979) found that pollen beetle adults were more abundant at field edges than centres while their larvae were often somewhat more evenly distributed. Similar invasive strategy was also apparent for the seed weevil, stem weevil and the pod midge (Free & Williams, 1979). Thus in a nutshell, the designer must know his trees well and yet recognize that, in consort, all the biophysical and socio-economic variables implicitly or otherwise presented in Fig. 1 have decisive influence(s) on the primary insect infestation patterns and distributions on agricultural crop plant components of agroforests. Such an integrated synthesis of a design strategy is important for insect pest management. Ahead of time, for example, centres for pre-control effort can readily' be identified at the design stages of an agroforest so that time, money and labour may be economized should the need for such an effort arise in future.

CONCLUSIONS Wind influences dispersion and resource exploitation in the majority of winged insects. Wind, for such insects, controls primary infestation patterns under various cropping systems, depending on the complexity and diversity of the systems. In monocultural systems, distributions of insect infestations are obscure and random because of the relatively uniform crop canopies which, in most cases, remain permanently within the depth of flight boundary layers of many insect pests. Windbreaks of woody plants within traditional agroecosystems and agroforests emphasize the roles of local winds on insect infestations. Nevertheless, the scattered shelter zones produced by remnant trees and other natural physical barriers within traditional agroecosystems still do encourage randomness in insect infestation patterns which are relatively difficult to predict accurately. But

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the geometrical patterning of w o o d y plants in most agroforests suggests that the qualities and distributions of wind-shadow produced are quite predictable and can be used to synthesize design strategies for agroforests which consciously take into account insect pest problems. Thus, aerodynamic behaviours of wind as modified by tree crop canopies and their indirect, subsequent influences on insect pest infestations must be diligently studied.

ACKNOWLEDGEMENT I would like to thank one of the referees whose guiding comments made it possible for me to revise this paper. Persons who read and commented on the first draft are equally thanked.

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