MANAGEMENT OF THE ENVIRONMENT
TO FAVOR PEST CONTROL
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Managing the environment is important to increase the efficacy of natural enemies, which depend on production technologies such as varietal development, cropping systems, tillage practices and chemical inputs. Late trends in agriculture have, nevertheless, been toward decreasing environmental heterogeneity, increasing fertilizer and pesticide input, increasing mechanization and decreasing genetic diversity (USDA 1973, Bottrell 1980, Whitham 1983, Altieri & Anderson 1986, Altieri & Letourneau 1999). Such creates agricultural environments that impede pest population regulation by natural enemies. The current emphasis on IPM, the increasing restrictions on various pesticides and growing public concern about pesticide contamination, as well as increased production costs, justify increased research efforts for long term alternatives to the current trends. Although agroecosystems devoted food and fiber production have been stressed, these same systems frequently generate pests that are of human and veterinary health concern, such as mosquito, gnat and fly outbreaks. Numerous research has been conducted to document the importance of manipulating environmental properties of crop fields to make them more favorable to natural enemies and less amenable for insect pests, since van den Bosch & Telford (1964) presented their classical chapter that encouraged biological control in agroecosystems (van Emden & Williams 1974, Perrin 1980, Cromartie 1981, Thresh 1981, Altieri & Letourneau 1982, 1984; Price & Waldbauer 1982, Risch et al. 1983, Herzog & Funderburk 1985). For pests of medical and veterinary importance environmental management is essential to the maximized performance of parasitoids and predators (Please refer to Selected Reviews & Detailed Research ) Since the mid 1970's most effort has been directed to analyzing the effects of reduced tillage and vegetational diversification of agroecosystems. Research on other types of cultural manipulation such as strip-harvesting, trap cropping, use of nests or artificial shelter, etc., has been scarce, except for the use of food sprays (Hagen 1986) and kairomones (Lewis et al. 1976, Nordlund et al. 1981a,b, 1987) that enhance the activity of specific natural enemies.
There has been much research on multiple cropping systems and their effect on insect dynamics (Root 1973, Litsinger & Moody 1976, Perrin 1977, Altieri et al. 1978, Perrin & Phillips 1978, Bach 1980a,b; Risch 1980, 1981; Andow 1983b, Letourneau & Altieri 1983, Altieri & Liebman 1986). These studies provide a basis for designing crop systems with vegetational attributes that enhance reproduction, survival and efficacy of natural enemies. However, because agricultural land use is driven principally by economic forces, pest control plans are seldom made on the basis of habitat management. In developed countries farmers reduce unit production costs by increasing farm size and becoming more specialized, with the consequence that environmental manipulation strategies with demonstrated effectiveness under experimental conditions, such as cotton/alfalfa strip cropping for Lygus management in cotton (Stern et al. 1964), or the use of Rubus plantings around vineyards for conservation of grape leafhopper parasitoids (Doutt & Nakata 1973), have not been adopted on a regional scale. The political and economic context of modern farming does not support the maintenance of landscape diversity, which is one of the main obstacles to the implementation of many of the alternative strategies to pesticides.
The effective environment of an organism has been characterized by Rabb et al (1976) as weather, food, habitat (shelter, nests) and other organisms. Environmental management for biological control is concerned with the functional environment, i.e., the physical and biotic elements that directly or indirectly impact survival, migration, reproduction, feeding and the behaviors associated with these life processes. Although pest populations can be controlled directly through cultural control methods that modify the habitat, the main thrust of this section is conservation (maintenance of natural enemy abundance and diversity) and enhancement (increased immigration, tenure time, longevity, fertility and efficiency) strategies that can be used to manipulate natural enemies in agroecosystems. Habitat management is directed at (1) enhancing habitat suitability for immigration and host finding, (2) providing alternative prey/hosts during times when pests are scarce, (3) providing supplementary food (food sprays, nectar and pollen for predators/parasitoids), (4) maintenance of noneconomic levels of the pest or alternative hosts over long periods to ensure continued survival of natural enemies and (5) providing refugia for mating or overwintering. Cropping techniques that enhance parasitoids through these five processes have been reviewed by Powell (1986) and shown in table form by Altieri & Letourneau (1999).
Approaches to manipulating natural enemies include several levels, from agroecosystem processes to eco-physiological features of individual organisms. The number of elements that can be manipulated and their degree of flexibility depend on characteristics of the agroecosystem. The role, methods and future directions of environmental management as a preventative control strategy are detailed after Vandermeer & Andow (1986) in the following sections.
A unique set of agroecosystems are found in different regions, which result from local climate, topography, soil, economic relations, social structure and history. A number of farming features can be modified and some can impact the dynamics of insect populations. The agroecosystems of a region often include both commercial and local use agricultures, which rely on technology to a different extent depending on the availability of land, capital and labor. Some technologies in modern systems aim at efficient land use, such as reliance on biochemical inputs, while others reduce labor or mechanical inputs. On the other hand, resource poor farmers usually adopt low technology, labor intensive practices that optimize production efficiency and recycle scarce resources (Mattson et al. 1984). Area wide environmental management techniques are difficult to design and implement because of differences in climate, agricultural products and economic and political structure of each agricultural system. Many farming systems are in transition, with changes forced by shifting resource needs, unequal resource availability, environmental degradation, economic growth or stagnation, political change, etc. Strategies amenable to labor intensive operations will be radically different from those designed for mechanized, large scale operations. Specialization and concentration of crops are the most important factors limiting the application of many environmental management options for a particular region.
Farms may be classified by type of agriculture or agroecosystem even though there are many individual differences among farms in a region. Functional grouping is essential for devising areawide habitat management strategies. Norman (1979) listed five criteria that can be used to classify agroecosystems in a region: (1) the types of crop and livestock, (2) the methods used to grow the crops and produce the stock, (3) the relative intensity of use of labor, capital and organization, and the resulting output of product, (4) the disposal of the products for consumption (whether used for subsistence or supplement on the farm or sold for cash or other goods), and (5) the structures used to facilitate farming operations. Using these criteria Giggs (1974) recognized seven main types of agricultural systems in the world: (1) shifting cultivation systems, (2) semi-permanent rain-fed cropping systems, (3) permanent raid-fed cropping systems, (4) arable irrigation systems, (5) perennial crop systems, (6) grazing systems, and (7) systems with regulated farming (alternating arable cropping and sown pasture). Systems 4 and 5 evolved into habitats which are much simpler in form and poorer in species than the others, which can be considered more diversified, permanent and less disturbed and consequently inherently containing elements of natural pest control. It is obvious that modern systems require more radical modifications of their structure to approach a more diversified, less disturbed state. If it is argued that such modifications are not possible in large scale agriculture due to technical or economic factors, then there is a strong conservative argument in favor of small, multiple use farms.
An obvious form of environmental management concerns vegetational designs across appropriate levels of scale. AT the regional level landscape vegetation mosaics influence the distribution of food and shelter resources and consequently, colonization patterns of insects (Andow 1983b). At a smaller scale, herbivores and their natural enemies respond to localized patterns of plant spacing, plant structure and plant species (or varietal) diversity. Environmental components and their management in agroecosystems have three main dimensions: temporal, spatial and biological. Other means of biotic management through inundative releases and classical biological control are considered in other sections. Mechanical modes of environmental management, such as cultivating, mowing and harvesting affect the structure and permanence of the habitat and thus the life processes of insects in agroecosystems. Chemical inputs, such as the periodic application, water, fertilizers, behavior modifying agents and the pesticides affect the rates of growth and survival of pests and natural enemies. Biotic, physical/chemical and mechanical manipulations are imposed upon agroecosystems often as means to achieve objectives unrelated to insect pest management, but the possible range of environmental manipulations designed for higher yields can be broad enough to incorporate tactics which simultaneously improve pest control.
Management of Vegetation.--Monocultures which are frequently disturbed often favor the rapid colonization and growth of herbivore populations. Initial conditions of natural enemy-free space and high abundance of pests further reduces the ability of natural enemies to regulate them (Price 1981). These negative factors can be minimized or eliminated by providing continuity of vegetation (and the associated food and shelter) in time and space, thereby aiding natural enemies. Studies documenting direct behavioral and physiological effects of plants on natural enemies are numerous (e.g., van Emden 1965, Leius 1967, Campbell & Duffey 1979, Nettles 1979, Altieri et al. 1981, Letourneau & Altieri 1983, Boethel & Eikenbary 1986, Letourneau 1987). Entomophages are sometimes more abundant in the presence of certain plants, even in the absence of hosts or prey, or they are attracted or arrested by chemicals released by the herbivore's host plant or other associated plants. Some parasitoids prefer particular plants over others (Monteith 1960, Shahjahan 1974, Nettles 1979). Other authors recognized that parasitism of a pest was higher on some crops than on others (Read et al. 1970, Martin et al. 1976, Nordlund et al. 1985, Johnson & Hara 1987).
Noncrop plants within and around fields can also benefit biological control agents (Altieri & Whitcomb 1979a,b; Barney et al. 1984, Norris 1986). Rapidly colonizing, fast growing plants offer many important requisites for natural enemies such as alternate prey or hosts, pollen or nectar, and microhabitats which are not available in weed free monocultures (van Emden 1965, Doutt & Nakata 1973) but these interactions can be difficult to define and to implement in control programs (Flaherty et al. 1985). Outbreaks of some kinds of crop pests are more apt to occur in weed free fields than in weed diversified crop systems (Dempster 1969, Flaherty 1969, Root 1973, Smith 1976a, Altieri et al. 1977). Crop fields with dense weed cover and high diversity usually have more predaceous arthropods than do weed free fields (Pimentel 1961, Dempster 1969, Flaherty 1969, Pollard 1971, Root 1973, Smith 1976b, Speight & Lawton 1976). Carabids (Dempster 1969, Speight & Lawton 1976, Thiele 1977), syrphids (Pollard 1971, Smith 1976b), and coccinellids (Bombosch 1966, Perrin 1975) are abundant in weed diversified systems. Relevant examples of cropping systems in which the presence of specific weeds has enhanced the biological control of particular pests are numerous. The potential for managing weeds as useful components of agroecosystems is great, but not all weeds promote biological control (see Powell et al. 1986).
Leius (1967) found that the presence of wild flowers in apple orchards resulted in an 18X increase in parasitism of tent caterpillar pupae over nonweedy orchards; parasitism of tent caterpillar eggs increased 4X, and parasitism of codling moth larvae increased 5X. A cover crop of bell beans, Vicia faba L. in rain fed apple orchards in northern California decreased infestations by codling moth. This lower moth infestation was correlated significantly with increased numbers of predators in the Aranae, Coccinellidae, Syrphidae and Chrysopidae, which were present on the trees (Altieri & Schmidt 1985). Similar observations were made by Dickler (1978) in Germany. In New Jersey peach orchards, control of the oriental fruit moth increased in the presence of ragweed, Ambrosia sp., smart weed, Polygonum sp., lambsquarter, Chenopodium album L., and goldenrod, Solidago sp. Such weeds provided alternate hosts for the parasitoid Macrocentrus ancylivorus Rohwer (Bobb 1939). O'Connor (1950) suggested the use of a cover crop in coconut groves in the Solomon Islands to improve the biological control of coreid pests by an ant, Oecophylla smaragdina subnitida Emery. In Ghana, coconut served this purpose by providing sufficient shade for cocoa to support high populations of Oecophylla longinoda Latreille which maintained the cocoa crop free of cocoa caspids (Leston 1973). Annual crops diversified with cover crops also suffer less damage. Brust et al. (1986) reported dramatically higher predation rates of Lepidoptera larvae (black cutworms, Agrotis ipsilon (Hufnagel), armyworms, Pseudaletia unipunctata Haworth, stalk borers, Papaipema nebris (Guenée) and European corn borers, Ostrinia nubilalis (Hübner) tethered to corn sown into a grass/legume mixture than to corn in monoculture. Carabid beetles were more abundant to the living mulch system and were among the larval predators in both systems.
Because farming in a region differs in energy inputs, levels of crop diversity and successional stages, variations in insect dynamics may occur that are difficult to predict. However, low pest potentials may be expected in agroecosystems that show traits as follows: (1) high crop diversity through mixtures in time and space (Cromartie 1981, Altieri & Letourneau 1982, Risch et al. 1983, Andow & Risch 1985, Nafus & Schreiner 1986). (2) Discontinuity of monoculture in time through rotations, use of short maturing varieties, use of crop-free or host free periods, etc. (Stern 1981, Lashomb & Ng 1984). (3) Small scattered fields creating a structural mosaic of adjoining crops, and uncultivated land which potentially provide shelter and alternative food for natural enemies (van Emden 1965, Altieri & Letourneau 1982). Pests also may proliferate in these environments depending on plant species composition (Altieri & Letourneau 1984, Collins & Johnson 1985, Levine 1985, Slosser et al. 1985, Lasack & Pedigo 1986). But the presence of low levels of pest populations and/or alternate hosts may be necessary to maintain natural enemies in the area. (4) Farms with a dominant perennial crop component. Orchards are considered to be more stable as permanent ecosystems than are annual crop systems. Because orchards suffer less disturbance and are characterized by greater structural diversity, possibilities for the establishment of biological control agents are generally higher, especially if floral undergrowth diversity is encouraged (Huffaker & Messenger 1976, Altieri & Schmidt 1985). Sometimes orchard sanitation practices may interfere with the performance of natural enemies, as is the case with sanitation to remove mummied almond fruit from almond and walnut trees that serve as overwintering reservoirs for parasitized hosts (Legner 1983a). (5) High crop densities and the presence of tolerable levels of weeds (Shahjahan & Streams 1973, Altieri et al. 1977, Sprenkel et al. 1979, Mayse 1983, Andow 1983a, Buschman et al. 1984, Ali & Reagan 1985). (6) High genetic diversity resulting from the use of variety mixtures or several lines of the same crop (Perrin 1977, Whitham 1983, Gould 1986, Altieri & Schmidt 1987).
The above generalizations can serve in the planning of a vegetation management strategy in agroecosystems; but they must take into account local variations in climate, geography, crops, local vegetation, inputs, pest complexes, etc., which might cause increases of decreases in the potential for pest development under some conditions. The selection of component plant species also can be critical. Systematics studies on the quality of plant diversification with respect to the abundance and efficiency of natural enemies are needed. While 59% of the 116 species of entomophages in documented studies reviewed by Andow (1986) exhibited increased abundance when plant species were added to the system, 10% decreased in abundance and 20% were variable, sometimes increasing and other times decreasing. Nafus & Schreiner (1986) found lower parasitism rates in intercropped corn. The addition to squash decreases the abundance of Coleomegilla maculata (DeGeer) on squash because of a nonuniform distribution of prey (Andow & Risch 1985). However, Orius tristicolor (White), a generalist predator, is more abundant on squash when corn is interplanted, and plant architecture and the nonuniform distribution of prey are beneficial (Letourneau 1988). Plant density and diversity may interact negatively to determine ground beetle emigration rates (Perfecto et al. 1986). Mechanistic studies to determine the underlying elements of plant mixtures that enhance or disrupt colonization and population growth of natural enemies allow a more precise planning of cropping schemes and increase the chances of a desired effect beyond the current levels.
Management of Crops With Mechanical Devices.--Manipulating the environment with mechanical devices may disturb the system depending on its severity and frequency. Low input, perennial systems would present an extreme contrast to mechanized annual crop production systems, for example. But slight modifications in cultural practices for sowing, maintaining and harvesting annual crops can effect substantial changes in natural enemy populations which bring them nearer to those observed in less disturbed perennial counterparts (Arkin & Taylor 1981, Barfield & Gerber 1979, Blumberg & Crossley 1983, Herzog & Funderburk 1985).
Cultivation & Habitat Disturbance.--Modern tillage practices reflect attempts to limit mechanical disturbance of the soil; and there is an emphasis on surface tillage and no tillage as alternative to plow tillage in order to control soil erosion, enhance crop performance, use energy more efficiently (Sprague 1986) and reduce soil breeding chloropid eye gnats (Legner 1970 ). Minimum tillage systems can conserve and enhance natural enemies of important pests (Legner 1970 , House & All 1981, Luff 1982, Blumberg & Crossley 1983, All & Musick 1986), altho each case must be considered independently.
Plowing, disking and other manipulations of the soil or breeding habitat can affect ground or waste-dwelling arthropods, whether they inhabit the soil consistently or intermittently (Legner 1970 , Legner et al. 1973-1980). The extent of direct mortality depends on their distribution with respect to soil depth and their phenologies. Less directly put potentially as important effects are caused by the removal of resources and natural enemies associated with living undergrowth and plant residues. The impact of natural enemies on crop pests in such systems, and the casual links between tillage practices, numbers of natural enemies, and level of biological control has been shown in only a few cases (Risch et al. 1983, Letourneau 1987).
Significantly higher densities of carabids, including Amara spp., Pterostichus spp. and Amphasia spp occurred in no tillage systems and were the major factor reducing black cutworm damage below that achieved in conventional corn systems (Brust et al. 1985). Other studies show that herbivore damage is reduced in no tillage fields despite similar predator abundance in tilled and nontilled fields. For example, reduced rootworm, Diabrotica spp., damage to corn in nontilled fields compared to plowed fields reflected lower herbivore densities (Stinner et al. 1986). Although spider density was highest in nontilled systems, predators in general did not exhibit higher densities. Probably efficiency rather than abundance of predators/parasitoids are enhanced and the vegetative component may be important by providing alternative resources to entomophages. Foster & Ruesink (1984) showed that the flowering weeds associated with reduced tillage in corn are important nectar sources that increase survival and fecundity of Meteorus rubens (Nees) an important parasitoid of the black cutworm.
Ants are generalist predators sensitive to tillage practices in agroecosystems (Risch & Carroll 1982). Altieri & Schmidt (1984) reported greater species richness, abundance and predation pressure in uncultivated orchard systems than in those cultivated twice in six weeks. Both lack of nest disturbance and habitat suitability due to vegetational cover may be important causes of greater ant abundance. Similar results were predicted for a highly effective predator of bollworm, Iridomyrmex pruinois (Rogers) in Arkansas cotton fields (Kirkton 1970) based on field observations. Carroll & Risch (1983) and Letourneau (1983) sampled ant activity in lowland tropical Mexico where farming practices are in transition between slash-and-burn and mechanized cropping practices. The number of ant species at tuna baits in maize fields was similar whether they had been plowed or sown into slash (20-23 spp.). But in central Texas, spring plowing decreased ant species richness from 12 species to 2 species. Among the species that were no longer present at baits after plowing were those that prey on Solenopsis invicta Buren queens.
Pests can be suppressed directly by plowing the soil (Watson & Larsen 1968) and burying stubble (Holmes 1982). Talkington & Berry (1986) significantly reduced the adult emergence of the pyralid moth pest Fumibotys fumalis (Guenée), in peppermint fields by burying the prepupae into the soil; tillage depth was directly correlated with control. In locations where natural enemies are not effective, deep burial of infested stubble may be necessary (Umeozor et al. 1985). However, a study by Telenga & Zhigaev (1959) on the beet weevil, Bothynoderes punctiventris Germer, shows how differential effects on pests and their natural enemies can be achieved through carefully planned tillage practices. Although >90% of the weevil eggs were destroyed by deep plowing, surface tillage with a disk increased the survival of a parasitoid on the eggs, which caused a greater level of pest control. Nilsson (1985) found that an average of 4X as many parasitoids of Meligethus sp. pollen beetles emerged from fallow fields or from plots of rape that had direct drilling of winter wheat than emerged from disk harrowed or plowed plots. Although the effect of these practices on parasitization were not studied, a regional use of direct-drilling was recommended. Studies in northern Florida by Altieri & Whitcomb (1979a,b) have shown that weed species composition changes markedly according to the date of plowing. Early winter plowing stimulated populations of golden rod, Solidago altissima L. and 58 predator species which feed on the aphids, Uroleucon spp, and other herbivores associated with this weed. However, plowing in mid-autumn caused camphor weed populations to be enhanced along with the 30 predator species associated with herbivores of this weed.
Mowing, Harvesting & Weed Control.--When crops are pruned or mowed, arthropods may move from the cut plant material and there will be a period of new growth. These can have important consequences on the performance and synchrony of natural enemies. Weeding can also stimulate crop colonization by associated arthropods, the extent to which movement will occur depending on distance and arthropod mobility, but some weeding operations leave associated arthropods intact and promote such movement. When patches of stinging nettle, Urtica dioica L., are cut in late spring, predators are forced to move into crop fields (Perrin 1975). Also Coccinellidae have been observed to move to orchard trees in southeastern Slovakia when grass weed cover was cut (Hodek 1973).
Alfalfa strip-cutting systems typically illustrate how natural enemy movement prompted by vegetation cutting can occur. Van den Bosch & Stern (1969) compared densities of several predators, including Geocoris pallens Stal, Nabis americoferus Caryon, Orius tristicolor, Chrysoperla carnea (Stephens), and Hippodamia spp. in strip-cut and solid cut fields. Movement out of the field was uncommon even for these mobile predators in strip-cut fields; most moved onto adjacent plants so that on a field wide basis these predators were conserved. Strip cutting also reduced mortality of Aphidius smithi Sharman & Rao by providing shelter from adverse physical conditions and host scarcity. Host availability for the parasitoid, Cotesia medicaginis (Muesebeck) in alfalfa was altered through a different mechanism, however. Oviposition rates of the alfalfa butterfly, Colias philodice eurytheme Godart, peak on new growth following harvest, which causes periodicity in the availability of early instar larvae. Strip cropping can spread the vulnerable stages more evenly over time and thus favor the maintenance of A. medicaginis populations over the season.
When fire is used to prepare land for cropping by the "slash and burn" practice or to reduce crop residue, the affects on resident natural enemies and incoming colonists can be serious. Burning of old fallow vegetation in a tropical slash and burn system decreased ant abundance and foraging activity for more than four months (Saks & Carrol 1980). Although fire has been used as a tool for direct control of pests (Komareck 1970), generalizations on its effect on natural enemies are not possible. An isolated study showed that controlled burning increased spider and ant densities and biomass due to increased food supply for herbivores in the form of succulent plant growth after the burn (Hurst 1970).
Chemical Usage.--Although the influence of water and fertilizer applications on herbivores is complex (Scriber 1984, Louda 1986), fertilizer and herbivory levels may be causally related through changes in plant quality or phenology that affect the dynamics of predator/prey and host/parasitoid interactions. However, pesticides have direct detrimental effects on natural enemies and their use in environmental management must be limited to situations where they are timed carefully or selectively applied. Perhaps the use of behavior modifying chemicals (Lewis & Nordlund 1985) will provide new tools for the manipulation of biological control agents, but to date practical deployment has not resulted (Chiri & Legner 1983, 1986).
Fertilizer.--Changes on the physiological conditions of crops caused by soil amendments may have consequences for pest management, which depend on soil variability, the growth, developmental and biochemical responses of the plant, the direct effects of such changes on herbivores and the secondary impact on natural enemies. Much work has been done on herbivore response to fertilizers that increase nitrogen levels in plants. Mattson (1980) believed that foliage N-level is a major regulator of herbivory rate. Although insects often improve their survival, fecundity and growth rates when plant quality is increased (higher N), general statements on the direct responses of herbivores to nitrogen fertilizer are not possible because of the array of responses by different species (Scriber 1984). Experiments on links between soil amendments and pest management relate to the effects on the pest via their response to resistant and susceptible varieties under conditions of different sources or levels of Ca, Mg, N, P, K or S (Kindler & Staples 1970, Culliney & Pimentel 1986, Shaw et al. 1986, Manuwoto & Scriber 1984). Thus the natural enemy's environment is affected by soil amendments through changes in plant quality as well as by the concomitant changes in the herbivores.
The direct effects of fertilizer on biological control are not well known. Many herbivores exhibit marked increases in population growth on nitrogen enriched hosts. There is an obvious concern for the ability of natural enemies to track their prey/hosts under conditions. There were no differences in biological control of mites detected on apple trees treated with three levels of nitrogen fertilizer (Huffaker et al. 1970). Although the fecundity of Panonychus ulmi (Koch) increased with the nitrogen level up to a 4X increase, when Amblyseius potentillae (Garman) predators were not present, the predators were able to compensate for most of the increased prey density. However, fertilized cotton plots exhibited higher levels of Heliothis zea (Boddie) than did controls despite significantly higher population densities of Hippodamia convergens Guerin-Meneville, Coleomegilla maculata langi Timberlake and Orius insidiosus (Say) in fertilized cotton (Adkisson 1958). Chiang (1970) showed that fertilized corn fields (50 tons manure/acre) had significantly fewer (ca. 1/2) corn rootworms than did unfertilized controls. Although ground beetles and spiders were not affected, the populations of phytophagous and predaceous mites were 3X higher in manure treatment plots. Through three seasons of field and laboratory experiments Chiang (1970) concluded that mit predation accounted for 20% control of corn rootworm under natural field conditions and 63% control when manure was applied. Other effects of fertilizers on natural enemies may be predicted based on the combined information of relevant studies. For example it is known that the parasitoid Diaretiella rapae (McIntosh) attacks the green peach aphid Myzus persicae (Sulzer) more readily when the aphid is associated with Brassica spp. (Read et al. 1970), the mustard oils in crucifers serving as attractants. It has also been shown that some glucosinolates are inversely related to nitrogen level (Wolfson 1980), and thus soil fertility may have profound effects on pest control by limiting the production of semiochemicals that play an important role in mediating interactions between plants, herbivores and natural enemies.
The frequency and levels of fertilizer applications can modify the synchrony of predators with their prey. Low nutritive quality of host plants may cause immature herbivores to develop more slowly, and thus increase their availability to natural enemies (Feeny 1976, Moran & Hamilton 1980, Price et al. 1980). A predaceous pentatomid was found to regulate more efficiently the Mexican bean beetles on nutritionally poor plants than on highly fertilized ones (Price 1986). Host plant phenology can also be driven by fertilizer inputs, and Hogg (1986) suggested that the timing of square availability was one factor influencing predation and parasitism rates of H. zea in cotton.
Changes in nutritive quality of host plants as influenced by fertilizer may indirectly affect the survival and reproduction of natural enemies by determining prey quality. Although direct examples of fertilizer effects have not been demonstrated, nitrogen content is known to be an important aspect of prey quality. Nitrogen content may be responsible for higher egg production by H. convergens when fed apterate instead of alate green peach aphids (Wipperfürth et al. 1987). Analagous effects may occur in the case of prey of different quality due to host plant conditions. Zhody (1976) observed that size, fecundity and longevity of Aphelinus asychis (Walker) was dependent on the food composition of the host Myzus persicae. But low quality food can also impair the ability of a host to encapsulate a parasitoid (El-Shazley 1972a,b). Nutrients in the host plant can also modify toxic effects to parasitoids (Duffey & Bloem 1986) and influence their sex ratio (Greenblatt & Barbosa 1981). Host size is often an important determinant of egg fertilization by ovipositing females (Charnov 1982). Although studies on direct effects of nitrogen on crop architecture and subsequent effects on searching efficiency are not available, some studies indicate that these interactions can occur. The sex ratio of Diadegma reared from larvae of Plutella xylostella L. from field plots over a wide range of nitrogen fertilizer inputs showed a significant trend for female bias in heavy fertilized plots.
Soil nutrient levels are known to influence plant size, leaf area, canopy closure and crop architecture, and these conditions define searching area for natural enemies (Kemp & Moody 1984). Predator/prey or parasitoid/host contact rates are a function of habitat preference, searching area, prey density and dispersion patterns. Fye & Larsen (1969) found that the searching efficiency of Trichogramma spp. was dependent on structural complexity. Hutchison & Pitre (1983) did not find this effect with Geocoris punctipes (Say) on H. zea, however. Shady conditions resulting from overgrowing vegetation reduce parasitism levels of Pieris spp. (= Artogeia spp.) by Cotesia glomerata (L.) (Sato & Ohsaki 1987) by deterring the parasitoid.
The levels of key chemical constituents in the soil can indirectly affect natural enemies by influencing weed composition in a field. In Alabama fields with low soil potassium were dominated by buckhorn plantain, Plantiago lanceolata L. and curly dock, Rumex crispus L., while fields with low soil phosphorus were dominated by showy crotalaria, Crotalaria spectabilis Roth, morning glory, Ipomoea purpurea Roth, sicklepod, Cassia obtusifolia L., Geranium carolinianum L. and coffee senns, Cassia occidentalis L. (Hoveland et al. 1976). Soil pH can influence the growth of weeds, e.g., weeds of the genus Pteridium occur on acid soils while Cressa sp. inhabits only alkaline soils. Other species of Compositae and Polygonaceae are found growing in saline soils (Anon. 1969).
Water.--Plant quality and RH at the field level can be influenced by flooding fields, draining land and furrow, drip or sprinkler irrigation. The desert valleys of southeastern California are suitable habitat for the predaceous earwig Labidura riparia (Pallas) due to favorable conditions produced by irrigation (van den Bosch & Telford 1964). Much of the experimental work on the effects of plant stress from water conditions has targeted herbivores (Miles et al. 1982, Bernays & Lewis 1986, Louda 1986). Water availability can affect palatability, feeding duration, developmental time, migration, survival and fecundity of plantfeeders. Therefore, many important effects of water conditions on natural enemies are indirect and are mediated through changes in host/prey abundance and dispersion or through qualitative changes. For example, rape plants under drought conditions had increased proline levels and an associated shift in the balance of free amino acids (Miles et al. 1982). Cabbage aphids reached adulthood faster on stressed plants, and availability of suitable hosts for parasitoids might thus be decreased both by the duration of vulnerable stages and if the parasitoids require slower development than the host, if plants are water stressed.
The direct effects of water include mortality during irrigation and impacts of RH. Ferro & Southwick (1984) and Ferro et al. (1979) reviewed the importance of RH on small arthropods. Crop architecture and watering regimes cause large deviations from ambient temperature and humidity levels (Ferro & Southwick 1984) within foliage boundary layer microhabitats. Irrigation of soybean caused a substantial decrease in canopy temperature and a 16% increase in RH at 15 cm above the ground (Downey & Caviness 1973). Prolonged periods of such irrigation effects can have important consequences for natural enemies because developmental time and therefore population growth and synchrony are related to temperature and RH. This may be illustrated in the case of the tachinid Eucelatoria armigera (Coquillett), which completes development at different rates depending on temperature and host species (Jackson et al. 1969). Holmes et al. (1963) showed that parasitism levels of the wheat stem sawfly by Bracon cephi (Gahan) were enhanced by soil moisture and temperature levels that slow plant ripening. Force & Messenger (1964) showed that a few degrees dramatically affect changes of the innate capacity for increase (r) in parasitoids under laboratory conditions. Cotesia medicaginis reaches its maximum longevity at 55% RH; longevity decreased markedly at levels above and below this value (Allen & Smith 1958). However, it was not deemed an important factor in determining parasitism levels of Colias spp. larvae in alfalfa. But it is known that armored scale parasitoids in arid citrus groves require irrigated conditions for maximum biological control (DeBach 1958b). The vertical profile and general microclimate depend not only on water inputs but on mulching, row direction, windbreaks and crop spacing (Hatfield 1982). The severity of effects caused by drought conditions depends on many factors, including availability of free water and nectar in the habitat. Bartlett (1964) reported that caged Microterys flavus (Howard) was able to function well at extremely low RH if provided with honey and water.
Semiochemicals.--The knowledge that parasitic insect behavior is influenced by chemicals produced by their hosts stimulated considerable interest in the use of semiochemicals for manipulating predators and parasitoids in the field, especially for aggregating and/or retaining released parasitoids in target areas (Gross 1981). The various opportunities for and limitations of manipulating natural enemies with semiochemicals were reviewed by Vinson (1977), Nordlund et al. (1981a,b, 1988), Powell (1986) and Hagen (1986). Lewis et al. (1976) suggest that host or prey selection is the most important step in the searching behavior of entomophagous insects that can be manipulated to improve biological control. Semiochemicals should be used to increase effective establishment of imported species, improving performance and uniform distribution of released species throughout a target area and optimizing abundance and performance of naturally occurring natural enemies (Greenblatt & Lewis 1983). It is possible to devise three main habitat management technique with semiochemicals: (1) strategies directed at improving habitat characteristics such as the use of semiochemicals to make crops more attractive or to define a more complex mosaic of local search areas (Altieri et al. 1981). Gardner & van Lenteren (1986) nevertheless give an exception. (2) Enhancing host plant characteristics; breeding programs directed at improving chemical attractiveness of crops or crops with extrafloral nectaries. (3) Mimicking high pest densities through applications of diatomaceous earth or artificial eggs impregnated with kairomones (Gross 1981).
Drift of Pesticides.--Low level inputs of insecticides to nontarget areas result from aerial applications. Half the material applied to a field under ideal conditions can drift a considerable distance downwind (Ware et al. 1970). Although a great deal is known about the effects of direct spraying of various insecticides on natural enemies, there is not much experimental work to determine the effects of low level inputs. Biological control can be disrupted given sufficient frequency, intensity and toxicity of sprays (Ridgway et al. 1976, Riehl et al. 1980). The ratio of natural enemies to herbivores was increased by low, drift-level concentrations of carbaryl, and arthropod abundance dropped significantly more in an old field than it did in a corn monoculture. It was suggested that low concentrations of insecticides have different effects on herbivores and natural enemies depending on whether the nontarget habitat is a crop field or a field of natural vegetation which serves as a source of colonizers. However, such impacts cannot be predicted from knowledge of effects at high concentrations (Risch et al. 1986). Drift of chemicals may be minimized by making applications when winds are less than 2 m/sec, using adjuvants, formulating inert emulsions and using large droplet sizes (Gebhardt 1981). Windbreaks surrounding field and regional wide spray synchrony are forms of cooperative efforts for drift reduction of the effects of low level pesticide applications.
The application of herbicides to crop fields can have nontarget effects similar to low-level insecticide application. Baker et al. (1985) showed that Orius spp. and Nabis spp. densities were decreased by monosodium methanearsenate, but not the abundance of spiders, Geocoris spp., Hymenoptera and coccinellids. Herbicides may also modify weed species composition in fields and thereby affect natural enemies.
Other Pollutants (Dust).--Dust and pollutants of different kinds may influence the efficiency of predators and parasitoids. Environmental management includes consideration of the placement of the sources and control of pollutant influx with respect to agricultural fields. It has long been known that some pest outbreaks are caused or enhanced by dust on crop foliage. Bartlett (1951) found that many inert dusts rapidly killed Aphytis chrysomphali (Mercet) and Metaphycus luteolus (Timberlake). DeBach (1985a) demonstrated an increase in California red scale populations on citrus trees in response to road dust. Mechanisms may be mechanical interference or desiccation (Edmunds 1973). It is possible also that leaf temperature, which can be raised 2-4°C by dust cover (Eller 1977) is a factor. Planned placement of roads and timing of cultivation can reduce the level of dust on crops. Strawberry growers in California profit from daily or twice daily watering of roadways through the reduction in losses from mites, as predaceous mites are apparently inhibited by dust.
Gaseous air pollutants are more difficult to detect and to control. Sulphur dioxide is a common effluent that has known negative effects on a variety of organisms (Petters & Mettus 1982), including honeybees (Ginevan et al. 1980). But acute exposure of female Bracon hebetor (Say) to sulphur dioxide in air causes no reduction in fertility and fecundity. Petters & Mettus (1982) suggested that damage to parasitic wasps may develop in the earlier stages or behavioral avoidance of contaminated areas may explain reports of lower parasitoid and higher herbivore levels near sources of sulphur dioxide pollution. Melanic morphs of the generalist coccinellid predator Adalia bipunctata (L.) occur disproportionately often in the vicinity of coal processing plants in Great Britain. Although earlier investigators suggested a mechanism involving selective toxicity of air pollutants, Muggleton et al. (1975) attributed the differences to sunshine levels. Whether or not the coloration of such predators affects their efficiency as biological control agents is unknown. Other sources of contamination include auto traffic, drainage from selenium rich soils (Gerling 1984), and ozone (Trumble et al. 1987). Literature stresses effects on herbivores, and little is known about effects on natural enemies. Lead as a contaminant from auto exhaust has been shown to concentrate in higher trophic levels (Price et al. 1974). Some pollutants are inadvertently added to the crop with soil amendments, such as sludge, manure and chemical fertilizer (Wong 1985). Culliney et al. (1986) found a general response of low arthropod diversity when sludge containing heavy metals and toxic chemicals was applied to cole crops.
Insights into the biological mechanisms for environmental management that enhances biological control can be obtained from an examination of host selection processes of entomophages, which includes host or prey habitat location, host or prey location and host or prey acceptance (Vinson 1981). Designing crop habitats for effective biological control requires an understanding of such mechanisms. During migration and habitat location the effective environment may be the local area, a regional landscape or a series of distant habitat patches with long distances between them. The interplay of colonizer source location, wind patterns, vegetation texture and host or prey density becomes important on a large scale. Maximum levels of natural control require at the onset both sufficient numbers of natural enemies and temporal synchrony of these invasions. Regional environmental management for enhancing the success of habitat location by natural enemies should focus on the arrangement of colonizer sources in relation to target sites of potential pest problems as well as on timing of natural enemy colonization. Rabb (1978) addressed these needs when he criticized the propensity of single commodity, closed system approaches to pest management in research and decision making as deficient for problems which demand attention to large unit ecosystem heterogeneity.
Natural enemies vary in their dispersal range, and migration often occurs in high currents along paths of turbulent convection. Even weak flying insects can disperse over long distances and across wide areas by exploiting the ephemeral but very structured nature of air movement (Wellington 1983). For example, robust hosts and minute parasitoids can exhibit coupled displacement in long distance migration, as shown by the Australian plague locust Chortoicetes terminifera Walker and its egg parasitoid Scelio fulgidus Crawford which disperse independently on wind currents to the same location (Farrow 1981). Cumulative numbers over a growing season may be irrelevant if immigration rates of natural enemies are very slow in relation to rising levels of the pest (Doutt & Nakata 1973, Letourneau & Altieri 1983, Williams 1984). Information on source constitution, phenology and flight patterns are necessary to design and manage regional scale agroecosystems for optimal biological control. Flight capacity studies and mathematical models to describe movement patterns based on continuous diffusion or discrete random walk equations have focused on predicting dispersal and migration of herbivores (Okubo 1980, Stinner et al. 1983, 1986). Biological information coupled with predictive models of natural enemy movement may aid in predicting synchrony (Duelli 1980), but many times synchronies are difficult to achieve because local species are adapted to exploit natural conditions of prey or host phenologies. For example, coccinellid beetles in California estivate during times of prey availability; irrigated crops provide a continuous food supply that was not available in an area before agricultural expansion had occurred (Hagen 1962).
While locating hosts or prey, factors such as the physical texture of plant surfaces, structural attributes of plants, microclimatic conditions and patch heterogeneity interplay. Flaherty (1969) showed enhanced control of herbivorous mites on grape vines with Johnson grass cover. The grass acted as a source of predaceous mites. In this study involving prey location, and in the habitat location phase study of Doutt & Nakata (1973), the cumulative total number of natural enemies was not as important as the temporal synchrony with growing herbivore populations. During host or prey acceptance and predation or parasitism, environmental factors operate indirectly through their effects on host or prey behavior, host or prey quality and alter levels of vulnerability of natural enemies to mortality factors. Examples of the mechanisms of host or prey selection on all levels of natural enemy behavior were given by many authors.
Activities other than those directly associated with predation or parasitism are migration to overwintering sites, mating, and the acquisition and use of resources other than the primary prey or hosts. The interdependence and variability of resource needs and factors such as proximity and availability of resources in time become vital aspects of the environment. These are factors of habitat suitability for natural enemies. A reduction of the relative energy expenditure needed, in a particular environment, to fulfill the resource needs of a particular parasitoid/predator will increase its efficiency as a biological control agent. Conservation of natural enemies through habitat management techniques adapted to the prevailing agronomic schemes can be of great benefit. Small changes in agricultural practices may increase natural enemy populations or enhance efficiency. But predators and parasitoids are extremely diverse and each family represents a particular range of responses to environmental modification. There are numerous examples of habitat management techniques that have been shown to increase the effectiveness an abundance of specific predator groups.
Natural Enemy-Free Space.--Probably the most general level of theory to guide habitat management for biological control is that of ecological and/or evolutionary escape from predators/parasitoids. Price (1981) acknowledged in his theory of natural enemy-free space, that pest irruption is a likely consequence of agricultural practices that foster the spatial and temporal isolation of herbivores from their natural enemies. Pest introduction to a novel environment is a classic example (Price 1981, Altieri & Letourneau 1982, 1984; Risch 1987). Temporary release of pests also occurs under conditions of insecticide caused pest resurgence and secondary pest outbreaks. Evolutionary changes in native crop pests (Host shifts) is still another process that may result in a reduction of predation/parasitism.
Island Biogeographic Theory.--Cultivated areas are insular in nature, which has motivated several analogies regarding crops as islands available for colonization by arthropods (Strong 1979, Price & Waldbauer 1982, Simberloff 1985). The development of arthropod communities in crops was analyzed using MacArthur & Wilson's (1967) theory of island biogeography, which allows the prediction of colonization rates and mortality/emigration rates, on a comparative basis, with respect to crop area, distance from the sources of colonizers, and crop longevity (assuming that the system has aspects of equilibrium). The species composition, structure and abundance of arthropods colonizing a crop field are the result of highly dynamic processes and the assumption of equilibrium is often inappropriate, however. But some predictions from the theory seem possible.
One example is that species richness is positively correlated to size on oceanic islands. Similarly in mainland communities, the number of herbivores associated with a plant is a positive function of the local area planned to or covered by that species (Strong 1979). Larger host islands probably collect more individuals by random probability of encounter. Also, patch detection by dispersers may increase with size. The effect of an increase in the number of herbivores with an increase in size is important for consideration in pest management strategies. But any increase in species diversity must be defined by the proportion in each trophic level, and if possible by the component species' biologies before it can be analyzed for pest management potential. MacArthur & Wilson's (1967) model treated all members of s species source pool as equivalent colonizers. The application of this theory to dynamic and temporary crop islands requires the consideration not only of the number of species and pattern of occurrence, but the order of colonizer establishment by trophic level (Altieri & Letourneau 1984, Robinson & Dickerson 1987).
Extinction rates depend upon resource availability in a system. Because the plants are supplied to the system or reset at certain intervals (Levins & Wilson 1980), the resource base may be more predictable for herbivores at least early in the season. The immigration rates of natural enemies to large expanses of monoculture may be similarly increased, though spread from the edges may be slow and thus favor the development of herbivore populations. The equilibrium theory of biogeography does not allow for comparisons of single, large crop fields versus a network of several small fields of the same total area, yet the contrasting designs are likely to differ in terms of suitability for biological control (Price 1976).
Even though most theory based on island community development poses questions and organizes thought on crop design, the barriers to its application are (1) frequent disturbance of most crop fields reduces the rigor and applicability of equilibrium models; (2) the few current empirical data available on diversity, size and distance relationships do not constitute a sufficient basis for environmental design recommendations (Simberloff 1985); (3) the theory does not distinguish pests and beneficial organisms (Stenseth 1981); (4) economic impact of changing island size must be viewed as exceedingly risky until demands for more certainty in the theory are met (Simberloff 1985). However, Liss et al. (1986) presented a modification of the MacArthur & Wilson (1967) model that incorporates colonizer source composition and changes in island habitats over time.
Consumer Dynamics.--Studies of consumer dynamics become important after the natural enemies are within the habitat of their prey or hosts, in order to predict the outcome of their interactions. Trophic interaction studies in manipulated and natural systems have focused on two trophic levels, such as plant-herbivore, host-parasitoid and predator-prey. Theory and data both demonstrate the regulation of populations at the lower trophic level (plant, prey or host) by natural enemies (Clark & Dallwitz 1975, Mattson & Addy 1975, Murdoch & Oaten 1975, Podoler & Rogers 1975, Morrow 1977, Gilbert 1978, Hassell 1978, May & Anderson 1978, Clark & Holling 1979, Murdoch 1979, McClure 1980, Kareiva 1982). On the other hand, natural enemies have been ineffective in other cases studied (Southwood & Comins 1976, Strong et al. 1984, Walker et al. 1984). The effectiveness of natural enemies as regulators of herbivore populations depends not only on behavioral and developmental responses of individual predators and on responses of the entire population to changes in prey or host densities (Murdoch 1971, Murdoch & Oaten 1975, Fox & Murdoch 1978), but also on variation in plant parameters such as density, secondary compounds and associated plants species. The ability of natural enemies to regulate the herbivores depends on the herbivore population's intrinsic growth rate (r), which in turn reflects the quality of the plant diet. Small changes in r caused by slight differences in plant quality, such as variety, secondary chemistry, nutrients, may determine whether or not parasitoids or predators can control the herbivore populations (Lawton & McNeill 1980, Price et al. 1980). The effectiveness of regulation also reflects subtle differences in the timing of population events in both predator and prey populations (Hassell 1978, May & Anderson 1978). Theory and data on interactions involving three trophic levels in a complex habitat are ultimately more suitable as a basis for environmental management strategies (Price 1986, Duffey & Bloem 1986, Barbosa & Letourneau 1988). Therefore, the goal of such preemptive measures of pest control is to avoid the provision of enemy-free space in agricultural environments and instead to present pests simultaneously with deleterious effects caused by their natural enemies and with selectively defensive or suboptimal properties of their food plants. Studying systems as communities of at least three trophic levels can contribute an understanding of complex interactions that is different from that likely to be gained purely as a byproduct of results from two level studies (Orr & Boethel 1986).
Vegetation Diversity & Patch Size.--Two hypotheses were proposed by Root (1973) to explain the tendency for low herbivore abundance in diverse vegetation. The Resource Concentration hypothesis, which predicts that many herbivores, especially those with a narrow host range, are more likely to find, survive and reproduce on hosts that are in pure or nearly pure stands. The Enemies hypothesis incorporates the third trophic level that Root (1973) predicted that vegetation would provide more resources for natural enemies (e.g., alternate hosts, refugia, nectar and pollen) and thus herbivore irruption would be rapidly checked by a higher diversity and abundance of natural enemies. Sheehan (1986) extended the resource concentration concept to predict that specialist natural enemies will respond to mixed vegetation differently, and probably less favorably, than will generalist predators and parasitoids, because of the importance of alternate prey for generalists. The designation of host/prey specialization categories, however, tends to rely only on one aspect of the resource spectrum of parasitoids and predators (Letourneau 1987). A range of species characteristics, such as relative vagility, resource needs, and habitat location cues may determine the response of parasitoids and predators to vegetational diversity.
Maintaining heterogeneity within an agroecosystem may also affect the success of establishment of imported biological control agents. The debate over the degree to which ultimate levels of regulation are attained by single versus multi species releases in classical biological control continues, but analyses of environmental factors as raw materials or as constraints are rarely considered (Beirne 1985). Factors such as species richness, climatic gradients and disturbance levels are important in assessing the susceptibility of large scale communities to biological invasion (Fox & Fox 1986).
Optimal Foraging.--During the host/prey selection process, natural enemies exhibit a chain of responses to stimuli. The objectives of biological control are to exploit natural processes that allow maximum prey encounter and foraging rates by natural enemies, and therefore, this body of theory is useful for predicting enhancement mechanisms and for evaluating the consequences of under and overexploitation.
The aggregation of foraging parasitoids in patches of higher host density has been a critical feature thought to be responsible for successful biological control (Beddington et al. 1978, may & Hassell 1981). Models of optimal patch use predict predation/parasitism levels between patches, based on host/prey densities (see Cook & Hubbard 1977, Waage 1979, Iwasa et al. 1984), but the power of these models varies. Murdoch et al. (1985) examined the importance of this searching behavior using the successful olive scale/Aphytis paramaculicornia DeBach & Rosen - Coccophagoides utilis Doutt system. These parasitoids do not aggregate in areas of high host density. Waage (1983) did find that Diadegma spp. attacking Plutella xylostella (L.) aggregated in patches with greater host density, yet the proportion of hosts parasitized at high host densities was not greater. Roland (1986) found similar results with Cyzenis albicans; whether or not the eggs are clumped, the level of parasitism is similar. Predictive models can be used to clarify the mechanisms involved in natural enemy behavior and their importance. It might be possible to take advantage of the simple rules that foragers use for decisions on how long to remain in a patch, which hosts or prey to seek and accept, when and where they will oviposit and especially for hymenopterous parasitoids, what the sex ratio will be. If these decisions are made in response to environmental cues, then they are potential field tools (Kareiva & Odell 1987). Dicke et al. (1985) found that searching eucoilid parasitoids remained longer in a patch with moderately higher kairomone concentrations regardless of the actual density of Drosophila melanogaster Meigen. Charnov & Skinner (1985) recommended careful reflection of both the proximate causes of such responses and the evolutionary causes as complementary approaches that enhance theory and application.
It is also necessary to consider the ultimate population effects on natural enemies given habitat manipulations that exploit behavioral cues and maximize prey reduction. A recent example giving particular attention to predator fitness shows that although juvenile mantids exhibit a strong Type II functional response, such behavior rapidly increases beyond the maximum gain in characteristics related to fitness (Hurd & Rathet 1986). In any case, natural enemy response to environmental manipulation should benefit through life table studies over many generations (Hassell 1986) and optimal foraging modes that include longer term population changes.
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