Habitat Management Crop Structure and Diversity

Habitat management is viewed as a strategy aimed at designing and constructing "phytocenotic architecture" dominated by plants that support populations of natural enemies (Altieri and Whitcomb, 1979; Altieri, 1983). Diversification of habitat is achieved through crop structure, protective refugia, occurrence of alternative prey/host, and supplementary food resources (nectar, pollen). Crop structure is the agroecosystem and its specific characteristics, its biotic composition, seasonality, etc. Protective refugia are defined as habitats in which beneficials can survive critical periods of the year (principally summer and winter periods) to disperse later to crops. Protected refugia can include a wide array of plant types and setting, e.g., rangeland, weedy field margins, autumn-sown crops, etc.

Alternative prey/hosts and their availability or proximity to crops are important at times when the target pest is low in numbers. Alternative hosts can improve the synchrony between natural enemies and the target pests. For a given beneficial, the alternative host might be a nonpest species feeding on wild plants or it might be a pest species different from the target on another crop (van den Bosch and Telford, 1964; Powell, 1986). For some aphid parasitoids, utilization of alternative hosts in proximity to the target pest and crop is known as multilateral control or the multilateral control approach (Stary, 1972; 1978). Such an approach takes advantage of the oligophagous host range of the bioagent. Switching from one host to another can be effective or ineffective depending upon the biotype or species specificity of the bioagent (Gordh, 1977). Population genetics in relation to host alternation is currently of high research interest. Until recently, host alternation was based solely on field observation and laboratory transfers; now, studies on population molecular genetics are further clarifying the species or species strains of key importance and their host alternation dynamics (Unruh et al., 1983; Nemec and Stary, 1985).

Sources of food such as nectar and pollen are requisite for hymenopterous parasitoid (van Emden, 1962) and syrphid (Hickman and Wratten, 1996) adults to ensure effective reproduction. More often than not, flowering plants in or even around agriculture crops for such uses are not always readily available (van Emden, 1962; Altieri and Whitcomb, 1979; Altieri and Letourneau, 1982; Powell, 1986).

The composition, seasonality, field size, and location of crops and noncrops all affect biodiversity. Semiperennial and perennial crops are generally classified as more stabilizing for biotic diversity in comparison with annuals; nonetheless, population drift can take place across all settings (Gross, 1987; Andow, 1991). Gliessman (1987) reported that whenever two or more crops are planted together, there is increased potential for species interactions, and this would include beneficials.

Diverse landscape mosaics in and around small-sized fields enhance the chances for greater diversity of beneficials. Nonetheless, relatively high biodiversity is thought possible even in intensely cultivated areas as long as crops are arranged together with patches of natural or seminatural areas (Duelli et al., 1989; 1990). Among the most difficult environments for biocontrol to succeed in are the annual crop monocultures. These usually lack the resources for the natural enemies to be efficacious, are grown using cultural practices (e.g., mowing of alfalfa) that often damage the natural enemy populations, and are present for only part of the year (Rabb et al., 1976; Powell, 1986). In such circumstances, it may be necessary through habitat management to maintain small populations of the target pest to ensure survival of the key beneficials (Powell, 1986). Volunteer crops along field margins may be useful in this regard.

Preservation, establishment, or sustainment of small heterogeneous strip habitats, in or neighboring crop farmlands, adds to the overall biodiversity of farm or area (Nentwig, 1993). Also, strip farming, especially intercropping, increases natural enemy opportunities for predation and parasitism over that of strict monocultures (Powell, 1986).

Cultural practices such as full field mowing of perennial legumes and grassy meadows hinder bioagent survival and success. These negative effects can be partially offset by strip mowing which affords arthropods an opening to retreat to uncut portions (Stern et al., 1964; Müller-Ferch and Mouci, 1995) and, thus, increases or strengthens the population stability of the beneficials (Schlinger and Dietrick, 1960; van den Bosch et al., 1967; Stary, 1970b).

Heterogeneous herbaceous strips constitute more suitable habitat for field species than strips of shrubs or trees that tend to harbor ecologically different forest species (Nentwig, 1988). Selected weed species used in strips within crop fields have been shown to attract and aid in the conservation of beneficials (Stary, 1964; van Emden, 1965; Gliessmann, 1987; Nentwig, 1988; 1992; 1993; 1994; 1995; Frei and Manhart, 1992; Weiss and Nentwig, 1992; Hausmann, 1996). Wyss (1995) showed that in apple orchards certain flowering weed strips resulted in more aphidophagous predators and fewer aphid pests than like areas without weeds. Nectar-bearing plants (Chumakova, 1977) and rich undergrowth of wild flowers (Leius, 1967) showed similar beneficial effects. Many natural enemies occur commonly in association with wild or natural habitat not always classified as weeds (van Emden, 1965). Tillage practices (no-till, minimum, conventional), tillage timing, mowing, or other agronomic practices can influence, sometimes significantly, the performance, success, maturation, and dispersal of beneficials (Bugg and Ellis, 1990; Bugg, 1992). Molthan and Ruppert (1988) demonstrated that flowers in wide boundary strips attracted beneficials, some being especially attractive and nutritionally suitable. They further recommended protection or arrangement of boundary strips in the framework of agricultural extensification.

Cultures of medicinal, culinary, and ornamental herbs, such as sweet fennel (Foeniculum vulgare) and spearmint (Menta spicata), grown in organic market gardens near various vegetable and tree crops are known to attract several adult entomophagous Hymenoptera and flower-visiting beneficials (Sawoniewicz, 1973; Bugg and Wilson, 1989; Bugg et al., 1989; Maingay et al., 1991; Bugg and Waddington, 1994). Some studies on herb attractiveness have listed not only the sampled species, but also have determined or attempted to determine their significance and effect on pests in nearby crops (Bugg and Waddington, 1994 ).

Pollen can serve as a supplemental or essential food source for beneficials. For example, Ouyang et al. (1992) report that pollen can positively affect polyphagous predacious mites, especially during periods when their arthropod prey is scarce. Green manure crops such as faba bean (Vicia faba) can manifest similar positive effects on beneficials (Bugg and Ellis, 1988; Bugg et al., 1989). And, habitat manipulations, such as the addition of mulch and flowers, may enhance spider densities and lower the number of pest insects in a mixed vegetable system (Riechert and Bishop, 1990).

Cover crops in general are known to affect a number of phenomena in orchards. They may harbor pest species, but they can also lead to increased numbers of insect natural enemies and heightened pest biocontrol (Bugg et al., 1990; Bugg and Waddington, 1994). Some covers, or mixtures of cover crops, constitute field insectaries and may be marketable as "insectary crops" or crops that support high densities of beneficials. Different covers may require different management protocols, depending upon whether they are supplementary (alternative prey or hosts, pollen) or complementary (nectar, honeydew) food sources (Bugg, 1992). For a detailed review of cover crop management in temperate zone orchard crops (almond, pecan, walnut, apple, cherry, peach, and citrus), see Bugg and Waddington (1994).

Field margins and crop edges can be highly supportive of beneficials, but their relative abundance varies depending upon the mix of plants at the margins and the adjacent crop (Dennis and Fry, 1992). Some studies have shown that field margins can increase the diversity of arthropods within the crop and that movement between the margin and the field can be significant. Margin habitats provide the stability needed for species that would otherwise not survive across all seasons. The landscape matrix of field margins can be vital for effective field dispersal and conservation success (Röser, 1988; Dennis and Fry, 1992).

Crop edges adjacent to hedges and broad grassy strips bear a rich fauna of beneficials and should not be treated with pesticides or receive fertilizer (van den Bosch and Messenger, 1973; Morris and Weeb, 1987; Basedow, 1988; Holtz, 1988; Klingauf, 1988; Welling and Kokta, 1988). Dover (1991) and Samways (1993) showed that 6-m-wide edges around cereal fields receiving reduced and selective pesticides helped conserve beneficials. Welling and Kokta (1988) reported that wide headlands with a large source of flowering plants guaranteed nutrition for flower-visiting beneficials, served as a refuge for different species (before and after harvest), and acted as a bridge between isolated biotypes.

The plants that compose hedges, hedgerows, and windbreaks vary widely, as does their significance as reservoirs for beneficials (Solomon, 1981). Wide hedgerows or windbreaks composed of trees and shrubs appear to function as a type of biocorridor across the landscape (Forman and Baudry, 1984); the associated beneficials are in part forest-edge species. The hedgerows and windbreaks, together with boundary strips and unsprayed field margins, represent a functional part of the agroecosystem, influencing positively the beneficials (Knauer, 1988), and thus should be encouraged on farms (Basedow, 1988). A number of papers cover hedgerow/windbreak habitats in detail, including the role of beneficials and IPM (Lewis, 1969; Zwölfer et al., 1984; Stechman and Zwölfer, 1988; Welling et al., 1988; Häni, 1989).

Food Sprays and Semiochemicals

Food requirements of predaceous species sometimes vary between life stages. Larval stages may be carnivores, while adults may feed on nectar, honeydew, and pollen. For hymenopterous parasitoids, nectar and pollen requirements are common for adults, but not for parasitic larvae. With some crops, nectar, pollen, and honeydew sources are insufficient or unavailable. By providing an artificial food supplement, beneficials may be retained, arrested, or stimulated to oviposit. Treatments may consist of yeast, sucrose solution, or artificial honeydew (Hagen and Bishop, 1979; Gross, 1987).

Natural enemies are known to respond to a number of environmental cues in the course of locating desired habitats, plants, prey/hosts, and the opposite sex. Behavior-controlling chemicals (semiochemicals) are rather species specific. In theory, synthetically derived semiochemicals may be used to attract increased numbers of natural enemies into a crop, prolong their searching activity, and improve their performance (Vinson, 1977; 1981; Lewis, 1981; Nordlund et al., 1981; Powell, 1986; Gross, 1987; McMurtry et al., 1995).

Modification of Chemical Pest Control Practices

Biodiversity of natural enemies in agroecosystems may be substantially affected by the use of pesticides. Nonselective treatments are toxic to beneficials. They decrease populations, contributing to pest outbreak. For this reason and others, pesticides have begun to be used with greater care. Emphasis is beginning to center more on the development and use of selective pesticides, on target-directed applications, and on applications timed to avoid the direct treatment of beneficials. Information on pesticidal effects on beneficials is extensive.

In some respects, the value of beneficials has been heightened by the overuse of pesticides leading to chemical resistance, secondary pest outbreaks, and environmental pollution. IPM concepts and strategies, from the earliest discussions, have centered on selective treatments to protect beneficials as key components in integrated control (Stern et al., 1959; Smith and Reynolds, 1966; Croft, 1990; McMurtry et al., 1995).

Nonselective pesticides directly and negatively affect natural enemies, sometimes even the larval stages within a host, e.g., parasitoids. Pesticides, both selective and nonselective, indirectly impact beneficials by diminishing prey/host populations and, in turn, force surviving beneficials to disperse to other communities to find food (Petr and Dlouhy, 1992). In a few cases, pesticide-affected systems have led to changes in gene diversity of beneficials through selection of pesticide-resistant strains.

Biocontrol is not the primary approach for some agroecosystems, and may never be, but it is or could be the key component in many agroecosystems. In most natural environments, biocontrol provides common, if not perennial regulation. With the removal of pesticides, diversity of beneficials and restoration of biocontrols are possible (Hagen et al., 1971). Where market demands require blemish-free products or where farm economics dictate treatment to protect investments, chemicals will likely remain a standard defense.

Conservation activities targeting beneficials through habitat diversification may be adversely disrupted by herbicides. Way and Cammell (1981) suggested that insect communities, including natural enemies, in and around agroecosystems are affected more by herbicides than by pesticides. Similarly, fertilizer, especially in heavy dosage, can adversely affect conservation efforts.

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