tinuous application. Sheehan and Lewis (unpublished data) found that application of the attractive plant volatile caryophyllene to cotton in small experimental patches increased retention of released M. croceipes without interfering with efficiency. The approach of adding foraging stimuli may be more relevant to short- term foraging (i.e., establishment upon release) than to retaining parasitoids in fields for a long time.
Manipulation of Plant Qualities. Variation in plant quality resulting from plant breeding and fertilization may be manipulated to advantage. Many, perhaps most, elements of plant resistance incorporated into crop plants during breeding programs are likely to be detrimental to parasitoids. For example, sticky trichomes entrap adult parasitoids as well as herbivores ( Rabb and Bradley 1968); nectariless cotton (developed to foil adult moth feeding) also deprives parasitoids of needed food for flight; and allelochemicals designed to reduce herbivore feeding may be passed on to developing parasitoids ( Campbell and Duffey 1979). However, carefully selected traits, such as Bt expression in cotton, may enhance parasitoid activity by extending the window of host vulnerability. Behavioral responses of insects to endogenous plant protectants are complex and need more study ( Gould 1988).
This chapter emphasizes intrinsic variations in the effectiveness of natural enemies. Further, it stresses the interconnectedness of natural enemies with other components of the ecosystem, the dependence of natural enemies on other components of the ecosystem, and the dependence of natural enemies on varied extrinsic resources within the cropping system and in the surrounding vegetation. An understanding of these intrinsic and extrinsic variables and their interdisciplinary management is required in order to ensure reliable performance of natural enemies.
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