Intraspecific Variation in Levels of Pesticide Resistance in Field Populations of a Parasitoid, Aphytis melinus (Hymenoptera: Aphelinidae): The Role of Past Selection Pressures

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Abstract
Thirteen populations of Aphytis melinus DeBach, a key biological control agent of California red scale, Aonidiella aurantii (Maskell), were collected from the citrus-growing regions of California. Each population’s history of exposure to insecticides was estimated by determining history of insecticide use at both local (in-grove) and regional (county-wide) geographical scales. Concentration/mortality regressions for five insecticides widely used in citrus were estimated for the populations. For each chemical, substantial variability existed in the responses of different populations. LC50’s were correlated with both in-grove and county-wide pesticide use histories; patterns of variability were best explained by results of a multiple regression analysis that combined the influences of these two histories. Roles of food limitation, migration, and host distribution in determining patterns and rates of evolution of pesticide resistance in arthropod biological control agents are discussed.