Selection for pesticide resistance in Aphytis

Field-collected populations of Aphytis holoxanthus DeBach (Hymenoptera: Aphelinidae), a parasite of Chrysomphalus aonidum (L.) (Homoptera: Diaspididae), were tested for malathion and for azinphosmethyl tolerance, and selected for increased resistance. Initial tolerance for malathion was not significantly different between populations, and could not be significantly increased by 30 selection cycles. Two field populations showed relatively high levels of azinphosmethyl tolerance, which were further increased by selection to 24–48 times that of a susceptible laboratory population.The differences in response to the two organophosphorus insecticides are discussed in relation to differences in their modes of application, detoxification by insects and the genetic bottleneck experienced by an imported species.