GEOGRAPHICAL AND TEMPORAL VARIATION IN RESPONSE TO INSECTICIDES IN VARIOUS LIFE STAGES OF APHIS-GOSSYPII (HOMOPTERA, APHIDIDAE) INFESTING COTTON IN CALIFORNIA

Five California populations of cotton aphid, Aphis gossypii Glover, were tested for variability in their responses to the organophosphorus insecticides oxydemeton-methyl, chlorpyrifos, and dicrotophos; the pyrethroid biphenate; and the chlorinated hydrocarbon endosulfan. In the dipped-leaf contact bioassay, alatiform nymphs were significantly more tolerant to all five insecticides compared with apterous adults. A subsequent geographical survey for resistance with single discriminating concentrations revealed that 6 of 13 early-season A. gossypii populations had significantly greater survival when exposed to one or more of the organophosphates or endosulfan than the other populations. These populations were collected primarily from the eastern range of the major cotton-growing region of California. A survey of aphid response to pesticides over time showed that survival declined in some of the populations as the season progressed. Populations that had significant survival at the discriminating concentrations as adults also showed improved survival of offspring exposed to residues.