Cotton aphids, Aphis gossypii Glover, were collected for 3 yr from cotton fields in Alabama and Texas, and screened for resistance to four pyre throed and eight oregano phosphate insecticides. Resistance ratios were based on Comparisons of the LC50s of field populations with the LC50s of a susceptible laboratory population. During 1988, resistance to the pyrethroids bifenthrin and cypermethrin was low (<20-fold) to moderate (20- to 50-fold). Resistance to the organophosphates ranged from 1- to 8S-fold, and was highest to methamidophos. During 1988, resistance to several insecticides increased rapidly within fields soon after insecticide applications. Resistance during 1989 was low or not detectable; the highest resistance was to phosphamidon (≈6-fold). During 1990, resistance was widespread but generally occurred at low levels. Resistance ratios and slopes of the probit regressions were variable among years within insecticides, and resistance was not always consistent among insecticides within an insecticide class, even when aphid populations were collected within the same general areas. Multiple mechanisms of resistance may be involved, and we conclude that resistance has developed on a relatively small scale.
Document Type: Research Article
Publication date: February 1, 1992
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