Abstract
Carboxylesterase isozymes responsible for organophosphate resistance in the cotton aphid, Aphis gossypii Glover, were evaluated by inhibitory experiments on isoelectric focused bands. Differences in the carboxylesterase band patterns were detected between organophosphate-susceptible and resistant clones, and the activity of all bands was equally inhibited by fenitroxon in both clones. The activity was also inhibited by carbaryl, methomyl and K2 (2-phenoxy-4H-1,3,2-benzodioxaphosphorin 2-oxide) in a resistant clone. Fenitroxon showed the highest inhibitory activity. On the other hand, in the peach-potato aphid, Myzus persicae (Sulz.), used as a comparative aphid species, no difference was detected in the carboxylesterase band patterns between organophosphate-susceptible and resistant clones. A single band, however, showed different activity between the clones. The band, whose activity was inhibited by fenitroxon, may be the isozyme responsible for insecticide resistance in M. persicae. These results indicate that all carboxylesterase isozymes, but not the particular isozyme in M. persicae, are related to organophosphate resistance, and that overall enzyme activity determines the degree of resistance in A. gossypii.
