Carboxylesterase of the Cotton Aphid, Aphis gossypii GLOVER(Homoptera:Aphididae), Responsible for Fenitrothion Resistance as a Sequestering Protein

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Abstract
Carboxylesterase activity of the cotton aphid (Aphis gossypii GLOVER), using naphth-1-yl acetate as a substrate, varied greatly among the clones and was closely correlated to the degree of organophosphorus insecticide resistance. Approximately 90% of the carboxylesterase activity was distributed in soluble fraction. The soluble fraction of both the susceptible and the resistant clones exhibited limited hydrolytic activity against fenitroxon, whereas that of the resistant clones showed significant sequestering activity proportional to carboxylesterase activity. Both the carboxylesterase activity and the fenitroxon sequestering activity were markedly inhibited by 2-phenoxy-4H-1, 3, 2-benzodioxaphosphorin 2-oxide (K-2) but not by S, S, S-tributyl phosphorotrithioate (DEF) and iprobenfos (IBP, Kitazin P(R)). Among these three esterase inhibitors, only K-2 showed synergistic action on the toxicity of fenitrothion. Furthermore, the fenitroxon sequestering activity was markedly reduced by coexistence with an excess amount of naphth-1-yl acetate. Based on these results, it was concluded that the carboxylesterase of A. gossypii had a role in fenitrothion resistance as a sequestering protein.