Levels of insecticide resistance, carboxylesterase activity, carboxylesterase expression, and the cDNA sequence of carboxylesterase gene were investigated in malathion resistant and susceptible strains of cotton aphids, Aphis gossypii (Glover). The resistant strain (MRR) exhibited 80.6-fold resistance to malathion compared to the susceptible strain (MSS) in cotton aphids. Five substrates, α-naphthyl acetate (α-NA), β-naphthyl acetate (β-NA), α-naphthyl propionate (α-NPr), α-naphthyl butyrate (α-NB), α-naphthyl caprylate (α-NC) and S-methyl thiobutyrate (S-MTB) were used to determine carboxylesterase activity in MRR and MSS strains of cotton aphids. Carboxylesterase activity was significantly higher in MRR strain than in MSS strain, 3.7-fold for α-NA, 3.0-fold for β-NA, 2.0-fold for α-NPr, 2.9-fold for α-NB and 1.6-fold for α-NC, While for S-MTB, there was nearly no difference between the two strains. Two site mutations (K14Q and N354D) with high frequency were also found by sequence analysis in the MRR strain, compared with the MSS strain. The levels of gene expression for carboxylesterase of both MRR and MSS strains were determined by real-time quantitative PCRs. Compared with the MSS strain, the relative transcription levels and gene copy numbers of the carboxylesterase were 1.99- and 4.42-fold in the MRR strain, respectively. These results indicated that the increased expression of the carboxylesterase resulted from the increased transcription levels of carboxylesterase mRNA and gene copy numbers and combined with the site mutants might play role in cotton aphid resistance to malathion.
