Isolation of an esterase conferring insecticide resistance in the mosquito Culex tarsalis

Malathion resistance in a strain of Culex tarsalis mosquitoes is due primarily to the activity of a malathion carboxylesterase (MCE). The resistant strain was 150 times more resistant to malathion than the susceptible strain and was weakly resistant to malaoxon and carbaryl, but not to any other insecticide tested. The phenotype could be reversed with the carboxylesterase inhibitor triphenylphosphate, but no synergism was observed with either the phosphatase or polysubstrate monooxygenase inhibitors, NaF and piperonyl butoxide. MCE is expressed throughout development and is most concentrated in the gut tissues of the larvae. Subcellular fractionation indicated that MCE was localized primarily in the mitochondria of resistant insects and the cytoplasm of susceptible insects. The enzyme was purified to homogeneity from both strains, and has a molecular weight of 59,000. However, chromatofocusing indicated that resistant insects have two MCEs with pIs of 6.8 and 6.2, while susceptible insects possessed only one MCE with a pI of 6.8. The MCE unique to the resistant strain hydrolysed malathion 18 times faster than the MCE common to both strains, suggesting that malathion resistance in C. tarsalis is due to the presence of a qualitatively different esterase in the resistant strain.