Abstract
Insecticide-resistant strains of the brown planthopper, Nilaparvata lugens, from four locations all had a single diffuse elevated esterase band on native polyacrylamide gel electrophoresis. The white backed planthopper Sogatella furcifera had two elevated esterases with lower relative mobilities than the N. lugens esterase, while the insecticide-susceptible N. bakeri and the grass-associated sibling species of N. lugens sensu lato all had a single low intensity staining esterase with a lower relative mobility than the resistance associated N. lugens esterase. All the esterases were inhibited by pre-incubation with 0.1 mM paraoxon, but were not affected by permethrin up to its solubility limit, indicating their possible role in organophosphorus, but not pyrethroid insecticide, resistance. Partial purification of the elevated esterase from insecticide resistant Sri Lankan N. lugens showed that despite its diffuse nature on gels it purified as a single isoform, with a specific activity of 5.85 mumol min- 1 mg- 1 and an estimated molecular weight of approximately 60 kDa. Insecticide resistance was conferred by this elevated esterase through rapid binding and slow turnover of the carbamate or the insecticidally active oxon analogues of the phosphorothioates, i.e. sequestration rather than metabolism is the primary resistance mechanism. In contrast to earlier studies on the elevated esterases of N. lugens from Japan and the Philippines, we were unable to detect any malathion metabolism by this esterase, nor did malathion inhibit the esterase, although it was sensitive to inhibition by malaoxon. The elevated N. lugens esterase did not cross-react with antisera raised to the elevated mosquito or aphid esterases. The efficacy of the partially purified N. lugens esterase in insecticide sequestration and turnover is compared with other well characterized amplified esterases from mosquitoes and aphids.
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