Characterization of the elevated esterase-associated insecticide resistance mechanism in Nilaparvata lugens (Stal) and other planthopper species

Insecticide-resistant strains of the brown planthopper, Nilaparvata lugens, from four locations all had a single diffuse elevated esterase band on nat ive polyacrylamide gel electrophoresis. The white backed planthopper Sogate lla furcifera had two elevated esterases with lower relative mobilities tha n the N. lugens esterase, while the insecticide-susceptible N. bakeri and t he grass-associated sibling species of N. lugens sensu late all had a singl e low intensity staining esterase with a lower relative mobility than the r esistance-associated N. lugens esterase. All the esterases were inhibited b y pre-incubation with 0.1 mM paraoxon, but were not affected by permethrin up to its solubility limit, indicating their possible role in organophospho rus, but not pyrethroid insecticide, resistance. Partial purification of th e elevated esterase from insecticide resistant Sri Lankan N. lugens showed that despite its diffuse nature on gels it purified as a single isoform, wi th a specific activity of 5.85 mu mol - min(-1) mg(-1) and an estimated mol ecular weight of approximately 60 kDa. Insecticide resistance was conferred by this elevated esterase through rapid binding and slow turnover of the c arbamate or the insecticidally active oxon analogues of the phosphorothioat es, i.e. sequestration rather than metabolism is the primary resistance mec hanism. In contrast to earlier studies on the elevated esterases of N. luge ns from Japan and the Philippines, we were unable to detect any malathion m etabolism by this esterase, nor did malathion inhibit the esterase, althoug h it was sensitive to inhibition by malaoxon. The elevated N. lugens estera se did not cross-react with antisera raised to the elevated mosquito or aph id esterases. The efficacy of the partially purified N. lugens esterase in insecticide sequestration and turnover is compared with other well characte rized amplified esterases from mosquitoes and aphids.