Evaluation of esterases hydrolyzing α-naphthyl acetate as markers for malathion resistance in Culex quinquefasciatus.

Early detection of insecticide resistance is essential to develop resistance countermeasures and depends on accurate and rapid biological and biochemical tests to monitor resistance and detect associated mechanisms. Many such studies have measured activities of esterases, enzymes associated with resistance to ester- containing insecticides, using the model substrate, α-naphthyl acetate (α-NA). However, in the field, pests are exposed to ester-containing insecticides such as malathion, that are structurally distinct from α-NA. In the current study, malathion resistance in C. quinquefasciatus (3.2- to 10.4-fold) was highly associated with esterase activity measured with either α-NA (R = 0.92) or malathion (R = 0.90). In addition, genes encoding two esterases (i.e., EST-2 and EST-3) were over-expressed in field- collected strains, but only one (EST-3) was correlated with malathion hydrolysis (R = 0.94) and resistance (Rs = 0.96). These results suggest that, in the strains studied, α-NA is a valid surrogate for measuring malathion hydrolysis, and that heightened expression of an esterase gene is not necessarily associated with metabolic resistance to insecticidal esters.