Tissue specific esterase and malathion carboxylesterase activity in larvae of malathion-resistant Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae)

Abstract

Conditions were optimized for measuring esterase activity in midgut and fatbody of single fifth instar larvae of Plodia interpunctella (Hübner) using α-naphthyl acetate as a substrate. Specific activities in midgut and fatbody from Savlab, the susceptible strain, were equal when α-naphthyl acetate, α-naphthyl butyrate, β-naphthyl acetate, and β-naphthyl butyrate were the substrates. Activity in midgut preparations from resistant insects was generally equal to midgut activity from susceptible individuals, while activity in fatbodies from resistant individuals was significantly lower toward all four naphthyl esters. Metabolism of malathion to the carboxylesterase products, α- and β-malathion monoacid, was higher in midgut than fatbody of resistant insects. Both tissues from resistant insects metabolized malathion at higher rates than comparable tissues from susceptible larvae. These findings are discussed in terms of the altered aliesterase theory of insecticide resistance.