INVITRO DEGRADATION OF SOME ORGANOPHOSPHORUS INSECTICIDES BY SUSCEPTIBLE AND RESISTANT DIAMONDBACK MOTH

In vitro degradation of several organophosphorus insecticides by larval homogenates of susceptible, parathion- and methyl parathion-selected strains of diamondback moth, Plutella xylostella (L.), was measured in an attempt to assess the role of glutathione S-transferase in resistance. The action of glutathione S-transferase was confirmed by the requirement of reduced glutathione as the cofactor and the inhibitory effects of diethyl maleate and N-ethylmaleimide. The involvement of other detoxifying enzymes, microsomal P450 monooxygenases and hydrolases, was also examined with a pertinent cofactor or inhibitor. Glutathione conjugation was confirmed as a major detoxifying reaction for parathion and methyl parathion, and a considerably higher degradation of both insecticides was found in the resistant than in the susceptible strains. A much reduced degradation of paraoxon and, especially, methyl paraoxon by glutathione S-transferase was observed in these strains. Low levels of cross-resistance to and limited glutathione conjugation of several other organophosphorus insecticides, i.e., diazinon, azinphosmethyl, tetrachlorvinphos, and prothiofos, were detected in the two resistant strains. Existence of isozymes of glutathione S-transferase in diamondback moth larvae is proposed. The degradation of malathion in all strains tested was mainly mediated by carboxylesterase. © 1991.