Metabolism of carbaryl by insecticide-resistant and -susceptible western corn rootworm populations (Coleoptera : Chrysomelidae)

Metabolic and integumental mechanisms of carbaryl resistance were investiga ted using [C-14] carbaryl in three western corn rootworm populations from N ebraska (two resistant and one susceptible). In diagnostic concentration bi oassays of carbaryl toxicity, mortality was 94% for the susceptible populat ion, and 63 and 29% for the resistant populations; confirming the presence of resistance as previously characterized. Penetration of carbaryl through the integument of the three populations was not different, suggesting that there are no integumental barriers involved in the resistance. Iii vivo dis tribution studies did not-identify any notable differences between populati ons except increased excretion rates in the resistant populations. In vivo metabolism studies identified more substantial increases in the disappearan ce of carbaryl and in naphthyl acetamide formation for the resistant popula tions. In vitro microsomal metabolism of carbaryl resulted in increased NAD PH-dependent disappearance of carbaryl and in formation of naphthyl acetami de and naphthol for both resistant populations. In vitro metabolism of carb aryl by soluble proteins identified increased naphthol formation (via hydro lysis) in both resistant populations, and increased polar metabolites in th e presence of reduced glutathione for one of the resistant populations. Bas ed on their apparent lack of interaction with the parent compound carbaryl, glutathione S-transferases appear to have an exclusive role in conjugation of secondary metabolites. Although qualitative differences are apparent be tween the two resistant populations studied, results indicate the primary i mportance of both cytochrome P450 monooxygenases and esterases in carbaryl resistance. (C) 1999 Academic Press.