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

Metabolic and integumental mechanisms of carbaryl resistance were investigated using [¹⁴C] carbaryl in three western corn rootworm populations from Nebraska (two resistant and one susceptible). In diagnostic concentration bioassays of carbaryl toxicity, mortality was 94% for the susceptible population, 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. In vivo distribution studies did not identify any notable differences between populations except increased excretion rates in the resistant populations. In vivo metabolism studies identified more substantial increases in the disappearance of carbaryl and in naphthyl acetamide formation for the resistant populations. In vitro microsomal metabolism of carbaryl resulted in increased NADPH-dependent disappearance of carbaryl and in formation of naphthyl acetamide and naphthol for both resistant populations. In vitro metabolism of carbaryl by soluble proteins identified increased naphthol formation (via hydrolysis) in both resistant populations, and increased polar metabolites in the presence of reduced glutathione for one of the resistant populations. Based 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 between the two resistant populations studied, results indicate the primary importance of both cytochrome P450 monooxygenases and esterases in carbaryl resistance.