Fipronil metabolism, oxidative sulfone formation and toxicity among organophosphate- and carbamate-resistant and susceptible western corn rootworm populations

Fipronil toxicity and metabolism were studied in two insecticide-resistant, and one susceptible western corn rootworm (Diabrotica virgifera virgifera, LeConte) populations. Toxicity was evaluated by exposure to surface residues and by topical application. Surface residue bioassays indicated no differences in ®pronil susceptibility among the three populations. Topical bioassays were used to study the relative toxicity of ®pronil, ®pronil‡ the mono-oxygenase inhibitor piperonyl butoxide, and ®pronil's oxidative sulfone metabolite in two populations (one resistant with elevated mono-oxygenase activity). Fipronil and ®pronil-sulfone exhibited similar toxicity and application of piperonyl butoxide prior to ®pronil resulted in marginal effects on toxicity. Metabolism of [C]®pronil was evaluated in vivo and in vitro in the three rootworm populations. In vivo studies indicated the dominant pathway in all populations to be formation of the oxidative sulfone metabolite. Much lower quantities of polar metabolites were also identi®ed. In vitro studies were performed using sub-cellular protein fractions (microsomal and cytosolic), and glutathione-agarose puri®ed glutathione-Stransferase. Oxidative sulfone formation occurred almost exclusively in in vitro microsomal reactions and was increased in the resistant populations. Highly polar metabolites were formed exclusively in in vitro cytosolic reactions. In vitro reactions performed with puri®ed, cytosolic glutathione-Stransferase (MW=27kDa) did not result in sulfone formation, although three additional polar metabolites not initially detectable in crude cytosolic reactions were detected. Metabolism results indicate both cytochromes P450 and glutathione-S-transferases are important to ®pronil metabolism in the western corn rootworm and that toxic sulfone formation by P450 does not affect net toxicity. # 2000 Society of Chemical Industry