Interaction of fipronil and the red imported fire ant (Solenopsis invicta): Toxicity differences and detoxification responses

Fipronil (FIP), a phenyl-pryazole pesticide, has been widely used for crop protection due to its broad insecticidal spectrum, especially for urban insect management. FIP also serves as the active ingredient of major baits used for the control of the red imported fire ant (RIFA; Solenopsis invicta). Although a vast majority of laboratory-based research has been performed using worker ants as a model, limited information is available regarding the toxicity of FIP in individuals from different castes and developmental stages. In this study, we investigated the interaction between FIP and this important pest, including FIP toxicity and transformation, RIFA enzyme activity and responses to FIP exposure. The topical and feeding toxicity of FIP in five adult castes, worker larvae and worker pupae were determined and compared. Topical toxicity assays showed that there were significant differences in FIP toxicity among adult workers (LD50 = 1.17 mu g/g), larvae (LD50 = 1891.00 mu g/g) and pupae (LD50 = 23981.00 mu g/g). Although, no obvious differences in topical toxicity were observed among the adult castes, the differences in feeding toxicity were significant. For example, the LC50 value for the workers was 3.96 fold lower than that for soldiers at 24 h, and the LC50 value was slightly lower for male alates than for female alates at day 3 and day 4, respectively. The activities of detoxification enzymes in individuals of different castes and developmental stages were investigated with or without FIP treatment. Cytochrome P450 activity was approximately 24-fold higher in larvae than in workers, and adult workers exhibited 4-fold higher FIP-induced cytochrome P450 activity than individuals from other adult castes. In addition, in vitro experiments demonstrated that FIP was transformed into FIP-sulfone, and this process may be primarily mediated by RIFA P450(s).