Environmentally relevant neonicotinoid concentrations increase mosquito larval tolerance to two widely used larvicides

Widespread occurrence of neonicotinoid insecticides in surface waters is a growing concern as aquatic invertebrates are highly sensitive. Mosquito larvae are important and ubiquitous aquatic invertebrates that are food for wildlife but are more important as disease vectors. Chemical and biological insecticides are applied to surface waters to abate mosquito populations and reduce disease transmission. Two widely used larvicides include the bacterial insecticide, Bti, and the organophosphate insecticide, temephos. To date, there are no reports on the interactive effects of neonicotinoids and these larvicides. Studies have reported that xenobiotic exposure can induce enzymes in mosquito larvae, which can enhance or dampen tolerance to subsequent xenobiotic exposure. Because Bti and temephos have different modes of action, we hypothesized that neonicotinoid exposure would increase tolerance to temephos but decrease tolerance to Bti. We exposed newly hatched Aedes albopictus to several concentrations of the neonicotinoid, thiamethoxam. When larvae developed to late instars they were subsequently exposed to either Bti or temephos. Larvae exposed to thiamethoxam only were observed for effects on development rate, adult size, and sex ratio—life history traits that influence disease transmission dynamics. Our results indicate that higher concentrations of thiamethoxam interact with Bti and temephos synergistically, but at particular lower concentrations thiamethoxam pre-exposure increases larval tolerance to both larvicides. When tested alone, thiamethoxam altered the sex ratio by disproportionately killing female larvae. Understanding the influence of neonicotinoids on mosquitoes and their control is vital in predicting disease transmission and ensuring the efficacy of mosquito control programs.