Carry-over effects of Bacillus thuringiensis on tolerant Aedes albopictus mosquitoes

Abstract Background: The biological larvicide, Bacillus thuringiensis subsp. israelensis (Bti), represents a safe and effective alternative to chemical insecticides for mosquito control. Efficient control of mosquitoes implicates continuous and extensive application of Bti. This massive use of Bti imposes strong selective pressure, but the complex mode of action of the numerous synergistic Bti endotoxins disfavors emergence of resistance. Despite resistance to Bti has not been identified at the population level in nature, some larvae can survive Bti exposure suggesting tolerance mechanisms. Here we investigated whether Bti tolerant Aedes albopictus larvae experience any fitness costs and how this tolerance affects adult phenotypes that are relevant for arboviral transmission. Methods: We exposed Ae. albopictus larvae to lethal concentration of Bti and studied the fitness and gut microbiota of tolerant larvae and their adult counterparts. We further compared the expression of nine key immunity genes in the gut of Bti-tolerant larvae vs. those not exposed to Bti. Results: Our results showed that Bti exposure has multifaceted impacts on Ae. albopictus mosquitoes during both larval and adult stages. The carry-over effect of Bti exposure on tolerant larvae manifested in reduced adult emergence rate, shorter lifespan and decreased fecundity. Bti also alters gut microbiota of both larvae and adults. We observed higher microbial diversity in Bti-tolerant larvae and changes in the richness of core microbiota. Bti infection and the altered microbiota triggered immune responses in the larval gut. Conclusions: The observed reduction in mosquito fitness and changes in the microbiota of adults emerging from tolerant larvae could negatively influence mosquito vectorial capacity. Understanding these impacts is crucial for evaluating the broader implications of Bti-based insecticides in mosquito control programs.