Sex-specific responses of mosquitoes to Bti pesticide and odonate predators

Mortality can counterintuitively enhance individual survivorship and increase population size via overcompensation, whereby extrinsic mortality reduces a population prior to the onset of density-dependent resource competition and yields more survivors than would be expected without the early-acting extrinsic mortality. In the context of vector control, pesticide-induced overcompensation in larval mosquito populations may result in larger adult populations of large-bodied mosquitoes with increased pathogen-spreading capabilities. Laboratory and theoretical studies have identified factors causing overcompensation in mosquito populations, but whether it is likely to occur in natural systems affected by multiple sources of extrinsic (e.g., predators, pesticide) and intrinsic mortality (e.g., interspecific and intraspecific competition) is unclear. We investigated interactive effects of odonate naiad predators (Aeshnidae, Coenagrionidae, Libellulidae) and the pesticide Bacillus thuringiensis israelensis (Bti) on individual and population-level responses of Culex pipiens mosquitoes in a field mesocosm experiment. We hypothesized that Bti applied to early-instar larvae would induce a release from density dependent resource competition sufficient to cause overcompensation, manifesting as a positive effect of Bti on adult production and body size (a proxy for fitness), but the effect would be mitigated by selective predation. We found that Bti reduced larval abundance by ~90% within 48 hours but did not induce an overcompensatory response in adult production or fitness. Predators had no significant effect on larvae within 48 hours, but they did have sex-specific effects on the abundance and size of adult mosquitoes. Predators reduced adult male production, increased adult female body size, and produced a female-biased sex ratio. Rather than mitigating the effects of Bti, the interaction of predators and Bti synergistically decreased adult male size. We discuss the roles of competition, phenotypic plasticity, selective predation, and sublethal Bti exposure potentially contributing to these results, emphasizing the complex interactions among sources of extrinsic and intrinsic mortality and their divergent effects across taxa, sexes, and environments that have important implications for both theoretical and applied ecology.