Adaptation to agricultural pesticides may allow mosquitoes to avoid predators and colonize novel ecosystems.

Human activities such as the application of agrochemicals may detrimentally disturb natural ecosystems, generating novel selection pressures. Here we examine how pesticides may influence community composition using the aquatic communities within bromeliad phytotelmata, and how adaptive responses to pesticides may influence community-level patterns. We first quantified the composition of macroinvertebrate communities from pesticide-free and pesticide-exposed locations. Complementary manipulative experiments where bromeliads were transplanted between pesticide-free and pesticide-exposed sites were then performed. Finally, pesticide bioassays on the most common predators (Mecistogaster modesta damselflies) and prey (Wyeomyia abebela mosquitoes) assessed a potential evolutionary mechanism that may influence community compositional differences. Our field survey revealed differences in W. abebela and M. modesta abundances between pesticide-free and pesticide-exposed areas. Our transplant experiment suggested compositional differences were not due to physical differences between bromeliads from different locations. Pesticide bioassays revealed that M. modesta from pesticide-free locations had higher innate pesticide tolerances than W. abebela from pesticide-free areas, but M. modesta larvae showed no evidence of adapted resistance as none were found where pesticides were used. Conversely, W. abebela larvae from pesticide-exposed locations had higher pesticide tolerances than individuals from pesticide-free sites, suggesting an adaptive response. This evolved resistance to pesticides may, therefore, allow W. abebela to colonize habitats free of the dominant predator in the system, explaining the higher W. abebela abundances in pesticide-exposed areas than in pesticide-free locations. We suggest that the total effect of novel stressors is driven by interactions between ecological and evolutionary processes.