Biotic interactions govern genetic adaptation to toxicants

The genetic recovery of resistant populations released from pesticide exposure isaccelerated by the presence of environmental stressors. By contrast, therelevance of environmental stressors for the spread of resistance duringpesticide exposure has not been studied. Moreover, the consequences ofinteractions between different stressors have not been considered. Here we showthat stress through intraspecific competition accelerates microevolution,because it enhances fitness differences between adapted and non-adaptedindividuals. By contrast, stress through interspecific competition or predationreduces intraspecific competition and thereby delays microevolution. This wasdemonstrated in mosquito populations (Culex quinquefasciatus)that were exposed to the pesticide chlorpyrifos. Non-selective predation throughharvesting and interspecific competition with Daphnia magnadelayed the selection for individuals carrying theace-1^R resistance allele. Under non-toxicconditions, susceptible individuals without ace-1^Rprevailed. Likewise, predation delayed the reverse adaptation of the populationsto a non-toxic environment, while the effect of interspecific competition wasnot significant. Applying a simulation model, we further identified howmicroevolution is generally determined by the type and degree of competition andpredation. We infer that interactions with other species—especiallystrong in ecosystems with high biodiversity—can delay the development ofpesticide resistance.