Insecticides are rapidly failing to control mosquito populations around the world. Thus, It is critical to understand the spread of insecticide resistance and use resistance management strategies (RMS). However, to do so, we need to know the selective pressures acting on mosquitoes both in the presence of a range of insecticide dosages, as well as in the absence of insecticides. We generated genetic crosses of three Aedes aegypti mosquitoes, both resistant and wildtype at the kdr V1016I + F1534C, and exposed these to different dosages of insecticides to determine the selective pressure at high and waning insecticide dosages. We further assessed how this selection is dependent on temperature. Next, we determined the cost of resistance associated with these resistance mutations, both at optimal conditions and under temperature stress of colder and hotter environments. Finally, we assessed the selection acting on these kdr mutations in field populations in Maricopa County, Arizona. We found that the double homozygous mutation is incomplete recessive and has a huge selective advantage at high insecticide dosages. We did not find evidence for a fitness cost associated with these mutations at optimal temperatures, but a potential fitness cost at high temperatures. These data predict a rapid selection for double mutants that is unlikely to revert in the absence of insecticides. Indeed, the field data show a rapid increase of double mutants in Maricopa County over the past five years. Overall, these results suggest that RMS that assume fitness costs are unlikely to be effective on these mosquitoes.
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