Tieu_2017_J.Pest.Sci_90_773

The aphid Aphis gossypii Glover is an important pest of Australian cotton and has developed resistance to many chemicals used for its control. Its resistance management is partially based on chemical rotation that relies on insecticide resistance being associated with fitness costs. Therefore, understanding fitness costs associated with insecticide resistance is critical to its sustainable resistance management. We studied the fitness cost of pirimicarb resistance in A. gossypii caused by a single mutation in the acetylcholinesterase gene ACE1 by mixing different ratios of susceptible and resistant aphids. This was achieved by establishing A. gossypii populations of a known starting allele frequency in aphid proof cages and measuring allele frequency change over time via qPCR. Unlike traditional cohort fitness studies, we used competitive fitness as a measure of relative fitness of resistant versus susceptible aphids in the same environment. We demonstrate that competitive fitness measured in this study is an accurate predictor of overall relative fitness. We found that pirimicarb resistance had a significant fitness cost in the presence of susceptible aphids in the absence of insecticide pressure and that the fitness cost was related to the initial resistance allele frequency. By using the competitive fitness measure and knowing the initial allele frequency, it is possible to predict the likely time from resistant to an essentially susceptible population. As resistance was stable in the absence of susceptible competition, we recommend the use of resistance management tactics that do not completely eliminate the susceptible genotype such as complimentary integrated pest management.