Lack of evidence for fitness tradeoffs in soybean aphid (Hemiptera: Aphididae) with resistance to pyrethroid insecticides in the Upper Midwestern United States

More than 20 years after its discovery in the United States, soybean aphid, Aphis gylcines Matsumura, remains the most economically damaging arthropod pest of soybean in the upper Midwestern US. While several insecticidal modes of action are approved for foliar use in soybean, aphid management has relied heavily on foliar-applied pyrethroids and organophosphates. In 2015, reports of reduced field efficacy within the pyrethroid class of insecticides began to emerge, and resistance was later confirmed in several laboratory assays. While insecticide resistance can provide important benefits for an insect pest in agricultural fields that are treated with insecticides, theory predicts that this resistance can impart fitness tradeoffs in the absence of insecticides when compared to susceptible individuals. The overexpression of detoxification genes involved with metabolic resistance can be energetically costly resulting in the resource reallocation at the expense of other life processes, while target-site mutations result in the loss of or altered function of the affected genes and may extend pleiotropic effects that can impact additional phenotypic traits. In this study we collected phenotypically resistant and susceptible field populations and compared them to a laboratory susceptible population over several fitness parameters including the intrinsic rate of population increase. Overall, there were no clear fitness advantages or disadvantages associated with the pyrethroid resistant phenotype under the parameters we examined in these experiments, which has worrisome implications for the longevity of pyrethroids as an effective management tool for soybean aphid.