First characterization of Vip3Aa resistance in beet armyworm (Lepidoptera: Noctuidae) collected from cotton...

Beet armyworm, Spodoptera exigua Hübner, is a sporadic pest of cotton (Gossypium hirsutum L.) that was once widespread in the crop across much of the US Cotton Belt. During 2021, we collected beet armyworm larvae from a field in North Carolina of transgenic cotton producing the Cry1Ac + Cry2Ab2 + Vip3Aa19 insecticidal proteins with unexpected injury to the plants. In diet-based bioassays with Vip3Aa39 protein, the colony collected from cotton in North Carolina displayed 813-fold resistance to Vip3Aa compared to a field-collected colony from Florida. We then performed reciprocal crosses and bioassays, which determined that the resistance was autosomal, recessive, and monogenic. Furthermore, the resistance likely had fitness costs, supported by the loss of resistance through rearing in the laboratory, requiring re-selection to restore it. Although beet armyworm is not currently a threat to Bt cotton in the United States, our finding of Vip3Aa resistance demonstrates that this pest has the capacity to evolve resistance under field conditions. This finding highlights the potential for beet armyworm to re-emerge as a significant pest if agronomic or climatic conditions shift to favor its establishment and spread. Moreover, the emergence of resistance in this historically sporadic pest should serve as a broader warning that Bt resistance can evolve and persist in non-target or secondary pests.