Developing a Cotton-Based Method for Bt Resistance Monitoring

Plant-based methods for detecting resistance in Bt crops can provide researchers a way to evaluate resistance without using diet-based bioassays that require protein or lyophilized tissue.  However, plant-based bioassays for Helicoverpa zea in Bt cotton face a potential challenge.  When H. zea larvae feed on squares, flowers, or bolls, penetration into the carpel or boll wall triggers ethylene production, leading to abscission.  Therefore, quantifying relative feeding amounts can be difficult.  We have developed a method to quantify bract feeding on bolls from H. zea.  Populations of H. zea were collected from the field across the Southeast and screened for resistance to Cry1Ac, Cry2Ab2, and Vip3A using protein or lyophilized leaf tissue diet-based bioassays.  These populations were then subjected to an additional plant-based bioassay using cotton bolls from non-Bt, Cry1Ac + Cry1F, Cry1Ac + Cry2Ab2, and Cry1Ac + Cry1F + Vip3Aa19 cotton, and bract feeding will be measured.  We hypothesized that the relative survival in the diet-based bioassays should reflect feeding in the boll bioassay.