Impact of seed blend and structured refuge on Helicoverpa zea (Lepidoptera: Noctuidae) resistance evolution parameters

Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), an important pest in the southeastern US, has field-evolved resistance to Cry toxins in corn and cotton; as a result new cultivars now include Vip3A toxins. Insecticide resistance management programs in this region require plantings of separate structured non-Bt corn, but planted area of structured refuge is low. An alternative to separate corn structured refuge is blended refuge, where non-Bt seeds are planted with Bt seeds. However, this pest may be exposed to higher sublethal doses of Bt in blended refuge, which has the potential to accelerate resistance. Fitness costs that may be associated with resistant insects developing on Bt crops include reductions in pupal weight. In 2019 and 2020, nine field trials were conducted in Florida, Georgia, North Carolina, and South Carolina to evaluate the impact of Bt and non-Bt plants in blended and structured refuge treatments on H. zea pupal survival and weight. Treatments consisted of Bt plants expressing either Cry1Ab + Cry1F or Cry1Ab + Cry1F + Vip3A. Our results showed a significant difference of pupal mortality and weight across treatments. Overall, results indicated cross-pollination between Bt and non-Bt ears on seed blend plots. Results of this study are important as they will inform regulatory agencies regarding effective refuge size of non-Bt plants under different refuge strategies. In addition, empirical data of fitness costs associated with resistance further refine IRM modeling and assist the development of better tools to conserve the effectiveness of Bt crops in the agricultural sector.