Impact of Blended and Structured Non-Bt Refuge on Helicoverpa zea (Lepidoptera: Noctuidae) Pupal Depth, Weight and Survival in Corn.

Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), an important pest of the southern U.S. cotton, has developed resistance to Cry1A and Cry2A toxins expressed in corn and cotton, and these are now being planted with pyramided varieties with Vip toxins. Since both crops express toxins in the same family, corn acts as a selective filter for H. zea in cotton. Resistance management programs include areas with non-Bt refuge, but grower’s compliance to this is very low. An alternative for this issue is the adoption of blended refuge. However, because corn cross pollinates, this pest may be exposed to sublethal doses of Bt, which has the potential to accelerate resistance. Some of the effects of Bt crops on insects that are associated with Bt resistance include fitness costs such as pupal depth and weight. In 2019, field trials were conducted in Florida, Georgia, North Carolina and South Carolina to evaluate the impact of blended refuge (20% non-Bt + Bt 2- or 3-toxin hybrid), structured refuge (non-Bt) and Bt pure stand (2- or 3-toxin hybrid) treatments on H. zea resistance parameters. Generalized linear mixed models were used to test the effect of treatment on pupal depth and weight with state and replication as random variables. Treatment means were compared using Friedman test and post hoc multiple comparisons. Our results showed that H. zea pupated deeper in the blended refuge with Bt 2-toxin hybrid and it was not different from the non-Bt structured refuge treatment. Also, pupae collected from the non-Bt structured refuge were the heaviest and it was followed by the Bt 2-toxin pure stand treatment. Our results will be discussed on terms of implications of blended refuge strategies on resistance management programs and the sustainability of Bt toxins on managing this important pest in the southern U.S. Cotton Belt.