Helicoverpa zea Boddie (Lepidoptera: Noctuidae) Remains Susceptible to Vip3A Across the Southeastern USA: Eight Years of Diet-Overlay Monitoring Highlight the Need for Annual Benchmarking

Helicoverpa zea Boddie (corn earworm or bollworm) is a primary target of transgenic crops expressing insecticidal Cry and Vip3Aa proteins from Bacillus thuringiensis (Bt). Widespread resistance to Cry toxins has increased reliance on Vip3Aa. We evaluated Vip3Aa susceptibility in 71 field populations of H. zea collected across the southeastern USA from 2018 through 2025 using diet-overlay bioassays of F1 progeny. Vip3Aa1 was used in 2018 and Vip3Aa39 from 2019 through 2025. Median lethal concentrations (LC50) were estimated and resistance ratios calculated relative to two susceptible laboratory colonies, a field-collected susceptible and the commercially available Benzon colony. Resistance ratios depended strongly on the susceptible reference used. Three populations evaluated in 2018 had resistance ratios ≥ 10 relative to the field-collected susceptible colony, whereas all Benzon-referenced resistance ratios were <1.3. Benzon-referenced resistance ratios increased significantly over time, whereas field-collected-susceptible-referenced resistance ratios decreased. In contrast, field-population LC50 values showed no temporal trend from 2019 through 2025, when all populations were evaluated with Vip3Aa39. These results provide no evidence of a sustained regional decline in Vip3Aa susceptibility in this region, but they highlight the importance of stable susceptible benchmarks, protein quality control, and complementary approaches for long-term resistance monitoring.