The Impacts of Introgression on the Biology and Management of Helicoverpa zea

Abstract

This thesis examined the biological and pest-management consequences of introgression between Helicoverpa zea (corn earworm) and its invasive sister species, Helicoverpa armigera. Recent research has shown that H. armigera has introduced the CYP337B3 allele into some North American populations of H. zea. This allele is associated with strong resistance to pyrethroid insecticides in H. armigera. Because H. zea is already a major agricultural pest and has evolved resistance to several Bt toxins, the spread of additional resistance traits could have important implications for crop protection and integrated pest management (IPM).The first objective of the thesis was to determine whether introgression increases pyrethroid resistance in H. zea. Laboratory colonies representing susceptible, non-admixed, and admixed (B3+) genotypes were established and screened using molecular methods. Caterpillars were exposed to multiple concentrations of λ-cyhalothrin and Warrior II, and mortality was recorded over time. The results demonstrated that admixed H. zea carrying the CYP337B3 allele were significantly more resistant to pyrethroid insecticides than non-admixed H. zea. Helicoverpa zea with the B3 allele were 7.86-fold more resistant to λ-cyhalothrin and a smaller increase in resistance to Warrior II. These findings provide the first direct evidence that introgression can increase pyrethroid resistance in H. zea. A second experiment evaluated whether the synergist piperonyl butoxide (PBO) could restore susceptibility by inhibiting cytochrome P450 enzymes. PBO produced only modest and inconsistent effects, suggesting that it is unlikely to provide a practical solution for managing resistance associated with the CYP337B3 allele. The second objective was to determine whether introgression influences cold tolerance. Because overwintering ability affects geographic distribution and seasonal population dynamics, the study compared the supercooling points (SCPs) of admixed and non-admixed H. zea pupae. Supercooling point is a commonly used indicator of freeze tolerance and represents the temperature at which hemolymph begins to freeze. Contrary to expectations, no significant difference in SCP was detected between admixed and non-admixed populations. Both groups had average SCP values near –18.5°C. These results suggest that the CYP337B3 allele does not alter cold tolerance, at least as measured by SCP. Additional analyses revealed significant differences between sexes, with male pupae exhibiting slightly lower SCP values than females. However, pupal weight did not differ significantly between sexes. The third objective was to investigate whether introgression alters attraction to commercial pheromone lures used to monitor H. zea populations. Monitoring programs rely heavily on pheromone traps to estimate pest abundance and guide management decisions. The thesis tested whether admixed and non-admixed moths responded differently to commonly used lure types. The results indicated that introgression did not significantly affect pheromone attraction. These findings suggest that existing pheromone-based monitoring programs should remain effective for tracking both admixed and non-admixed H. zea populations. Overall, this thesis demonstrates that introgression from H. armigera has already altered one important trait in H. zea: susceptibility to pyrethroid insecticides. In contrast, no evidence was found that introgression affects cold tolerance or attraction to pheromone lures. The study highlights the importance of continued monitoring of admixed populations and emphasizes the need for diversified IPM programs that reduce reliance on a single control tactic. As resistance to both Bt crops and pyrethroid insecticides continues to evolve, integrated approaches combining cultural, biological, and chemical controls will be essential for sustainable management of H. zea.