Physiological Factors Influencing Bacillus thuringiensis Susceptibility in Laboratory and Field-Collected Bollworm, Helicoverpa zea (Lepidoptera: Noctuidae)

Field-evolved resistance of Helicoverpa zea to crops expressing Cry insecticidal proteins from Bacillus thuringiensis (Bt) is widespread across the United States. To comparatively evaluate physiological factors associated with Bt susceptibility, we analyzed two laboratory strains (Benzon and SIMRU) and one field colony obtained from a commercial corn field near Pickens, Arkansas. Biochemical assays of larval midgut extracts showed that Pickens exhibited significantly altered activities of chymotrypsin-like proteases, aminopeptidase N (APN), and alkaline phosphatase (ALP) compared with the SIMRU or Benzon colonies, with differences varying by larval instar. In contrast, trypsin-like protease activities did not differ significantly among the three colonies. Gene expression analyses of ten serine protease genes and seven candidate Cry receptor genes (including cadherin, ATP-binding cassette family C2, ALP, and four APN genes) revealed significant transcriptional differences in the Pickens relative to the lab colonies. Collectively, these results suggest that chymotrypsin-like proteases may play an important role in the activation of Cry toxins in H. zea. Altered chymotrypsin and APN activities, together with differential gene expressions in the Pickens population, likely contribute to reduced Bt susceptibility. The biochemical and molecular differences provide insight into potential physiological factors underlying reduced Bt susceptibility and may inform future Bt resistance monitoring and management strategies.