Reduced susceptibility of Cry1Ac ‐ and Cry2Ab ‐resistant Helicoverpa armigera to transgenic Bt maize events and associated changes in midgut receptor gene expression

Abstract BACKGROUND Large‐scale cultivation of transgenic Bt crops effectively controls target pests but increases the risk of resistance evolution. Polyphagous pests such as Helicoverpa armigera may evolve resistance to Bt crops, and cross‐resistance among Bt proteins could hasten this process. This study evaluated the susceptibility of Cry1Ac‐resistant (BtR1.8, BtR3.6) and Cry2Ab‐resistant (2Ab5.0, 2Ab30) H . armigera to three Bt maize events (assessed with leaf, silk, and kernel), DBN9936 (expressing Cry1Ab), DBN3601T (Cry1Ab + Vip3Aa), and Ruifeng 125 (Cry1Ab/Cry2Aj). RESULTS Compared with the susceptible 96S strain, both Cry1Ac‐ and Cry2Ab‐resistant strains exhibited significantly reduced susceptibility to all three Bt maize events. In diet‐incorporated bioassays with lyophilized leaf powder, resistance rations based on EC 50 (the concentration preventing 50% of larvae from reaching the third instar within 7 days) ranged from 3.44‐ to 8.84‐fold for Cry1Ac‐resistant strains and from 2.20‐ to 9.53‐fold for Cry2Ab‐resistant strains (>12.81‐fold for 2Ab30 on Ruifeng 125). Bioassays using fresh silk and kernels confirmed significantly lower mortality in resistant strains compared with 96S. Notably, pyramided events (DBN3601T and Ruifeng 125) demonstrated higher insecticidal efficacy against resistant larvae than the single‐trait DBN9936, particularly in strains with lower resistance levels. Following Bt maize consumption, reverse transcription quantitative polymerase chain reaction (RT‐qPCR) analysis revealed significant downregulation of midgut receptor genes, including ABCC2 , ABCC3 , ABCA2 , APN1 , and APN2 . Specifically, ABCC2 was significantly downregulated in Cry1Ac‐resistant strains, and both ABCC2 and ABCA2 were downregulated in Cry2Ab‐resistant strains. CONCLUSION These results show that laboratory‐selected Cry1Ac/Cry2Ab‐resistant H . armigera have reduced susceptibility to Cry1Ab‐expressing maize tissue and to the pyramided events tested. Lower transcript levels of ABCC2 and ABCA2 were associated with the resistant phenotypes, but the direct association with the reduced susceptibility was not tested. Because the strains were selected in the laboratory, these findings provide information relevant to, but cannot be directly extrapolated to, insect resistance management strategies for Bt maize in China. © 2026 Society of Chemical Industry.