Characterization of <i>Helicoverpa armigera</i> ( <scp>Lepidoptera: Noctuidae</scp> ) resistance to <scp>Cry1Ac</scp> soybean in <scp>Brazil</scp> : inheritance patterns, cross‐resistance and fitness costs

BACKGROUND: Bt soybean varieties that express Cry1Ac protein are vital for controlling Helicoverpa armigera (Hübner) in Brazil, yet their widespread adoption in Brazil has increased the risk of Cry1Ac resistance evolution. This study assessed inheritance, cross-resistance, and fitness costs of H. armigera resistance to Cry1Ac using a near-isogenic resistant strain (Iso-Res) developed via backcrossing of a Cry1Ac-resistant strain with a susceptible strain (Sus). RESULTS: The resistance ratio of the Iso-Res strain estimated in diet-overlay bioassays using purified Cry1Ac was more than 300-fold. Reciprocal crosses showed that the inheritance was autosomal and incompletely dominant across all tested concentrations. However, the Sus and heterozygous strains were controlled when fed with leaf discs from Cry1Ac soybean, indicating that the functional dominance of resistance is recessive. Backcrosses of the heterozygous strains with the Sus strain indicated that the resistance is polygenic. Cross-resistance occurred between Cry1Ac and Cry1A.105, but not with Cry2Ab2. Soybean expressing Cry1Ac/Cry1A.105/Cry2Ab2 caused total mortality of all tested strains. The Iso-Res strain showed biological disadvantages compared with the Sus strain, including lower larval and pupal weight, longer development time, lower survival, and a 30% reduction in female fertility when reared on non-Bt soybean. CONCLUSIONS: The current low frequency of resistance to Cry1Ac soybean in H. armigera in Brazil is likely driven by the functionally recessive nature of the resistance, significant fitness costs and incomplete resistance. To sustain the long-term efficacy of Bt soybean technologies, management strategies must prioritize planting of structured refuge areas and pyramided traits that utilize proteins lacking cross-resistance to Cry1Ac. © 2026 Society of Chemical Industry.