Present scenario on the effectiveness of Bollgard-II (BG-II) cotton expressing the insecticidal crystal proteins against Helicoverpa armigera (Hübner) in India

The present study evaluated the mortality of Helicoverpa armigera neonate larvae in laboratory bioassay on Bt ( BG-II ) cotton tissues (leaves, squares, flowers, and bolls) expressing Cry1Ac and Cry2Ab toxins, across phenological stages from three major cotton-growing zones of India (North, Central and South) under irrigated and rainfed conditions during 2018–2023. The hybrids proposed under the All India Co-ordinated Research Project (AICRP) on Cotton, grown in replicated field trials across all three zones, were evaluated. Results revealed both spatial and temporal variations in the H. armigera larval mortality under laboratory conditions. The mean larval mortality (%) observed on leaf tissue in the North zone, irrigated crops, recorded consistently high mortality (approaching 100% at early crop growth stages) from 2018 to 2020. However, a slight decline was observed from 2021 to 2023, with the lowest mean mortality of 93.38% recorded in 2021. All mortality values recorded in the North zone were numerically non-significant. In Central zone, mean larval mortality (%) recorded on leaf tissue was 94.76% under irrigation and 96.01% under rainfed conditions, while in South zone mortality averaged 96.85% (irrigated) and 95.61% (rainfed). Mann–Kendall trend analysis also showed a statistically significant declining trend in the Central zone under both irrigated (τ = − 0.50, p = 0.0064) and rainfed (τ = − 0.73, p = 0.0004) conditions over 2018–2023. Among plant parts, flowers consistently recorded the lowest mortality (up to 81.6%). Mortality in bolls was generally high but showed greater variability between irrigation regimes. A clear numerical reduction in mortality was observed during later crop growth stages (> 90 days), which may be attributed to sublethal exposure to Cry proteins. Over six years, the overall decline in mortality trend from 97.8% (2018) to 94.1% (2023) may not be statistically significant except in the Central zone. As a precaution, it provides information on the survival of H. armigera on BG-II cotton. Nevertheless, as a proactive measure, these observations provide important information on the survival of H. armigera on BG-II cotton. Future studies should therefore focus on continued monitoring of survival patterns to support the long-term sustainability of BG-II cotton.