Bt cotton and modern insecticide adoption decreases Helicoverpa spp. population recruitment in a subtropical cropping system

Extensive crop sampling for Helicoverpa spp. pupae was undertaken (2017–2021) to provide insights as to how pest population dynamics have altered in a subtropical cropping system following the introduction of transgenic Bt cotton and newer generation insecticides for pulse production. Previously (1996–1999), a pattern of year-round population cycling and build-up was identified to occur between summer cotton (non-Bt) and winter chickpea, enabled by widespread resistance to broad-spectrum insecticides used at the time. Current pupae sampling was unable to recover pupae from Bt cotton, suggesting this crop had become a population sink rather than a source for Helicoverpa spp. Pupae were less abundant in pulses, with monthly counts varying from 0 to 3192/ha in chickpea being a fraction of previous densities that ranged from 10,000 to 100,000/ha whilst pupae were not detected in mungbean crops. Poor survival (0%–39.5%) of pupae collected from chickpea, likely due to sub-lethal insecticide exposure during the larval stage, further limited population recruitment. The highest densities of pupae (up to 18,666/ha) were routinely recovered from unsprayed pigeon pea grown as refuges for Bt cotton resistance management, although Tachinid spp. parasitoids caused increasing pupal mortality with refuge age. This study suggests that the high control efficacy afforded by transgenic Bt traits incorporated into cotton and newer insecticides used on pulse crops has provided a form of area-wide management that may explain suppression of Helicoverpa spp. pupae densities compared to levels previously recorded across the cropping system.