Dynamics of arthropod pest complex and their natural enemies in bt versus non-bt cotton fields

Abstract:

Bt cotton was supposed to simplify pest management, yet secondary pest outbreaks keep surprising growers who assumed the technology would handle everything. This research compared the arthropod pest complex and associated natural enemies in adjacent Bt (Cry1Ac) and non-Bt cotton fields at the Sylhet Institute of Agricultural Sciences, Bangladesh, during two kharif seasons (2022 and 2023). Weekly visual counts on 30 randomly tagged plants per field recorded sucking pests (aphids, jassids, whiteflies, thrips), lepidopteran larvae, predatory coccinellids, chrysopids and spiders. Total pest density was 58.3% lower in Bt plots (mean 4.37 per plant) than in non-Bt plots (10.49 per plant), driven almost entirely by a 79.2% reduction in lepidopteran larvae. Sucking pest numbers, however, did not differ significantly between the two systems (P = 0.41). Natural enemy density was also lower in Bt fields (2.73 vs. 6.21 per plant), following the reduced prey base. The predator-to-pest ratio stayed around 0.62–0.63 in both systems, indicating that biological control potential was proportionally maintained. Hemiptera dominated the sucking pest guild in both field types, accounting for 41.7% of non-target arthropod individuals. These findings suggest that while Bt cotton effectively suppresses target Lepidoptera, it does not relieve growers of the need to manage sucking pests, and conservation of natural enemies remains important.