Efficiency of cotton bollworm ( Helicoverpa armigera Hübner) control of different Bt cotton varieties in North China

Background The cotton bollworm ( Helicoverpa armigera ) is one of cotton’s most destructive insect pests in terms of yield and quality. Since 1997, China has grown commercially available transgenic Bacillus thuringiensis ( Bt ) cotton. We aimed to investigate the variation in resistance of transgenic Bt cotton varieties to cotton bollworm in North China. Methods Populations of cotton bollworm were monitored from 2008 to 2015 in environments where Bt cotton was planted adjacent to other non- Bt crops. The study included 197 Bt cotton varieties planted in 42 counties/locations in three provinces (Hebei, Shandong and Henan) of North China, which were evaluated through field investigations, bioassays, and enzyme-linked immunosorbent assays (ELISA). Results The average number of cotton bollworms never exceeded the action threshold (10 larvae per 100 cotton plants), however, their number reached 19.55 per 100 cotton plants in 2011. The ratios of damaged plants to total Bt cotton stem-tips, buds, and bolls was low except in 2010, when the ratios reached 1.82%, 2.09%, and 10.63%, respectively. The results of bioassay showed that the corrected mortality were higher at the second generation cotton bollworm stage than the third and fourth germination stages. Totally, Bt protein content declined sharply at the seedling stage from 2008 to 2015. Conclusions This study indicated that almost all Bt cotton varieties were capable to effectively control the populations of cotton bollworm in North China.