Long-term impacts of elevated carbon dioxide and transgenic Bt cotton on performance and feeding of three generations of cotton bollworm

The effects of elevated carbon dioxide (CO₂) on growth, metabolism, and performance of three generations of cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), were studied. The insects were continuously fed transgenic Bacillus thuringiensis (Berliner) (Bt) cotton [Cry1A(c)] grown in open-top chambers. Two levels of CO₂ (ambient and 2x ambient) and two cotton cultivars (non-transgenic Simian-3 and transgenic Bt GK-12) were used and bollworm larvae were reared on all four treatment combinations for three generations. CO₂ level and cotton variety significantly affected the growth and food digestibility parameters of H. armigera, with the exception of larval consumption rate for cotton variety. Overall, elevated CO₂ and transgenic Bt cotton both increased larval lifespan, food consumption rate, relative consumption rate, and approximate digestibility, while decreasing pupal weight, survival rate, fecundity, frass output, relative and mean relative growth rates (RGR/MRGR), and the efficiency of conversion of ingested and digested food (ECI/ECD). Moreover, there were significant CO₂*variety interactions on pupal weight and ECD, and CO₂*generation interactions on pupal weight, frass output, and MRGR. Furthermore, transgenic Bt cotton significantly decreased the population-trend index compared to non-transgenic cotton for the three successive bollworm generations, especially at elevated CO₂. Damage inflicted by the cotton bollworm on cotton, irrespective of the presence of insecticidal genes, is predicted to be higher under elevated CO₂ conditions because of individual compensatory feeding on host plants. Conversely, population abundance is presumed to be lower under elevated CO₂ compared to that under ambient CO₂, particularly in combination with transgenic technologies.