Effects of elevated CO2 and transgenic Bt cotton on plant chemistry, performance, and feeding of an insect herbivore, the cotton bollworm

Effects of elevated atmospheric CO2 (double-ambient CO2) on the growth and metabolism of cotton bollworm, Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae), fed on transgenic Bacillus thuringiensis (Berliner) (Bt) cotton [Cry1A(c)], grown in open-top chambers, were studied. Two levels of CO2 (ambient and double-ambient) and two cotton cultivars (non-transgenic Simian-3 and transgenic GK-12) were deployed in a completely randomized design with four treatment combinations, and the cotton bollworm was reared on each treatment simultaneously. Plants of both cotton cultivars had lower nitrogen and higher total non-structural carbohydrates (TNC), TNC:Nitrogen ratio, condensed tannin, and gossypol under elevated CO2. Elevated CO2 further resulted in a significant decrease in Bt toxin level in GK-12. The changes in chemical components in the host plants due to increased CO2 significantly affected the growth parameters of H. armigera. Both transgenic Bt cotton and elevated CO2 resulted in a reduced body mass, lower fecundity, decreased relative growth rate (RGR), and decreased mean relative growth rate in the bollworms. Larval life-span was significantly longer for H. armigera fed transgenic Bt cotton. Significantly reduced larval, pupal, and adult moth weights were observed in the bollworms fed elevated CO2-grown transgenic Bt cotton compared with those of bollworms reared on non-transgenic cotton, regardless of the CO2 level. The efficiency of conversion of ingested food and of digested food of the bollworm were significantly reduced when fed transgenic Bt cotton, but there was no significant CO2 or CO(2)x cotton cultivar interaction. Approximate digestibility of larvae reared on transgenic cotton grown in elevated CO2 was higher compared to that of larvae fed non-transgenic cotton grown at ambient CO2. The damage inflicted by cotton bollworm on cotton, regardless of the presence or absence of insecticidal genes, is predicted to be more serious under elevated CO2 conditions because of individual compensatory feeding on host plants caused by nitrogen deficiency.