No effects of elevated CO₂ on the population relationship between cotton bollworm, Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), and its parasitoid, Microplitis mediator Haliday (Hymenoptera: Braconidae)

Estimating the population consumption of an insect population under elevated CO₂ is an important step in understanding the effects of elevated CO₂ on herbivore-crop interactions. Two successive generations of cotton bollworm, Helicoverpa armigera Hübner, were reared on milky grains of spring wheat (Triticum aestivum L.) grown in open-top chambers under increased carbon dioxide (CO₂) concentration. H. armigera development, wheat consumption, and parasitism by Microplitis mediator Haliday were examined, as were the effects of elevated CO₂ on the wheat itself. We experimentally tested the hypotheses that, by quantifying the population consumption of H. armigera, elevated CO₂ enhanced the pest-control ability of M. mediator again H. armigera. Decreases in protein, total amino acid, and nitrogen (N) content were noted in spring wheat when grown in an elevated-CO₂ environment, as were increases in total non-structure carbohydrates (TNCs) and in the ratio of TNC to N. In the first generation of H. armigera reared under elevated CO₂, no significant changes were observed in population generation time (T) or in the intrinsic rate of increase (r m) between CO₂ treatments. However, in the second treatment generation, longer generation time resulted in a lower r m value. Elevated-CO₂ levels caused no significant changes in the H. armigera population's total wheat consumption. The rates of parasitism, cocooning, and emergence by M. mediator were also unaffected, as were its average weight and adult lifespan. As no significant changes in wheat consumption by H. armigera or in the parasitic rate of M. mediator were revealed, the results indicate that the population relationship between H. armigera and M. mediator is unlikely to vary due to future elevated atmospheric CO₂ concentrations.