Long-term effects of elevated CO2 on the nutrition provided to parasitoids by their herbivorous hosts

Abstract

Elevated concentrations of atmospheric carbon dioxide (CO₂), a consequence of anthropogenic global change, may profoundly interfere with natural ecological processes, perhaps even interactions across trophic levels. Even the survival prospects of organisms at higher trophic levels could be affected, as follows. We showed previously that the endoparasitoid Diaeretiella rapae, a specialized parasitoid of the cabbage aphid Brevicoryne brassicae, exposed to elevated CO₂ concentrations (800 ppm) for up to 10 weeks, performed far worse (e.g., lower survival and parasitism rates) compared with ambient CO₂ (400 ppm). To investigate whether these CO₂-related effects in the parasitoids were mediated by changes in the quality of the aphids as hosts, we measured the nutritional and energy content of cabbage aphids under the above conditions. Specifically, we measured lipid, protein and water-soluble carbohydrate concentrations. We tested the hypothesis that when insects feed from plants with altered nutritional content, they incur a cost in dealing with such biotic stresses. That cost reduces their metabolic well-being and, thus, reduces their nutritional status with respect to parasitoids developing at their expense. We found that CO₂ had significant effects on aphid body mass (i.e., wet weight) and chemical composition, with elevated CO₂ concentrations reducing aphid mass by more than 50%. Aphids grown under elevated CO₂ also had significantly reduced soluble carbohydrates but significantly more lipids, on a weight-per-weight basis, than aphids grown under ambient CO₂. A significant decrease in total energy reserves (i.e., the sum of total proteins, lipids, and water-soluble carbohydrates) thus typified aphids grown under elevated CO₂. Our results contribute to explaining the impaired performance of the aphid B. brassicae and its parasitoid D. rapae previously reported under elevated CO₂, and provide evidence that under future climate change, host plants might affect the development and performance of parasitoids through their impacts on the nutritional quality of their herbivorous hosts.