Impact of carbon dioxide concentration on the plant-aphid-natural enemies relationship

While the impact of climate change on plant or insect communities has been receiving increasing attention during the last decade, plant-insect interactions under a changing environment remain to be studied. These interactions are of importance as regard to the economically dimension of some crop plants and the associated species of insect pests. Insect-plant interactions are mediated by plant secondary metabolites, including volatile organic compounds (VOCs). Modifications in greenhouse gases concentrations, as predicted for 2100, could alter these chemically mediated interactions.
Here, we raise the hypothesis that a raise of carbon dioxide concentration (beyond 700 ppm) affects the volatile emission of Broad bean plants, with a cascade impact on insect pests and its natural enemies.
For this purpose, an experiment was made to determine the impact of CO2 concentration on aphid choice for their host plant. Choice tests were made on winged aphids between two plants grown under two CO2 concentrations. Differences were found in the choice made by the aphids. We decided then to generate odour samplings on Vicia faba plants reared under the same CO2 concentrations, hypothesizing that differences in plant semiochemicals may induce preferences for aphids. No differences were found when identifying the volatile organic compounds.
Because atmospheric carbon dioxide (CO2) concentration directly impacts plants physiology, we raise the hypothesis that elevated CO2 concentrations impact the quantity of honeydew produced by aphids, as well as the diversity and quantity of honeydew VOCs, with cascade effects on the foraging behavior of aphid natural enemies. Using solid-phase microextraction, we quantified the VOCs emitted by honeydew from pea aphids (Acyrthosiphon pisum Harris) reared either under 450 ± 50 ppm of CO2 (aCO2) or 800 ± 50 ppm of CO2 (eCO2). While the total amount of honeydew (honeydew release by 190 ± 50 individus in both conditions) is not impacted by the CO2 concentration, we found qualitative and quantitative differences in the semiochemistry of aphid honeydew between CO2 conditions. Three VOCs were not found in the honeydew of eCO2 aphids: 3-methyl-2-buten-1-ol, 2-methyl-1-butanol and isobutanol. However, no difference was observed in the searching and oviposition behavior of hoverfly (Episyrphus balteatus De Geer) females exposed to infested plants reared under both CO2 conditions, in a dual choice bioassay.