Elevated CO2 concentration impact the semiochemistry of aphid honeydew without cascade effect on an aphid predator

Honeydew is now considered a cornerstone of the interactions between aphids and their natural enemies. Its composition is impacted by the aphid host plant species and associated phloem sap. Bacteria activity occurring in the aphid honeydew typically results in the release of volatile organic compounds (VOCs) that are used by aphid natural enemies for prey location. 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. The present work focuses on one particular aspect of atmospheric changes and should be extended to other abiotic parameters, such as temperature.