Combined Elevation of Temperature and CO2 Impacts the Production and Sugar Composition of Aphid Honeydew.

Honeydew is the keystone of many interactions between aphids and their predators, parasitoids, and mutualistic partners. Despite the crucial importance of honeydew in aphid-ant mutualism, very few studies have investigated the potential impacts of climate change on its production and composition. Here, we quantified changes in sugar compounds and the amount of honeydew droplets released by Aphis fabae reared on Vicia faba plants under elevated temperature and/or CO2 conditions. Following the combined elevation of these two abiotic factors, we found a significant increase in the fructose content of A. fabae honeydew, accompanied by nonsignificant trends of increase in total honeydew production and melezitose content. The environmental conditions tested in this study did not significantly impact the other honeydew sugar contents. The observed changes may be related to changes in phloem composition under elevated CO2 conditions as well as to increases in aphid metabolism and sap ingestion under elevated temperatures. Although limited, such changes in aphid honeydew may concurrently reinforce ant attendance and mutualism under elevated temperature and CO2 conditions. Finally, we discuss the enhancing and counteracting effects of climate change on other biological agents (gut microorganisms, predators, and parasitoids) that interact with aphids in a complex multitrophic system.