Evidence for positive population‐level responses but not individual performance of sycamore aphids under elevated <scp> CO ₂ </scp>

Abstract While being one of the most destructive agricultural pests globally, aphids are also important components of herbivory networks in forest and woodland habitats. They are predicted to be among the few insect herbivore ‘winners’ under future elevated CO 2 (eCO 2 ) scenarios, but additional field experiments are needed to determine species‐specific responses, including impacts on fecundity and mortality at the population level. The impact of eCO 2 on populations of aphids on common sycamore, Acer pseudoplatanus, was investigated within a temperate woodland at the Birmingham Institute for Forest Research (BIFoR) Free‐Air CO 2 Enrichment (FACE) facility. A survey of the aphid species, Drepanosiphum platanoidis , Periphyllus testudinaceus and P . acericola , showed an increase in the abundance and population density of all three species under eCO 2 (150 ppm above ambient), although differences were only significant for D. platanoidis . The number of nymphs produced by individual D. platanoidis alates isolated in clip cages was not significantly affected by the eCO 2 . These results suggest that D. platanoidis could be a species of aphid that may increase in abundance under eCO 2 , but that population‐level responses are not driven by improved individual performance. Further experiments are needed to determine the relative contribution of competition and attack by natural enemies as drivers of population‐level effects on aphid populations under climate change.