Abstract
We investigated the effects of long‐term CO 2 enrichment on foliar chemistry of quaking aspen ( Populus tremuloides ) and the consequences of chemical changes for performance of the gypsy moth ( Lymantria dispar ) and susceptibility of the gypsy moth to a nucleopolyhedrosis virus (NPV). Foliage was collected from outdoor open‐top chambers and fed to insects in a quarantine rearing facility. Under enriched CO 2 , levels of leaf nitrogen declined marginally, levels of starch and phenolic glycosides did not change, and levels of condensed tannins increased. Long‐term bioassays revealed reduced growth (especially females), prolonged development and increased consumption in larvae fed high‐CO 2 foliage but no significant differences in final pupal weights or female fecundity. Short‐term bioassays showed weaker, and sex‐specific, effects of CO 2 treatment on larval performance. Correlation analyses revealed strong, negative associations between insect performance and phenolic glycoside concentrations, independent of CO 2 treatment. Larval susceptibility to NPV did not differ between CO 2 treatments, suggesting that effects of this natural enemy on gypsy moths are buffered from CO 2 ‐induced changes in foliar chemistry. Our results emphasize that the impact of enriched CO 2 on plant–insect interactions will be determined not only by how concentrations of plant compounds are altered, but also by the relevance of particular compounds for insect fitness. This work also underscores the need for studies of genetic variation in plant responses to enriched CO 2 and long‐term population‐level responses of insects to CO 2 ‐induced changes in host quality.