Abstract The effects of elevated CO2 on foliar chemistry of two tree species (Populus pseudo-simonii Kitag. and Betula platyphylla) and on growth of gypsy moth (Lymantria dispar L.) larvae were examined. Furthermore, we focused on the comparison of results on the growth responses of larvae obtained from two methods of insect rearing, the no-choice feeding trial performed in the laboratory or in situ in open-top chambers. On the whole, both primary and secondary metabolites in the leaves of the two tree species were significantly affected by main effects of time (sampling date), CO2 and species. Elevated CO2 significantly increased the C : N ratio and concentrations of the soluble sugar, starch, total nonstructural carbohydrates, total phenolics and condensed tannins, but significantly decreased the concentration of nitrogen. Higher contents of total phenolics and condensed tannins were detected in the frass of larvae reared in elevated CO2 treatments. Overall, the growth of gypsy moth larvae were significantly inhibited by elevated CO2 and CO2-induced changes in leaf quality. Our study did not indicate the two methods of insect rearing could influence the direction of effects of elevated CO2 on the growth of individual insects; however, the magnitude of negative effects of elevated CO2 on larval growth did differ between the two insect rearing methods, and it seems that the response magnitude was also mediated by larval age and host plant species.