This study was conducted to examine the effects of CO2-mediated changes in tree chemistry on the performance of the gypsy moth ((Lymantria dispar L.) and the parasitold Cotesia melanoscela (Ratz.). We used carbon-nutrient balance theory to develop hypotheses regarding changes in tree chemistry and the performance of both insects under elevated CO2. As predicted, levels of foliar nitrogen declined and concentrations of carbon-based compounds (e.g. starch and phenolics) increased under elevated CO2. Gypsy moth performance (e.g. growth, development) was altered by CO2-mediated changes in foliar chemistry, but the magnitude was small and varied across tree species. Larvae feeding on high CO2 aspen exhibited the largest reduction in performance, relative to larvae feeding on birch, oak, or maple. Parasitism by C. melanoscela significantly prolonged gypsy moth development and reduced growth rates. Overall, the effect of parasitism on gypsy moth performance did not differ between CO2 treatments. Altered gypsy moth performance on high CO2 foliage in turn affected parasitoid performance, but the response was variable: parasitoid mortality increased and adult female size declined slightly under high CO2, while development time and adult male size were unaffected. Our results suggest that CO2-induced changes in plant chemistry were buffered to the extent that effects on third trophic level interactions were weak to non-existent for the system examined in this study.