Atmospheric change and species invasions are arguably two of the most important factors affecting the long-term sustainability of natural ecosystems. We examined the independent and interactive effects of atmospheric carbon dioxide (CO2) and tropospheric ozone (O3) on the foliar quality of two host species and performance of an invasive folivorous insect. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown at the Aspen FACE research site in northern Wisconsin, USA, under all combinations of ambient and elevated CO2 and O3. We measured the effects of elevated CO2 and O3 on aspen and birch phytochemistry and on the survivorship, development time, growth, and fecundity of the gypsy moth (Lymantria dispar). Elevated CO2 had little effect on, whereas elevated O3 altered, the composite phytochemical profiles of aspen and birch. Nutritional quality in aspen and birch leaves was marginally affected by elevated CO2 and reduced by elevated O3. Both gases increased concentrations of phenolic and structural compounds in aspen and birch. Elevated CO2 offset reduced foliar quality under elevated O3, but only in aspen, and to a greater extent later than earlier in spring. Elevated CO2 generally had beneficial effects on, while elevated O3 detrimentally affected, gypsy moth performance. Elevated CO2 ameliorated most of the reductions in gypsy moth performance under elevated O3. Our findings suggest that atmospheric change can alter foliar quality in gypsy moth hosts sufficiently to influence gypsy moth performance, but that these responses will depend on interactions among CO2, O3, and tree species. Our findings also contrast with those of earlier studies at Aspen FACE, indicating that foliar quality responses to environmental change are likely influenced by tree stand age and longevity of exposure to pollutants to the extent that they affect plant-herbivore interactions differently over decadal time spans.