Both atmospheric [CO2] and average surface temperatures are predicted to increase with potentially different, additive or opposing, effects on leaf quality and insect herbivore activity. Few studies have directly measured the interactive effects of concurrent changes in [CO2] and temperature on insect herbivores. None have done so over the entire developmental period of a tree-feeding insect, and none have compared responses to low pre-industrial [CO2] and present day [CO2] to estimate responses to future increases. Eucalypt herbivores may be particularly sensitive to climate-driven shifts in plant chemistry, as eucalypt foliage is naturally low in [N]. In this study, we assessed the development of the eucalypt herbivore Doratifera quadriguttata exposed concurrently to variable [CO2] (290, 400, 650 μmol mol−1) and temperature (ambient, ambient +4 °C) on glasshouse-grown Eucalyptus tereticornis. Overall, insects performed best on foliage grown at pre-industrial [CO2], indicating that modern insect herbivores have already experienced nutritional shifts since industrialisation. Rising [CO2] increased specific leaf mass and leaf carbohydrate concentration, subsequently reducing leaf [N]. Lower leaf [N] induced compensatory feeding and impeded insect performance, particularly by prolonging larval development. Importantly, elevated temperature dampened the negative effects of rising [CO2] on larval performance. Therefore, rising [CO2] over the past 200 years may have reduced forage quality for eucalypt insects, but concurrent temperature increases may have partially compensated for this, and may continue to do so in the future. These results highlight the importance of assessing plant–insect interactions within the context of multiple climate-change factors because of the interactive and potentially opposing effects of different factors within and between trophic levels.