Doubling of the current atmospheric CO2 concentration, and an increase in global mean annual temperatures of 1.5-6 °C, have been predicted to occur by the end of this century. Whilst the separate effects of CO2 and temperature on plant-insect interactions have been examined in a number of studies, few have investigated their combined impact. We carried out a factorial experiment to explore the effect of a doubling of CO2 concentration and a 3°C temperature increase on the development of a complete generation of the leaf-miner, Dialectica scalariella, in the host plant Paterson's Curse, Echium plantagineum. Elevated CO2 increased biomass, reduced leaf N and increased C:N ratios in the host plants. Leaf thickness also increased under elevated CO2, but only in the high-temperature treatment. Female D. scalariella did not discriminate between plants grown at the different CO2 levels when ovipositing, despite the reduction in foliage quality under elevated CO2. Overall, the negative response of D. scalariella to elevated CO2 was greater than for many species of free-living insects, presumably because of the limited mobility imposed by the leaf-mining habit. Development was accelerated at the high temperature and slowed under elevated CO2. The net results was a reduction in development time of ∼14 days in the elevated CO2/high temperature treatment, compared to the ambient CO2/low temperature treatment. Larval survivorship and adult moth weight were both affected by a significant interaction between CO2 and temperature. At the low temperature, CO2 had little effect on survivorship, but at the high temperature, survivorship was significantly reduced under elevated CO2. Similarly, elevated CO2 had a stronger negative effect on adult moth weight when combined with the high-temperature treatment. A possible explanation for these results is that the high temperature accelerated insect development to such an extent that the larvae did not have sufficient feeding time to compensate for the poorer quality of the foliage. The frequency with which interaction between CO2 and temperature affected both plant and insect performance in this study highlights the need for caution when predicting the effects of future climate change on plant-insect interactions from single-factor experiments.