Interactive effects of elevated CO₂ and temperature on the leaf‐miner Dialectica scalariella Zeller (Lepidoptera: Gracillariidae) in Paterson's Curse, Echium plantagineum (Boraginaceae)

Doubling of the current atmospheric CO 2 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 CO 2 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 CO 2 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 CO 2 increased biomass, reduced leaf N and increased C:N ratios in the host plants. Leaf thickness also increased under elevated CO 2 , but only in the high‐temperature treatment. Female D. scalariella did not discriminate between plants grown at the different CO 2 levels when ovipositing, despite the reduction in foliage quality under elevated CO 2 . Overall, the negative response of D. scalariella to elevated CO 2 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 CO 2 . The net result was a reduction in development time of ∼14 days in the elevated CO 2 /high temperature treatment, compared to the ambient CO 2 /low temperature treatment. Larval survivorship and adult moth weight were both affected by a significant interaction between CO 2 and temperature. At the low temperature, CO 2 had little effect on survivorship, but at the high temperature, survivorship was significantly reduced under elevated CO 2 . Similarly, elevated CO 2 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 interactions between CO 2 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.