Interactive effects of pre-industrial, current and future [CO2] and temperature on an insect herbivore of Eucalyptus

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.