Understanding the direct and indirect effects of elevated [CO2] and temperature on insect herbivores and how these factors interact are essential to predict ecosystem-level responses to climate change scenarios. In three concurrent glasshouse experiments, we measured both the individual and interactive effects of elevated [CO2] and temperature on foliar quality. We also assessed the interactions between their direct and plant-mediated effects on the development of an insect herbivore of eucalypts. Eucalyptus tereticornis saplings were grown at ambient or elevated [CO2] (400 and 650 μmol mol−1 respectively) and ambient or elevated ( + 4 °C) temperature for 10 months. Doratifera quadriguttata (Lepidoptera: Limacodidae) larvae were feeding directly on these trees, on their excised leaves in a separate glasshouse, or on excised field-grown leaves within the temperature and [CO2] controlled glasshouse. To allow insect gender to be determined and to ensure that any sex-specific developmental differences could be distinguished from treatment effects, insect development time and consumption were measured from egg hatch to pupation. No direct [CO2] effects on insects were observed. Elevated temperature accelerated larval development, but did not affect leaf consumption. Elevated [CO2] and temperature independently reduced foliar quality, slowing larval development and increasing consumption. Simultaneously increasing both [CO2] and temperature reduced these shifts in foliar quality, and negative effects on larval performance were subsequently ameliorated. Negative nutritional effects of elevated [CO2] and temperature were also independently outweighed by the direct positive effect of elevated temperature on larvae. Rising [CO2] and temperature are thus predicted to have interactive effects on foliar quality that affect eucalypt-feeding insects. However, the ecological consequences of these interactions will depend on the magnitude of concurrent temperature rise and its direct effects on insect physiology and feeding behaviour.
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