The elevated concentration of atmospheric CO 2 may result in a decline of leaf nutritional quality (especially N) and an increase in some kinds of defensive secondary components (such as phenolics). The changes in the phytochemistry of trees, combined with the effect of elevated CO 2 per se, have a potential negative influence on insect herbivores. Here, we review the effect of elevated CO 2 on the performance of leaffeeding forest insects at individual-level and community-level. The elevated CO 2 per se have little influence on the metabolism of insects. Over half of the tree-insect experimental systems show that the performance of individual insect become poorer under high-CO 2 grown trees; but the others show that the insects have just little or no response to the treatments. The direction and magnitude of the changes in the performance of insects could be mediated by various factors. The effects of treatment are strongly species-dependent. The magnitude of changes in the phytochemistry, the sensitivity and adaptive capacity of insects to the poorer leaf quality, the differences in plant growth conditions and experimental methods, and the mediated effects of other environmental factors (such as soil nutrient availability, light, temperature, O 3 ) were all closely related to the final performance of insects. However, the larvae's consumption usually increased under enriched CO 2 treatment, which was widely thought to be a compensatory response to poorer plant quality. The experiments on forest community-level found identically a reduction in herbivory, which was contrary to the results from small-scale experiments. The changes in insect population and the actual response of consumption by leaffeeding forest insects under CO 2 enrichment remain unclear, and more field-based experiments need to be conducted.

No abstract

The effects of elevated CO2 on foliar chemistry of two tree species (Populus pseudo‐simonii Kitag. and Betula platyphylla) and on growth of gypsy moth (Lymantria dispar L.) larvae were examined. Furthermore, we focused on the comparison of results on the growth responses of larvae obtained from two methods of insect rearing, the no‐choice feeding trial performed in the laboratory or in situ in open‐top chambers. On the whole, both primary and secondary metabolites in the leaves of the two tree species were significantly affected by main effects of time (sampling date), CO2 and species. Elevated CO2 significantly increased the C : N ratio and concentrations of the soluble sugar, starch, total nonstructural carbohydrates, total phenolics and condensed tannins, but significantly decreased the concentration of nitrogen. Higher contents of total phenolics and condensed tannins were detected in the frass of larvae reared in elevated CO2 treatments. Overall, the growth of gypsy moth larvae were significantly inhibited by elevated CO2 and CO2‐induced changes in leaf quality. Our study did not indicate the two methods of insect rearing could influence the direction of effects of elevated CO2 on the growth of individual insects; however, the magnitude of negative effects of elevated CO2 on larval growth did differ between the two insect rearing methods, and it seems that the response magnitude was also mediated by larval age and host plant species.