Interactions between insect herbivores and plants grown under conditions of ambient and elevated CO2 were investigated by analysing data on 43 herbivores, representing 61 plant-herbivore interactions. Changes in herbivore performance in enhanced CO2 environments were correlated with changes in the quality of the host plants, measured as nitrogen content, water content, carbohydrate content and secondary plant compounds. The data were analysed to determine whether CO2 mediated effects on insect performance differed between feeding guilds (leaf-chewers, leaf miners, phloem-feeders (root and shoot), xylem-feeders, whole-cell-feeders and seed-eaters) or instar stage. Host-plant quality changed in elevated CO2; leaf nitrogen content decreased, on average, by 15% while carbohydrates increased by 47% and secondary plant compounds (phenolics) by 31%. Water content did not change. Of the variables measured, changes in nitrogen and carbohydrate levels only were found to be correlated with changes in food consumption. No differences were found in CO2-mediated herbivore responses on woody plant compared with non-woody plants. Insects from different feeding guilds respond to CO2 mediated changes in host-plant quality in various ways. Leaf-chewers generally seem able to compensate for the decreased nitrogen levels in the plant tissues by increasing their food consumption (by 30%) and with no adverse effects on pupal weights. Leaf-miners only slightly increase their food consumption. The negative effect on pupal weight suggests that their population dynamics may change over several generations. Limited data on seed-eaters suggest that enhanced CO2 conditions have no effect on these insects. Phloem-feeders and whole-cell-feeders are the only insects to show a positive CO2 response. Population sizes generally increased in elevated CO2 and development lime of phloem-feeders was reduced by 17%. Early instar larvae are restricted more by CO2 enhancement than late instars. Although changes in food consumption are similar, changes in development times are much more pronounced in young instars (18% vs 6%).
