PLANT-INSECT HERBIVORE INTERACTIONS IN ELEVATED ATMOSPHERIC CO2 - QUANTITATIVE-ANALYSES AND GUILD EFFECTS

Interactions between insect herbivores and plants grown under conditio ns of ambient and elevated CO2 were investigated by analysing data on 43 herbivores, representing 61 plant-herbivore interactions. Changes i n herbivore performance in enhanced CO2 environments were correlated w ith changes in the quality of the host plants, measured as nitrogen co ntent, water content, carbohydrate content and secondary plant compoun ds. The data were analysed to determine whether CO2 mediated effects o n insect performance differed between feeding guilds (leaf-chewers, le af miners, phloem-feeders (root and shoot), xylem-feeders, whole-cell- feeders and seed-eaters) or instar stage. Host-plant quality changed i n elevated CO2; leaf nitrogen content decreased, on average, by 15% wh ile carbohydrates increased by 47% and secondary plant compounds (phen olics) by 31%. Water content did not change. Of the variables measured , changes in nitrogen and carbohydrate levels only were found to be co rrelated with changes in food consumption. No differences were found i n CO2-mediated herbivore responses on woody plant compared with non-wo ody plants. Insects from different feeding guilds respond to CO2 media ted changes in host-plant quality in various ways. Leaf-chewers genera lly seem able to compensate for the decreased nitrogen levels in the p lant 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 t hat their population dynamics may change over several generations. Lim ited 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 genera lly increased in elevated CO2 and development lime of phloem-feeders w as reduced by 17%. Early instar larvae are restricted more by CO2 enha ncement than late instars. Although changes in food consumption are si milar, changes in development times are much more pronounced in young instars (18% vs 6%).