Indirect effects of elevated carbon dioxide on members of three herbivorous feeding guilds on soybean, Glycine max

Does elevated CO2 change insect preference for or performance on soybeans? We attempted to answer this question with a number of experiments, measuring herbivore gustatory and olfactory preferences for soybeans grown under elevated levels of CO2, and their longevity and fecundity with long-term feeding on soybeans grown under elevated CO2. While many herbivorous insect species feed readily on soybean plants growing in elevated CO2, we focused our studies on three of them: the introduced Japanese beetle, Popillia japonica, the introduced soybean aphid, Aphis glycines, and the native painted lady butterfly, Vanessa cardui.;Y-tube choice tests were used to measure herbivore olfactory preference. There was a significant P. japonica preference for damaged soybean foliage over undamaged soybean foliage grown under ambient levels of carbon dioxide. This was expected, as several studies have found that P. japonica orient toward volatiles released from damaged plants rather than towards volatiles released from undamaged plants. There was no significant P. japonica preference for damaged soybean foliage over undamaged soybean foliage grown under elevated CO2. This result suggests that there is a change in volatile production, either its chemical composition or amounts released, in plants grown under elevated CO2 conditions. There was a significant P. japonica preference for damaged leaves grown under elevated levels of CO2 over damaged leaves grown under ambient levels of CO2. There were no significant preferences exhibited by A. glycines or V. cardui for any of the olfactory choices offered.;We measured aphid population growth on plants grown under elevated and ambient levels of CO2. A. glycines population growth was higher on plants grown under elevated CO2. Soybeans grown under elevated levels of CO2 are warmer than soybeans grown under ambient levels of CO2, which may increase aphid population growth rate. Aphid performance on plants grown under elevated levels of CO 2 was not significantly different from aphid performance on plants grown under ambient levels of CO2, when the leaf temperatures were controlled. This finding suggests that the increased population growth of aphids on plants grown under elevated levels of CO2 is the result of the increased leaf temperature of those plants.;We measured V. cardui larval development and mortality on soybeans grown under ambient and elevated levels of CO2. There was no significant difference in development or survivorship in larvae reared on plants grown under ambient or on plants grown under elevated levels of CO2. Nitrogen fixation in soybeans may provide these larvae with enough protein to develop normally despite the change in the C:N ratio in plants grown under elevated CO2.;We measured the flavonoid content of leaves grown under ambient and elevated levels of CO2. The flavonol glycosides were significantly increased under elevated CO2, while the isoflavonoid glycosides were significantly decreased under elevated CO2. Isoflavonoids are produced for plant defense against biotic factors; a decrease in their levels could lead to a decrease in dietary stress in herbivores feeding on these plants. Flavonols such as quercetin are more often produced as a defense against abiotic stress, such as pollutants or UV light. Their antioxidant properties could provide additional protection against oxidative damage in herbivores feeding on these plants and increase herbivore longevity.;P. japonica prefers damaged leaves grown under elevated CO2 over damaged leaves grown under ambient CO2, which may be the result of increased complexity in volatile compounds in leaves grown under elevated CO2. Increased amounts of flavonols and decreased amounts of isoflavonoids in leaves grown under elevated CO2 may contribute to increased herbivore longevity and fecundity. A. glycines population growth is greater on soybeans grown under elevated levels of CO2, which may be the result of increased leaf temperatures. V. cardui larval development and mortality are not changed when caterpillars fed on soybean leaves grown under elevated levels of CO2, which may be the result of nitrogen fixation and consequent nitrogen enrichment in soybean foliage. (Abstract shortened by UMI.)...