Plant-Mediated Aphid Responses to Elevated CO2: The Impact of Phloem Phytochemistry, Endophyte Infection and Nitrogen Availability.
Rising atmospheric CO2 is expected to increase primary productivity resulting in changes in bottom-up ecosystem interactions. CO2-induced increases in photosynthesis are expected to change plant growth, morphology and metabolism. Such changes will alter the nutritional quality of plants for insect herbivores, particularly those involving resource allocation. While plant mechanisms underlying foliage feeder response to CO2 have been well documented, much less is known about the responses of insects feeding on a phloem diet. In this work I examined the current literature on responses to elevated CO2 using a quantitative meta-analysis of changes in insect life history traits and plant growth, nutrient allocation, morphology and metabolite responses. Insect response to rising CO2 was dependent on feeding guild, with foliage feeder performance decreasing, while phloem feeder response increased. Changes in plant parameters were found to occur under elevated CO2 and were dependent on plant functional group and interacting environmental variables. Plant-mediated Rhopalosiphum padi aphid responses to CO2 were measured on tall fescue and barley. Aphid abundance and density decreased on tall fescue and increased on barley plants in response to elevated CO2. The relative concentration of some essential amino acids decreased under elevated CO2 in tall fescue while most essential amino acids increased in barley, along with an increase in the total concentration of amino acids and the ratio of essential to non-essential amino acids. In tall fescue, aphid response was dependent on plant nitrogen and Neotyphodium endophyte status. Elevated CO2 was found to alter endophyte and alkaloid concentrations which may have implications for the grass-endophyte mutualism. On endophyte-infected grasses, CO2-induced decreases in alkaloids had no effect on aphids which were unsuccessful at colonizing endophyte-infected plants. The possible causal agents of CO2-induced changes in aphid performance are discussed with an emphasis on amino acids. Future studies which may further elucidate these underlying mechanisms are addressed.