Effects of elevated CO2 and O3 on leaf damage and insect abundance in a soybean agroecosystem

By altering myriad aspects of leaf chemistry, increasing concentrations of CO2 and O-3 in the atmosphere derived from human activities may fundamentally alter the relationships between insect herbivores and plants. Because exposure to elevated CO2 can alter the nutritional value of leaves, some herbivores may increase consumption rates to compensate. The effects of O-3 on leaf nutritional quality are less clear; however, increased senescence may also reduce leaf quality for insect herbivores. Additionally, changes in secondary chemistry and the microclimate of leaves may render plants more susceptible to herbivory in elevated CO2 and O-3. Damage to soybean (Glycine max L.) leaves and the size and composition of the insect community in the plant canopy were examined in large intact plots exposed to elevated CO2 (similar to 550 mu mol mol(-1)) and elevated O-3 (1.2* ambient) in a fully factorial design with a Soybean Free Air Concentration Enrichment system (SoyFACE). Leaf area removed by folivorous insects was estimated by digital photography and insect surveys were conducted during two consecutive growing seasons, 2003 and 2004. Elevated CO2 alone and in combination with O-3 increased the number of insects and the amount of leaf area removed by insect herbivores across feeding guilds. Exposure to elevated CO2 significantly increased the number of western corn rootworm (Diabrotica virgifera) adults (foliage chewer) and soybean aphids (Aphis glycines; phloem feeder). No consistent effect of elevated O-3 on herbivory or insect population size was detected. Increased loss of leaf area to herbivores was associated with increased carbon-to-nitrogen ratio and leaf surface temperature. Soybean aphids are invasive pests in North America and new to this ecosystem. Higher concentrations of CO2 in the atmosphere may increase herbivory in the soybean agroecosystem, particularly by recently introduced insect herbivores.