Development of gypsy moth larvae feeding on red maple saplings at elevated CO2 and temperature

Predicted increases in atmospheric CO2 and global mean temperature may alter important plant-insect associations due to the direct effects of temperature on insect development and the indirect effects of elevated temperature and CO2 enrichment on phytochemicals important for insect success. We investigated the effects of CO2 and temperature on the interaction between gypsy moth (Lymantria dispar L.) larvae and red maple (Acer rubrum L.) saplings by bagging first instar larvae within open-top chambers at four CO2/temperature treatments: (1) ambient temperature, ambient CO2, (2) ambient temperature, elevated CO2 (+300 mul l(-1) CO2), (3) elevated temperature (+3.5degreesC), ambient CO2, and (4) elevated temperature, elevated CO2. Larvae were reared to pupation and leaf samples taken biweekly to determine levels of total N, water, non-structural carbohydrates, and an estimate of defensive phenolic compounds in three age classes of foliage: (1) immature, (2) mid-mature and (3) mature. Elevated growth temperature marginally reduced (P <0.1) leaf N and significantly reduced (P <0.05) leaf water across CO2 treatments in mature leaves, whereas leaves grown at elevated CO2 concentration had a significant decrease in leaf N and a significant increase in the ratio of starch:N and total non-structural carbohydrates:N. Leaf N and water decreased and starch:N and total non-structural carbohydrates:N ratios increased as leaves aged. Phenolics were unaffected by CO2 or temperature treatment. There were no interactive effects of CO2 and temperature on any phytochemical measure. Gypsy moth larvae reached pupation earlier at the elevated temperature (female =8 days, P <0.07; male =7.5 days, P <0.03), whereas mortality and pupal fresh weight of insects were unrelated to either CO2, temperature or their interaction. Our data show that CO2 or temperature-induced alterations in leaf constituents had no effect on insect performance; instead, the long-term exposure to a 3.5degreesC increase in temperature shortened insect development but had no effect on pupal weight. It appears that in some tree-herbivorous insect systems the direct effects of an increased global mean temperature may have greater consequences for altering plant-insect interactions than the indirect effects of an increased temperature or CO2 concentration on leaf constituents.