Seedlings of loblolly pine Pinus taeda (L.), were grown in open-topped field chambers under three CO regimes: ambient, 150 μl l CO above ambient, and 300 μl l CO above ambient. A fourth, non-chambered ambient treatment was included to assess chamber effects. Needles were used in 96 h feeding trials to determine the performance of young, second instar larvae of loblolly pine's principal leaf herbivore, red-headed pine sawfly, Neodiprion lecontei (Fitch). The relative consumption rate of larvae significantly increased on plants grown under elevated CO, and needles grown in the highest CO regime were consumed 21% more rapidly than needles grown in ambient CO. Both the significant decline in leaf nitrogen content and the substantial increase in leaf starch content contributed to a significant increase in the starch:nitrogen ratio in plants grown in elevated CO. Insect consumption rate was negatively related to leaf nitrogen content and positively related to the starch:nitrogen ratio. Of the four volatile leaf monoterpenes measured, only β-pinene exhibited a significant CO effect and declined in plants grown in elevated CO. Although consumption changed, the relative growth rates of larvae were not different among CO treatments. Despite lower nitrogen consumption rates by larvae feeding on the plants grown in elevated CO, nitrogen accumulation rates were the same for all treatments due to a significant increase in nitrogen utilization efficiency. The ability of this insect to respond at an early, potentially susceptible larval stage to poorer food quality and declining levels of a leaf monoterpene suggest that changes in needle quality within pines in future elevated-CO atmospheres may not especially affect young insects and that tree-feeding sawflies may respond in a manner similar to herb-feeding lepidopterans.

Summary Few studies have investigated how tree species grown under elevated CO2 and elevated temperature alter the performance of leaf‐feeding insects. The indirect effects of an elevated CO2 concentration and temperature on leaf phytochemistry, along with potential direct effects on insect growth and consumption, may independently or interactively affect insects. To investigate this, we bagged larvae of the gypsy moth on leaves of red and sugar maple growing in open‐top chambers in four CO2/temperature treatment combinations: (i) ambient temperature, ambient CO2; (ii) ambient temperature, elevated CO2 (+ 300 μL L−1 CO2); (iii) elevated temperature (+ 3.5°C), ambient CO2; and (iv) elevated temperature, elevated CO2. For both tree species, leaves grown at elevated CO2 concentration were significantly reduced in leaf nitrogen concentration and increased in C: N ratio, while neither temperature nor its interaction with CO2 concentration had any effect. Depending on the tree species, leaf water content declined (red maple) and carbon‐based phenolics increased (sugar maple) on plants grown in an enriched CO2 atmosphere. The only observed effect of elevated temperature on leaf phytochemistry was a reduction in leaf water content of sugar maple leaves. Gypsy moth larval responses were dependent on tree species. Larvae feeding on elevated CO2‐grown red maple leaves had reduced growth, while temperature had no effect on the growth or consumption of larvae. No significant effects of either temperature or CO2 concentration were observed for larvae feeding on sugar maple leaves. Our data demonstrate strong effects of CO2 enrichment on leaf phytochemical constituents important to folivorous insects, while an elevated temperature largely has little effect. We conclude that alterations in leaf chemistry due to an elevated CO2 atmosphere are more important in this plant–folivorous insect system than either the direct short‐term effects of temperature on insect performance or its indirect effects on leaf chemistry.

Folivorous insect responses to elevated CO2‐grown tree species may be complicated by phytochemical changes as leaves age. For example, young expanding leaves in tree species may be less affected by enriched CO2‐alterations in leaf phytochemistry than older mature leaves due to shorter exposure times to elevated CO2 atmospheres. This, in turn, could result in different effects on early vs. late instar larvae of herbivorous insects. To address this, seedlings of white oak (Quercus alba L.), grown in open‐top chambers under ambient and elevated CO2, were fed to two important early spring feeding herbivores; gypsy moth (Lymantria dispar L.), and forest tent caterpillar (Malacosoma disstria Hübner). Young, expanding leaves were presented to early instar larvae, and older fully expanded or mature leaves to late instar larvae. Young leaves had significantly lower leaf nitrogen content and significantly higher total nonstructural carbohydrate:nitrogen ratio as plant CO2 concentration rose, while nonstructural carbohydrates and total carbon‐based phenolics were unaffected by plant CO2 treatment. These phytochemical changes contributed to a significant reduction in the growth rate of early instar gypsy moth larvae, while growth rates of forest tent caterpillar were unaffected. The differences in insect responses were attributed to an increase in the nitrogen utilization efficiency (NUE) of early instar forest tent caterpillar larvae feeding on elevated CO2‐grown leaves, while early instar gypsy moth larval NUE remained unchanged among the treatments. Later instar larvae of both insect species experienced larger reductions in foliage quality on elevated CO2‐grown leaves than earlier instars, as the carbohydrate:nitrogen ratio of leaves substantially increased. Despite this, neither insect species exhibited changes in growth or consumption rates between CO2 treatments in the later instar. An increase in NUE was apparently responsible for offsetting reduced foliar nitrogen for the late instar larvae of both species.

Predicted increases in atmospheric CO(2) 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 CO(2) enrichment on phytochemicals important for insect success. We investigated the effects of CO(2) 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 CO(2)/temperature treatments: (1) ambient temperature, ambient CO(2), (2) ambient temperature, elevated CO(2) (+300 microl l(-1) CO(2)), (3) elevated temperature (+3.5 degrees C), ambient CO(2), and (4) elevated temperature, elevated CO(2). 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 CO(2) treatments in mature leaves, whereas leaves grown at elevated CO(2) 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 CO(2) or temperature treatment. There were no interactive effects of CO(2) 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 CO(2), temperature or their interaction. Our data show that CO(2) or temperature-induced alterations in leaf constituents had no effect on insect performance; instead, the long-term exposure to a 3.5 degrees C 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 CO(2) concentration on leaf constituents.