LEAF AGE EFFECTS OF ELEVATED CO2-GROWN WHITE OAK LEAVES ON SPRING-FEEDING LEPIDOPTERANS

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 sh orter exposure times to elevated CO2 atmospheres. This, in turn, could result in different effects on early vs. late instar larvae of herbiv orous insects. To address this, seedlings of white oak (Quercus alba L .), grown in open-top chambers under ambient and elevated CO2, were fe d to two important early spring feeding herbivores; gypsy moth (Lymant ria dispar L.), and forest tent caterpillar (Malacosoma disstria Hubne r). Young, expanding leaves were presented to early instar larvae, and older fully expanded or mature leaves to late instar larvae. Young le aves had significantly lower leaf nitrogen content and significantly h igher total nonstructural carbohydrate:nitrogen ratio as plant CO2 con centration rose, while nonstructural carbohydrates and total carbon-ba sed phenolics were unaffected by plant CO2 treatment. These phytochemi cal changes contributed to a significant reduction in the growth rate of early instar gypsy moth larvae, while growth rates of forest tent c aterpillar were unaffected. The differences in insect responses were a ttributed 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 uncha nged 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 le aves substantially increased. Despite this, neither insect species exh ibited changes in growth or consumption rates between CO2 treatments i n the later instar. An increase in NUE was apparently responsible for offsetting reduced foliar nitrogen for the late instar larvae of both species.