Effects of Elevated CO2, Soil Nutrient Levels, and Foliage Age on the Performance of Two Generations of Neodiprion lecontei (Hymenoptera: Diprionidae) Feeding on Loblolly Pine

Abstract

We investigated how changes in loblolly pine needle phytochemistry caused by elevated CO2, leaf age, and soil nutrient levels affected the performance of 2 individual generations of the multivoltine folivorous insect pest Neodiprion lecontei (Fitch). In 2 feeding trials, mature needles produced in the previous (spring) and current (fall) year from seedlings grown in open-topped chambers under 4 CO2 and 2 soil nutrient levels were fed to 2 separate generations of redheaded pine sawfly larvae. Strong seasonal differences (i.e., spring versus fall) in leaf nutritional and defensive constituents resulted in significant between-generation differences in the growth, consumption, and growth efficiency of sawfly larvae. Enriched CO2-grown needles had higher levels of starch and starch/nitrogen ratios in older, overwintering spring needles, which were lower in leaf nitrogen and monoterpenes than younger, current year needles (fall). Overall, larval growth was higher and consumption lower on the fall needles, presumably because of higher levels of leaf nitrogen compared with the spring needles. The plant CO2 concentration significantly contributed to the larval consumption responses between seasons (significant CO2 × season interaction), demonstrating that the 2 sawfly generations were affected differently by CO2-induced phytochemical alterations in spring versus fall needles. The data presented here suggests that when investigating multivoltine folivorous insect responses to elevated CO2-grown tree seedlings in which multiple leaf flushes within a growing season expose insects to an array of leaf phytochemical changes, >1 insect generation should be investigated.