Effects of elevated CO2-grown loblolly pine needles on the growth, consumption, development, and pupal weight of red-headed pine sawfly larvae reared within open-topped chambers

Seedlings of loblolly pine, Pinus taeda, were grown in open-topped chambers under four levels of CO2: two ambient and two elevated. Larvae of the red-headed pine sawfly, Neoniprion lecontei, were reared from early instar to pupation, primarily on branches within chambers. Larval growth and mortality were assessed and leaf phytochemistry samples of immature and mature leaves collected weekly. Mature leaves grown under elevated CO2 had significant reductions in leaf nitrogen and increases in non-structural carbohydrate contents, resulting in foliage being a poorer food source for larvae, i.e. higher carbohydrate:nitrogen ratio. Nutritional constituents of immature needles were unaffected by seedling CO2 treatment. Volatile mono-and sesquiterpenes were unrelated to plant CO2 treatments for either leaf age class. Larval consumption of immature needles significantly increased on seedlings grown under CO2 enrichment, while mature needle consumption was not different between the treatments. The average weight gain per larva significantly declined in late instar larvae consuming elevated CO2-grown needles. In spite of this reduced growth, neither the days to pupation nor pupal weights were different among the CO2 treatments. This study suggests that enriched CO2-induced alterations in pine needle phytochemistry can affect red-headed pine sawfly performance, However, compensatory measures by larvae, such as choosing to consume more nutritious immature needles, apparently helps offset enriched CO2-induced reductions in the leaf quality of mature needles.