Effects of elevated CO2 on feeding preference and performance of the gypsy moth (Lymantria dispar) larvae

Multiple-choice assays were conducted in the laboratory to examine the effect of CO2-induced changes in leaf quality on growth, nutritional indices and preferences of the gypsy moth (Lymantria dispar) larvae. The tested tree species, one-year-old aspen (Populus pseudo-simonii Kitag.), two-year-old birch (Betula platyphylla) and three-year-old oak (Quercus mongolica Fisch.) were transplanted to open-top chambers at ambient or elevated CO2 (650 ppm) concentrations in May 2005. The present study was conducted in 2006. Leaves from the upper and lower crowns of each tree species were sampled and analysed, respectively. On both measurement dates (24 July 2006 and 16 August 2006), there were significant CO2 effects on levels of condensed tannin, starch, nitrogen and C : N ratio. But only on the latter date, were the concentrations of total phenolics, soluble sugar, carbon and total non-structural carbohydrates significantly affected by elevated CO2. Leaf dry weight content and specific leaf weight were almost completely unaffected by CO2 enrichment. Gypsy moth larvae exhibited a clear selectivity for tested leaf types (leaves from the upper and lower crowns of each tree species) even in their early instar stage, with the upper leaves of P. pseudo-simonii being the most preferable and the lower leaves of Q. mongolica being the least preferred. The changes in leaf quality significantly reduced larval growth and altered partial indices of insect performance (e.g., relative growth rate and relative consumption rate). However, at least in this short-term choice feeding assay (13 days), the CO2-induced changes in leaf quality had no significant effects on food preference behaviour of the gypsy moth larvae, neither within the limited range of host plants nor within the leaves at different canopy heights of the same tree species.