Atmospheric change alters foliar quality of host trees and performance of two outbreak insect species

This study examined the independent and interactive effects of elevated carbon dioxide (CO2) and ozone (O-3) on the foliar quality of two deciduous trees species and the performance of two outbreak herbivore species. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown at the Aspen FACE research site in northern Wisconsin, USA, under four combinations of ambient and elevated CO2 and O-3. We measured the effects of elevated CO2 and O-3 on aspen and birch phytochemistry and on gypsy moth (Lymantria dispar) and forest tent caterpillar (Malacosoma disstria) performance. Elevated CO2 nominally affected foliar quality for both tree species. Elevated O-3 negatively affected aspen foliar quality, but only marginally influenced birch foliar quality. Elevated CO2 slightly improved herbivore performance, while elevated O-3 decreased herbivore performance, and both responses were stronger on aspen than birch. Interestingly, elevated CO2 largely offset decreased herbivore performance under elevated O-3. Nitrogen, lignin, and C:N were identified as having strong influences on herbivore performance when larvae were fed aspen, but no significant relationships were observed for insects fed birch. Our results support the notion that herbivore performance can be affected by atmospheric change through altered foliar quality, but how herbivores will respond will depend on interactions among CO2, O-3, and tree species. An emergent finding from this study is that tree age and longevity of exposure to pollutants may influence the effects of elevated CO2 and O-3 on plant-herbivore interactions, highlighting the need to continue long-term atmospheric change research.