CO sub(2) and O sub(3) Effects on Host Plant Preferences of the Forest Tent Caterpillar (Malacosoma disstria)

The effects of elevated atmospheric carbon dioxide levels are frequently modulated by other environmental factors, including tropospheric ozone levels. In this study, the effects of elevated CO sub(2) and O sub(3) on host-plant preferences of forest tent caterpillar Malacosoma disstria larvae were examined. Two-choice bioassays were used to determine the relative preferences of fourth instar larvae for paper birch Betula papyrifera and two genotypes of quaking aspen Populus tremuloides grown under different CO sub(2) and O sub(3) conditions. The relationships between host-tree quality and larval preferences were examined in terms of the levels of foliar nutrients and secondary compounds. The study was conducted near Rhinelander, WI. Overall, elevated O sub(3) levels altered host-plant preferences more than elevated CO sub(2) levels, with larvae avoiding O sub(3)-exposed foliage, especially in aspen. Larvae did not distinguish between birch from the different fumigation treatments, but they clearly avoided aspen from the O sub(3) treatment, although elevated CO sub(2) levels increased the preference for aspen over birch. Tree responses were generally stronger to elevated O sub(3) than to elevated CO sub(2) levels, including the levels of phenolic compounds.