CO2 and O3 effects on host plant preferences of the forest tent caterpillar (Malacosoma disstria)

Elevated levels of CO2 and O-3 affect plant growth and phytochemistry, which in turn can alter physiological performance of associated herbivores. Little is known, however, about how generalist insect herbivores respond behaviorally to CO2- and O-3-mediated changes in their host plants. This research examined the effects of elevated CO2 and O-3 levels on host plant preferences and consumption of forest tent caterpillar (FTC, Malacosoma disstria Hbn.) larvae. Dual choice feeding assays were performed with foliage from birch (Betula papyrifera Marsh.) and aspen (Populus tremuloides Michx., genotypes 216 and 259). Trees were grown at the Aspen Free Air CO2 Enrichment (FACE) facility near Rhinelander, WI, USA, and had been exposed to ambient or elevated concentrations of CO2 and/or O-3. Levels of nutritional and secondary compounds were quantified through phytochemical analyses. The results showed that elevated O-3 levels increased FTC larval preferences for birch compared with aspen, whereas elevated CO2 levels had the opposite effect. In assays with the two aspen genotypes, addition of both CO2 and O-3 caused a shift in feeding preferences from genotype 259 to genotype 216. Consumption was unaffected by experimental treatments in assays comparing aspen and birch, but were increased for larvae given high O-3 foliage in the aspen genotype assays. Elevated levels of CO2 and O-3 altered tree phytochemistry, but did not explain shifts in feeding preferences. The results demonstrate that increased levels of CO2 and O-3 can alter insect host plant preferences both between and within tree species. Also, consequences of altered host quality (e.g., compensatory consumption) may be buffered by partial host shifts in situations when alternative plant species are available. Environmentally induced changes in host plant preferences may have the potential to alter the distribution of herbivory across plant genotypes and species, as well as competitive interactions among them.