Responses of plant phenology, growth, defense, and reproduction to interactive effects of warming and insect herbivory

Climate warming can modify plant reproductive fitness through direct and indirect pathways. Direct effects include temperature-driven impacts on growth, reproduction, and secondary metabolites. Indirect effects may manifest through altered species interactions, including herbivory, although studies comparing the interactive effects of warming and herbivory are few. We used experimental warming combined with herbivore exclusion cages to assess the interactive effects of climate warming and herbivory by Popillia japonica, the Japanese beetle, on flowering phenology, growth, defense, and lifetime reproduction of a biennial herb, Oenothera biennis. Regardless of temperature, herbivory delayed flowering phenology and, surprisingly, led to decreased levels of foliar defenses. At ambient temperatures, plants were able to compensate for herbivory by producing smaller seeds and increasing total seed production, leading to similar investment in seed biomass for plants exposed to and protected from herbivores. At elevated temperatures, plants had elevated total seed production, but herbivory had negligible impacts on flower and fruit production, and total lifetime seed biomass was highest in plants exposed to herbivores in warmed conditions. We speculate that warming induced a stress response in O. biennis resulting from low soil moisture, which in turn led to an increase in seed number at the expense of maternal investment in each seed. Plant-insect interactions might therefore shift appreciably under future climates, and ecologists must consider both temperature and herbivory when attempting to assess the ramifications of climate warming on plant populations.