Earth is changing at a rapid rate as a result of human activities, and we should expect to see substantial effects on interactions among organisms in ecological communities. For example, interactions between agents of disease and their host organisms are widely expected to change in the coming years. Here, I focus on interactions between insect herbivores and their parasites or pathogens, with particular emphasis on the role of plant chemistry in mediating host-parasite interactions. Human activities are causing significant changes in plant chemistry on the phytochemical landscape, with important implications for herbivore disease dynamics. Using examples from diverse systems, including gypsy moths in temperate forests and monarch butterflies on milkweed, I illustrate how changes on the phytochemical landscape are likely to influence parasite-host interactions.
Anthropogenic nutrient deposition, increasing global temperatures, changes in patterns of precipitation, and increases in atmospheric concentrations of carbon dioxide all influence the nutritional and medicinal chemistry of plants. In turn, changes in plant chemistry alter the disease dynamics of insect herbivores. For example, polyphenolic concentrations in forest trees, which vary with nutrient deposition, stand structure, and light availability, determine in part the spatial and temporal dynamics of gypsy moth and its nuclear polyhedrosis virus. Empirical and modeling studies suggest that human mediated changes in the phytochemical landscape will alter the spread and severity of gypsy moth outbreaks. Likewise, cardenolide concentrations in the foliage of milkweeds vary with nutrient deposition, water availability, temperature, and atmospheric carbon dioxide concentrations. In combination with foliar nutrient concentrations, cardenolide concentrations determine the medicinal quality of milkweed foliage for monarch butterflies, and subsequent effects on protozoan parasites. Overall, our studies suggest that (a) gypsy moth outbreaks should increase in severity under environmental change while (b) monarch butterflies will suffer increasingly deleterious effects of their protozoan parasites. I discuss the implications of this work more generally for disease dynamics under global environmental change.