Host plants and experimental warming impact fitness and infection outcomes in a migratory butterfly

Climate warming can impact insects and their parasites directly, but higher temperatures also influence parasitism indirectly through other species interactions, such as herbivory. Temperature can influence plant quality, which in turn can shape nutrition and parasite defenses of insect herbivores. Thus, understanding plant-mediated effects of warming on insect-parasite interactions will be critically important for insect conservation and pest insect management. We examined how warming and host plant species influenced parasitism of monarch butterfly (Danaus plexippus) larvae by the protozoan parasite Ophryocystis elektroscirrha (OE) in a field cage experiment. Cardenolides, a defensive chemical produced by milkweed host plants (Asclepias spp.), increase monarch tolerance and resistance to OE infection. We reared parasite-exposed and uninfected control monarch larvae on tropical milkweed (high-cardenolide) or swamp milkweed (low-cardenolide) in either ambient or elevated temperatures, measuring monarch development, survival, size, infection success and intensity. To assess whether temperature effects on infection or monarch traits can be informed by changes in milkweed plant nutrition or toxicity, we measured foliar carbon, nitrogen and cardenolides. We found that host plant species was an important predictor of monarch development and survival and that elevated temperatures reduced monarch tolerance to infection even when they were reared on high-cardenolide tropical milkweed. We also found that milkweed leaves had more nitrogen relative to carbon in temperature-elevated plots, indicating that despite improved host plant nutrition, monarchs experienced lower fitness and higher costs of infection. These findings will help predict the consequences of warming for monarch health across the insect’s range.