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

Climate warming can impact plant quality, which in turn shapes the nutrition and immune defence of insect herbivores. Understanding plant‐mediated effects of warming on insect‐parasite interactions could inform insect conservation and pest management, especially if plant‐mediated impacts differ among closely related plant species. 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, defensive chemicals produced by milkweed host plants, increase monarch tolerance and resistance to OE. We reared parasite‐exposed and uninfected control monarch larvae on tropical (high‐cardenolide, Asclepias curassavica ) or swamp (low‐cardenolide, Asclepias incarnata ) milkweed in either ambient or elevated temperatures, measuring monarch development, survival, size and OE infection success and intensity. To inform whether temperature effects on infection or monarch traits were influenced by altered milkweed nutrition or toxicity, we also measured how foliar carbon:nitrogen and cardenolides responded to temperature. We found that host plant species were an important predictor of monarch development and survival, and that elevated temperatures reduced monarch tolerance to infection by 22% even when reared on high‐cardenolide tropical milkweed. Milkweed leaves had more nitrogen relative to carbon in temperature‐elevated plots, revealing that, despite increased access to a limiting nutrient, monarchs experienced lower fitness and higher costs of infection. Still, temperature modified the expected role of tropical milkweed, which did not confer OE resistance to monarchs under elevated temperatures. These findings illustrate the potential for host plants to shape consequential insect‐parasite interactions under warming.