Resistance and Tolerance to Imperfectly Specialized Parasites: Milkweed Butterflies and Their Protozoan Parasites.

Understanding host specificity and cross-species transmission of parasites is crucial for predicting the risk and consequences of parasite spillover. We experimentally examined these dynamics in two closely related, sympatric, milkweed butterfly hosts: monarchs () and queens (). The debilitating protozoan () infects wild monarchs throughout their range, and similar neogregarine parasites have been reported from queens. We compared host resistance and tolerance to infection between hosts exposed to parasites of conspecific and heterospecific origin and examined whether differences in immune investment reflected variation in infection outcomes. Results showed that monarchs were highly susceptible to both conspecific and heterospecific parasites. In contrast, queens were susceptible almost exclusively to conspecific parasites. Queens showed greater tolerance to infection and greater immune defense in the form of melanization activity and concentration of encapsulating hemocytes. Additionally, monarch parasites caused higher pre-adult mortality and more wing deformities than queen parasites. Given that can reduce monarch abundance and migratory performance, quantifying cross-infection outcomes is important for conservation management of these two butterfly species. The greater susceptibility and costs of infection in monarchs suggest potential fitness trade-offs against resistance and tolerance to infection in migratory hosts and underscore the need to identify factors that limit hosts' adaptation to parasites.