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 (Danaus plexippus) and queens (D. gilippus). The debilitating protozoan Ophryocystis elektroscirrha (OE) 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 concentrations of encapsulating hemocytes. Additionally, monarch parasites caused higher pre-adult mortality and more wing deformities than queen parasites. Given that OE can reduce monarch abundance and migratory performance, and that monarchs overlap with butterflies that host similar parasites, 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 underscores the need to identify factors that limit hosts’ adaptation to parasites.

We performed controlled cross-infection experiments on monarch and queen butterflies using conspecific and heterospecific parasite strains. Caterpillars were assigned to receive either no parasites, parasites from queens, or parasites from monarchs. Within each host-parasite source combination, caterpillars were fed either swamp or tropical milkweed from the day of inoculation until pupation. This design resulted in 12 treatment combinations of butterfly species x parasite source x plant diet combinations. A subset of caterpillars was bled at the 5tth instar to estimate hemocyte concentration, hemocyte differentials, and the activity of the enzyme phenoloxidase as metrics of immune defense. We quantified measures indicative of host fitness and to inform analyses of host resistance and tolerance to infection.