“Stay-cation” monarch butterflies (Danaus plexippus) burdened by disease: Effects of altered host migration on parasite transmission
Animal migrations can have profound ecological consequences for species interactions. In particular, recent studies have highlighted how migration can influence infectious disease dynamics. Migratory animals might experience reduced infection risk if migrants leave behind contaminated habitats (migratory escape) or are weeded out during strenuous journeys (migratory culling). But migration is now a threatened phenomenon for many species. Climate change and habitat alteration have changed the timing and extent of many migrations, and some long-distance migrations have disappeared altogether. To investigate how changes in migration might affect infectious disease processes, we asked whether the altered migration of monarch butterflies (Danaus plexippus) affects the prevalence and virulence of the protozoan parasite Ophryocystis elektroscirrha (OE). Each fall, monarchs in eastern North America migrate up to 3000 km to central Mexico. However, in recent decades, some monarchs have remained throughout the winter in the southern U.S. to reproduce on exotic milkweeds. In this study, we (1) worked with citizen scientists through the program Monarch Health to determine the prevalence of OE across replicated samples of migratory vs. winter-breeding populations, and (2) inoculated monarchs with different strains of OE from migratory and winter-breeding populations to ask whether parasites from migratory sources are less virulent.
Results from over 800 monarch samples across 28 sites in the southern U.S. showed that winter-breeding (non-migratory) monarchs are over 6 times more likely to be infected with OE than monarchs that migrate long distances to Central Mexico. However, counter to our expectations, experimental data showed no significant difference in virulence of OE isolated from migratory and winter-breeding populations, indicating that high gene flow or insufficient time for divergence might limit evolutionary changes in the parasite. This presentation also outlines future areas of research aimed at understanding how the loss of migratory behavior might affect parasite transmission for monarchs and other migratory species.