Data from: Loss of migratory behavior increases infection risk for a butterfly host

Long-distance animal migrations have important consequences for infectious disease dynamics, in some cases lowering pathogen transmission by allowing animals to periodically escape contaminated habitats and weeding out infected individuals during strenuous journeys. Human activities cause some migratory animals to migrate shorter distances or form sedentary (non-migratory) populations. We focused on North American monarchs (Danaus plexippus) and a specialist protozoan parasite (Ophryocystis elektroscirrha) to investigate how the loss of migratory behaviour affects pathogen spread and evolution. Each year, monarchs traditionally migrate between eastern North America and central Mexico. However, some monarch populations have become non-migratory in the southern U.S. in response to the planting of exotic milkweed. We used field sampling and citizen science data to quantify infection prevalence among migratory and sedentary populations, and we experimentally tested virulence of parasites from these same areas. Prevalence was markedly higher among sedentary monarchs compared to migratory monarchs. These results indicate that altered migration greatly increases infection risk for monarchs. Parasite virulence differed among strains but was similar between migratory and non-migratory populations, suggesting changes in migratory behaviour have not led to evolutionary divergence in virulence. Possible explanations include insufficient time for divergence or parasite gene flow between migratory and sedentary populations.