Each year, billions of animals migrate long distances to track seasonal resources. In some cases, migration lowers parasite infection risk for populations by removing infected individuals or allowing a periodic escape from contaminated habitat. In recent decades, some migratory populations of birds, bats, and ungulates have become more sedentary as animals abandon or shorten their journeys in response to supplemental feeding. Exploring the consequences of reduced movement for host-parasite interactions is needed to predict future responses of pathogens to human-provided resources. Monarch butterflies (Danaus plexippus) and their specialist protozoan parasite Ophryocystis elektroscirrha provide a model system for examining how resources that alter migratory behaviors can influence infectious disease dynamics. Monarchs from eastern North America typically migrate over 4000 km each fall to central Mexico. This process annually reduces protozoan infection prevalence. However, some monarchs now forego migration to breed year-round in the southern U.S. on exotic milkweed, which can grow continuously and provide food for monarch caterpillars even during the winter (unlike most native milkweeds). We synthesize our work investigating (1) How do altered migratory behaviors, enabled by provisioned resources, influence infection prevalence and parasite virulence? (2) To what degree do provisioned, non-migratory animals pose transmission risk to migratory conspecifics?
We report recently published work based on citizen science data (from the Monarch Health program) showing that non-migratory monarchs in the southeastern U.S. face significantly higher risk of infection with Ophryocystis elektroscirrha parasites compared to migratory monarchs. Multiple lines of evidence suggest that year-round exotic milkweeds allow monarchs to skip migration and thus lose the associated mechanisms that reduce infection prevalence each year. We also report results from a similar study of infection risk in western North America, where a separate population of migratory monarchs overwinters along the coast and where year-round breeding also appears to be rising in southern California. Finally, we examine whether migratory and non-migratory monarchs overlap in habitat and exotic milkweed use in Texas. Such resident-migrant interactions could lead to parasite transmission and pose an additional challenge to migratory monarchs currently undergoing severe declines in North America. For monarchs, our findings show that the planting of exotic milkweeds alters migratory behaviors and increases parasite burdens for sedentary monarchs, which in turn places spring migratory monarchs at risk for acquiring parasites. Our findings underscore the potential for provisioned resources to alter host-parasite interactions.