Low prevalence of the parasite Ophryocystis elektroscirrha at the range edge of the eastern North American monarch (Danaus plexippus) butterfly population

Every year monarch butterflies (Danaus plexippus (Linnaeus, 1758)) from the eastern North American population migrate from Mexico to southern Canada in the spring. This northward migration has been shown to reduce monarch infection with the host-specific parasite Ophryocystis elektroscirrha McLaughlin and Myers, 1970 (OE); yet, the prevalence of OE at their range limits and the mechanism(s) responsible are unknown. We assessed OE infection levels of monarchs at the northern edge of the eastern population distribution around Ottawa, Ontario, Canada, and found extremely low levels of infection (∼1% with upper confidence intervals close to 3%). Low OE infection levels are likely due to low densities of monarchs in this region and (or) migratory escape effects, where migrating individuals leave behind areas with high density of conspecifics and high potential for parasite accumulation and transmission. Future work should aim to disentangle the relative contribution of these two mechanisms for governing the decrease in parasitism at the range limits of migratory populations.