Migratory behaviour and host-parasite co-evolution in natural populations of monarch butterflies infected with a protozoan parasite

The prevalence of the protozoan parasite, Ophryocystis elektroscirrha, varies dramatically among natural populations of monarch butterflies. One potential cause of this variation is that host resistance or parasite virulence differs among populations due to underlying variation in host migratory behaviour and parasite transmission. In this study, I examined the geographic variation in host and parasite characteristics using reciprocal cross-infection experiments, where monarchs from three North American populations were exposed to parasites from native and novel sources. I tested hosts and parasites from the following three populations: a continuously breeding population in southern Florida, a population in western North America that migrates relatively short distances and a population in eastern North America that migrates remarkably long distances. Cross-infection experiments using hosts and parasites from the eastern and western migratory populations demonstrated that western parasites caused higher mortality and parasite loads than eastern parasites, and eastern monarchs had higher survival and lower parasite loads than western monarchs. Eastern migratory and Florida resident monarchs performed similarly across all treatments, but parasites isolated from southern Florida caused higher parasite loads than those from the eastern population. Differences in parasite virulence among populations were also supported by sub-lethal effects of parasites on monarch wingspan, mass at emergence and rates of weight loss. Unlike other documented patterns of host–parasite specificity, O. elektroscirrha strains do not appear to be more infectious to their native hosts. Rather, geographic variation may be better explained by selection resulting from differences in host migratory behaviour and parasite transmission among populations. The results of this study are consistent with the hypothesis that seasonal, long-distance host migration is associated with higher host resistance and lower parasite virulence in North American monarch populations.