Associations between host migration and the prevalence of a protozoan parasite in natural populations of adult monarch butterflies

1. Monarch butterflies Danaus plexippus (L.) (Lepidoptera: Nymphalidae) are susceptible to infection by the obligate protozoan parasite Ophryocystis e lektroscirrha (McLaughlin and Myers) (Apicomplexa: Neogregarinida). Because monarchs form resident and migratory populations in different parts of the world, this host-parasite system provides the opportunity to examine how v ariation in parasite prevalence relates to host movement patterns. 2. Parasite prevalence was evaluated using 14 790 adult monarchs captured b etween 1968 and 1997. Comparison of three populations in North America indi cated that parasite prevalence is associated negatively with host dispersal distances. A continuously breeding, nonmigratory population in southern Fl orida showed high prevalence (over 70% heavily infected). The western popul ation migrates moderate distances to overwintering sites on the Pacific Coa st and has intermediate prevalence (30% heavily infected). The eastern migr atory population, which travels the longest distance to Mexican overwinteri ng sites, has exhibited less than 8% infection throughout the past 30 years . 3. Variation in parasite loads within North American migratory populations was investigated to determine whether the prevalence of heavy infection and average parasite loads declined during migration or overwintering. Average parasite loads of summer-breeding adults in western North America decrease d with increasing distance from overwintering sites. This suggests that hea vily infected monarchs are less likely to remigrate long distances in sprin g. No differences in the frequency of heavily infected adults were found am ong eastern or western North American monarchs throughout the overwintering period, however, suggesting that this parasite does not affect overwinteri ng mortality. 4. Changes in the prevalence of monarchs with low parasite loads demonstrat e that spore transfer occurs during migration and overwintering, possibly w hen adult butterflies contact each other as a result of their clustering be haviour. 5. This study of geographical and temporal variation in O. elektroscirrha a mong populations of D. plexippus demonstrates the potential role of seasona l migration in mediating interactions between hosts and parasites, and sugg ests several mechanisms through which migratory behaviour may influence par asite prevalence.