Long-distance animal migrations have important consequences for infectious disease dynamics. In some cases, migration lowers pathogen transmission by removing infected individuals during strenuous journeys and allowing animals to periodically escape contaminated habitats. Human activities are now causing some migratory animals to travel shorter distances or form sedentary (non-migratory) populations. We focused on North American monarch butterflies and a specialist protozoan parasite to investigate how the loss of migratory behaviours affects pathogen spread and evolution. Each autumn, monarchs migrate from breeding grounds in the eastern US and Canada to wintering sites in central Mexico. However, some monarchs have become non-migratory and breed year-round on exotic milkweed in the southern US. We used field sampling, citizen science data and experimental inoculations to quantify infection prevalence and parasite virulence among migratory and sedentary populations. Infection prevalence was markedly higher among sedentary monarchs compared with migratory monarchs, indicating that diminished migration increases infection risk. Virulence differed among parasite strains but was similar between migratory and sedentary populations, potentially owing to high gene flow or insufficient time for evolutionary divergence. More broadly, our findings suggest that human activities that alter animal migrations can influence pathogen dynamics, with implications for wildlife conservation and future disease risks.
Protozoan infection prevalence data from migratory and non-migratory monarchs
These data show the number of infected (and total) monarchs found per site when citizen scientists or the authors tested monarch butterflies for the protozoan parasite Ophryocystis elektroscirrha (OE). We were interested in differences in infection prevalence for migratory vs. sedentary (non-migratory) monarchs. Sites were located in one of several source populations: the monarchs' summer-breeding range (migratory monarchs), Mexico overwintering areas (migratory monarchs), winter-breeding areas (non-migratory monarchs), or at coastal overwintering areas. One line of data shows results from one site for one year of the study (year 1 = 2011-2012 and year 2= 2012-2013). For each site and year, geographic coordinates, observer initials, city, state, number of total samples, and number of infected samples are listed. We calculated prevalence for sites with 8 or more total samples per year. Prevalence is the proportion of total samples that were heavily infected with OE. Prevalence was arc-sine-square-root-transformed. Each site was also assigned to a sub-region, as detailed in the Electronic Supplementary Material.
Prevalence data, Satterfield, Maerz and Altizer 2014 site abbrev.xlsx
Virulence for parasite isolates collected from migratory vs. non-migratory monarchs
These data show results of an experiment to compare the virulence of Ophryocystis elektroscirrha (OE) parasites from migratory vs. non-migratory monarch butterflies. In the laboratory, monarch larvae (2nd instars) were inoculated with 10 parasite spores from one of 57 parasite isolates (indicated by parasite ID) and then reared to adulthood. The source population of each wild-collected parasite isolate is listed. Details on parasite origins are available in the Electronic Supplementary Material. Experimental monarchs originated from one of five monarch lineages (or families), which were randomly assigned to treatments. We inoculated 10 larvae for each isolate and also reared 80 additional control larvae which were not inoculated. Larvae were given fresh, uncontaminated milkweed following inoculation. Key dates, including date of inoculation, monarch pupation, monarch eclosion and monarch death are listed. When monarchs were in the pupa stage, we weighed each pupa (pupal weight in grams) and also noted signs of OE disease (dark patches on the pupa), ranking the pupal infection on a scale from 0 to 5. A pupal score of 0 indicates no dark patches due to OE on the pupa, a score of 1 indicates minimal patching, 2 indicates about 25% of the pupa shows dark patches, 3 indicates about 50%, 4 indicates about 75%, and 5 indicates about 100% of the pupa showing darkening due to OE spores under the cuticle. We pupal-scored each monarch starting on day 6 of pupation and repeated this daily until eclosion. Monarchs with a maximum pupal score of 0 or 1 were tested for the parasite after eclosion using the tape method described in the manuscript -- in order to verify absence of infection. All monarchs were classified as either uninfected or infected (infected = having a pupal score of 2 or higher, or an adult infection tape-score of 4 or higher). All adult monarchs were kept at 12C and day of death was noted. Adult longevity (days until death) was calculated and used as a proxy of parasite virulence (shorter adult longevity indicates higher parasite virulence). After death, we quantified the parasite load on each infected monarch's abdomen, as described in the manuscript. Parasite load was log-transformed. Monarch sex is also indicated.
Virulence data, Satterfield, Maerz, and Altizer 2014.xlsx