Seasonal Migrations for Monarchs Keep the Doctor Away

Because North American monarch butterflies (Danaus plexippus) can’t survive cold winters in the United States, they often migrate south in the fall to places such as Mexico. Now, a new study published in the journal PLOS ONE shows that these long-distance migrations may help lower the amount of infections in the butterflies.

A research team led by Sonia Altizer, an ecology professor at the University of Georgia, examined wild monarch butterflies in search of the parasite Ophryocystis elektroscirrha, a protozoan parasite that commonly infects monarchs. After collecting the butterflies from two wintering cites in central Mexico, they determined how many were infected with the protozoan parasite and compared the infection status of each butterfly with their hydrogen isotope measurements, which indicate the latitude of the butterflies’ origin when they began their migration.

“The chemical markers allowed us to estimate where the monarchs started and how far they travelled to reach the wintering sites in Mexico, something that would not be possible using other currently available methods,” Altizer said in a press release.

After analyzing their data, the team discovered that uninfected monarchs had lower hydrogen isotope values than butterflies that had been infected with the parasite. This lower hydrogen isotope value suggested that the uninfected butterflies traveled farther, from more northern latitudes in order to reach their wintering sites in Mexico.

The team concluded that these fall migrations may be helpful in lowering infection levels in North American monarchs. The authors suggest that recent observations of monarchs being sedentary may mean more risk of infection in the species.

Header Image: A monarch butterfly on a plant. Image Credit: Jim, the Photographer