Emory study finds monarch health tied to migration

Monarch butterflies in eastern North America have one of the longest migrations of any species, with a survival-of-the-fittest trek that can take them thousands of miles from Canada to Central Mexico. A new Emory University study has found that these journeys may be the key to maintaining healthy monarch populations at a time when habitat loss and other environmental issues could curb the ability of the butterflies to make the trip.

Emory researchers discovered that monarch butterflies infected with a protozoan parasite flew slower, tired faster and had to expend more energy flying than healthy monarchs. These results, published in the March issue of Ecology Letters, may explain why parasite burdens are much lower in migratory monarch populations compared to year-round residents — an effect that possibly occurs in other migratory species as well, explains Sonia Altizer, lead researcher of the study and an assistant professor of environmental studies at Emory.

“We know that several species of birds, insects and other animals undergo two-way migrations of several thousand miles or longer. These journeys can be thought of as animals essentially running a marathon every fall and spring. So if animals are infected with parasites, this would be like a distance runner trying to run a marathon with the flu. In this case, parasitized animals will drop out of the race, and across the whole population, prevalence of disease will decline,” Altizer says.

However, monarch migration in eastern North America is threatened by several environment factors such as habitat loss at wintering sites, climate warming trends and an increase of tropical milkweed species in milder climates. These dynamics could ultimately cause large migratory populations to be replaced with smaller remnants that stay put and breed year-round, she says.

“The results of our study add one more reason to protect monarch migration east of the Rockies. If this migration collapses due to climate warming, habitat loss, pesticide use or other reasons, we probably won’t lose monarchs as a species, but we’d be left with remnant, nonmigratory populations that are heavily infected with parasites, which could have several negative effects, from higher mortality rates, smaller body sizes and deformities, to more virulent strains of the parasite,” Altizer says.

Altizer’s experiments, conducted with graduate student Catherine Bradley, showed that parasitized monarch butterflies had 10-20 percent lower flight ability while on the “butterfly treadmill,” and that the parasite affected their ability to fly long distances. Although the infected monarchs looked the same, weighed the same and had nearly identical survival rates to adulthood when they were not migrating, the flight trials in the laboratory essentially “unmasked” an effect of the parasite that normally would have been hidden. “The experiment demonstrates that seemingly small effects of parasites on their hosts can have a larger impact when combined with the stresses of migration,” Altizer says.