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
Contact: Beverly Cox Clark
Emory University Health Sciences Center