In the migratory marathon, parasitized monarchs drop out early

A little-studied outcome of animal migration is whether these long journeys can limit the spread of parasites by weeding out diseased animals. Monarch butterflies in eastern North America fly up to thousands of kilometers from Canada to Central Mexico – one of the longest migrations of any insect species.

Emory University researchers found that monarchs infected with a protozoan parasite fly slower, tire out faster and expend more energy flying than healthy monarchs. These results, published in the March issue of Ecology Letters, could explain why parasite infections are much lower in migratory monarch populations compared to year-round residents that do not migrate.

Habitat loss at overwintering sites and climate warming trends, combined with an increase of tropical milkweed species in milder climates, could ultimately replace the large migratory populations with smaller remnants that breed year-round and do not migrate. Study results suggest that if migration is lost from this system, remaining populations will become heavily parasitized.

More generally, this study demonstrated that seemingly small effects of parasites on their hosts could have a much larger impact when combined with the stresses of long-distance migration.