University of Minnesota
Migrating helps monarch butterfly populations keep numbers of parasites low, a new study finds.
Keeping monarchs on the move
Why butterfly migrations may be crucial to their survival
By Deane Morrison
Like many human retirees, some monarch butterflies have taken up residence in Florida, where they need not migrate to find winter warmth.
Cushy as that life may seem, however, the Florida populations would probably enjoy better health if, like their northern cousins, they flew 1,500 miles south into central Mexico for the winter.
In a new study, University of Minnesota monarch researcher Karen Oberhauser and three colleagues showed that long migrations tend to rid butterfly populations of parasites. This implies that disruptions to migrations, whether by habitat loss, loss of nectar plants along the migration route, climate change, or other factors, can reduce the fitness of monarchs, other migrating insects, and even vertebrates.
"There's a lot of concern now that human activities are affecting animal migrations," says Oberhauser, an associate professor of fisheries, wildlife and conservation biology. "The take-home message here is that the migration is probably very important to the biology of the butterflies."
The work is published in the journal Ecology.
The researchers studied monarchs in the eastern United States and Canada to see how parasite loads varied with location and migratory status. The data came from hundreds of "citizen scientists" who collected parasites from butterflies and recorded data on the abundance of monarch eggs and larvae in 100–200 locations each year from 2006 to 2009.
The parasite in question spreads by spores living on the abdomens of adults. Spores can pass from adults to progeny when females lay eggs on the leaves of milkweed. Volunteers removed spores from adult abdomens with tape, causing no injury to the insects. Researchers then counted the spores under a microscope.
The researchers found that the proportion of monarchs carrying heavy parasite loads grew steadily as the breeding season progressed, partly due to rising densities of larvae, which seem to aid spore transmission. And in two out of three years examined, parasite prevalence dropped between its peak in the final summer breeding phase and the butterflies' wintering period in Mexico.
Also, as the monarchs migrated south, the proportion of insects carrying heavy parasite loads dropped significantly.
"We've shown that migration provides an opportunity to escape from habitats that might build up levels of diseases, and also to cull out susceptible individuals," says Oberhauser.
Thus, migration leaves populations smaller but healthier.
A global phenomenon
According to other studies, migratory insects account for more moving biomass than any group of migratory mammals or birds, including wildebeest and caribou, the researchers say. They cite studies showing that human interference, intentional or otherwise, with migration can increase parasite prevalence. For example, human interference with Pacific salmon migration led to young fish suffering higher infestations of sea lice.
Understanding how long-distance migrations affect the balance between hosts and pathogens is critical to predicting wildlife health. Threats such as deforestation of overwintering grounds, loss of habitat across the breeding range, and climate change all threaten monarchs, according to previous work by Oberhauser and others. And warming could lead more insects to stop migrating.
"If the large eastern migratory population declines and [populations of] year-round breeding monarchs expand, this could lead to greater disease prevalence and reductions in overall population health," the researchers warn.
Oberhauser's colleagues were first author Rebecca A. Bartel of the University of Georgia and Emory University; Jacobus C. de Roode of Emory; and Sonia M. Altizer of the University of Georgia.
Published in 2011