The monarch butterfly may not be endangered, but its migration is, researchers find

With vigorous debate surrounding the health of the monarch butterfly, new research from the University of Georgia may have answered the biggest question plaguing butterfly researchers. Why are the wintering populations declining while breeding populations are stable?

Published by the Proceedings of the National Academy of Sciences, the study suggests that monarchs are dying off during their fall migration south to Mexico.

Migrating monarchs don't fly at night, so they spend their evenings in bunches on trees or shrubs, known as roosts. The study relied on 17 years of data from more than 2,600 citizen scientist observations of monarch roosts along the butterfly's migration route.

The researchers found that roost sizes have declined by as much as 80%, with these losses increasing from north to south along the migration route.

"The monarchs are increasingly failing to reach their winter destinations," said Andy Davis, lead author of the study and an assistant research scientist in UGA's Odum School of Ecology. "Either they're losing their ability to migrate or they're losing their will to migrate."

And it's likely not because they've lost habitat or food supplies along the way. The study largely ruled out this possibility.

"How do you say that the monarch butterfly is going extinct in the winter while they're perfectly healthy in the summer?" said William Snyder, co-author of the study and a professor of entomology in UGA's College of Agricultural and Environmental Sciences. "This paper fills in that gap by saying the problem is that fall migration."

What's leading to the monarchs' migration decline?

The researchers relied on data from Journey North, an organization that uses citizen sightings of various animals to track wildlife migration patterns. The study covers 17 years of citizen-reported sightings of migratory roosts and their estimated sizes, along with analyses of landscape characteristics and climate data.

The study represents the most detailed and comprehensive picture to date of the health of the monarch fall migration in eastern North America.

The researchers found that the timing of the migration hasn't changed and, if anything, the route has become greener and warmer over time. Greener, warmer locations should have led to larger roost populations.

Instead, the researchers documented steady, dramatic declines in roost sizes over the migration route that were independent of climate and landscape factors.

Some scientists suggest that climate change is allowing monarchs to forgo their migration and spend their winters in warmer parts of the U.S. Due in part to warmer temperatures and increased planting of nonnative tropical milkweed -- milkweed is the only plant on which monarchs can lay their eggs and their caterpillars can feed -- these year-round resident monarchs don't need to travel all the way to Mexico.

But the present study shows that's likely not what's driving the decline along the migration route, as the drop in roost sizes remains relatively consistent along the whole path, even before the migration reaches these regions.

Increase in parasite, captive breeding may be to blame

The researchers point to other research that suggests there are two main culprits behind the monarch migration losses.

The first is the increasing prevalence of a debilitating monarch parasite, which has increased tenfold since the early 2000s. This increase corresponds with increased plantings of nonnative milkweeds throughout the flyway.

Many infected monarch caterpillars don't make it to adulthood. Those that do can't fly well and don't live long, possibly contributing to the fallout of butterflies along the migration path. Nonnative milkweeds have longer growing seasons, and as a result these plants may lead to more monarchs becoming infected with the parasite because the infectious parasite spores can build up on their leaves.

The second possible driver is the release of captive-reared butterflies by well-meaning people who are hoping to help the monarchs.

"All of the evidence we have shows that when monarchs are reared in a captive environment, either indoors or outdoors, they're not as good at migrating," Davis said.

How can people help 'save the monarchs'?

The researchers urge people to take a less is more approach when it comes to monarchs.

Although well intended, many of the things people do to try to help, such as planting nonnative milkweeds or raising monarchs for release in captivity, may actually be harming the monarch population by interfering with their ability to migrate long distances.

Planting flowering plants -- but not tropical milkweed -- along the migration path won't hurt and may provide a source of nectar for traveling butterflies.

"One of the best things people can do to ensure that the monarchs are as robust and healthy as possible is basically just leave the caterpillars alone in your backyard," Davis said. "Resist the temptation to bring them inside and protect them because it seems like Mother Nature does a better job at creating really healthy, robust migrators than we do."

Published in PNAS, the study was co-authored by Jordan Croy, a postdoctoral associate in UGA's College of Agricultural and Environmental Sciences.