The monarch butterfly may not be endangered, but research suggests its migration is

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 wintering populations decreasing while breeding populations are stable?

Published by Proceedings of the National Academy of Sciencesthe study suggests that monarchs die during their fall migration to southern Mexico.

Migratory monarchs don’t fly at night, so they spend their evenings in groups on trees or shrubs, called roosts. The study drew on 17 years of data from more than 2,600 citizen science observations of monarch roosts along the butterfly’s migration route.

The researchers found that roost sizes had decreased by up to 80%, with losses increasing from north to south along the migration route.

“Monarchs are increasingly failing to reach their winter destinations,” said Andy Davis, lead author of the study and an assistant research scientist at UGA’s Odum School of Ecology. “They either lose their ability to migrate or they lose their willingness to migrate.”

And it’s probably not because they lost their habitat or food supplies along the way. The study largely ruled out this possibility.

“How can we say that the monarch butterfly disappears in winter when it is perfectly healthy in summer?” said William Snyder, co-author of the study and professor of entomology in UGA’s College of Agricultural and Environmental Sciences. “This paper fills that gap by arguing that the problem lies in fall migration.”

What is leading to the decline in monarch migration?

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 observations of migratory roosts and their estimated sizes, as well as analyzes of landscape features and climate data.

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

The researchers found that the timing of migration did not change and, instead, the route became greener and warmer over time. Greener and warmer places should have led to larger roosting populations.

Instead, researchers have documented consistent and dramatic declines in roost sizes along the migration route, independent of climatic 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 United States. Partly because of warmer temperatures and increased planting of non-native tropical milkweed, milkweed is the only plant on which monarchs can lay their eggs and their caterpillars can feed: these year-round resident monarchs do don’t need to go all the way to Mexico.

But the present study shows that this is probably not what explains the decline along the migration route, because the decrease in roost size remains relatively constant throughout the route, even before migration reaches these regions.

Increase in parasites, captive breeding may be to blame

The researchers cite other research suggesting there are two main culprits for monarch migratory losses.

The first is the growing prevalence of a debilitating monarch parasite, which has increased tenfold since the early 2000s. This increase corresponds to an increase in non-native milkweed plantings along the flyway.

Many infected monarch caterpillars do not reach adulthood. Those that do cannot fly well and do not live long, perhaps contributing to butterfly spillovers along the migration path. Non-native milkweeds have longer growing seasons and, as a result, these plants can cause more monarchs to be infected with the parasite because the parasite’s infectious spores can accumulate on their leaves.

The second possible factor is the release of captive-bred butterflies by well-meaning people hoping to help monarchs.

“All the evidence we have shows that when monarchs are raised in a captive environment, whether it’s indoors or outdoors, they’re not as good at migrating,” Davis said.

How can people help “save the monarchs”?

Researchers are urging people to take a less-is-more approach when it comes to monarchs.

Although well-intentioned, many of the steps people take to try to help, such as planting non-native milkweed or breeding monarchs for release into captivity, may actually harm 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 can provide a source of nectar for traveling butterflies.

“One of the best things people can do to ensure monarchs are as hardy and healthy as possible is to simply leave the caterpillars alone in your yard,” Davis said. “Resist the temptation to bring them indoors and protect them, because it seems Mother Nature is doing a better job than we are at creating truly healthy and hardy migrants.”