Why are monarch butterflies vanishing in the winter?

In the midst of vigorous debate regarding the health of monarch butterflies, new research from the University of Georgia sheds light on a critical issue: while breeding populations remain stable, wintering populations are declining. 

Published in the Proceedings of the National Academy of Sciences, the study suggests that monarch butterflies are experiencing significant losses during their fall migration south to Mexico.

Declining roost sizes along migration route

Migrating monarchs rely on roosts – clusters on trees or shrubs where they rest overnight – as they journey southward. 

The researchers analyzed 17 years of data from over 2,600 citizen scientist observations of these roosts along the migration path. They discovered that roost sizes have declined by up to 80%, with the most substantial losses occurring further south along the 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 University of Georgia’s Odum School of Ecology. “Either they’re losing their ability to migrate or they’re losing their will to migrate.”

Monarchs going extinct in winter 

Contrary to what some might expect, the study largely ruled out habitat loss or food supply shortages along the migration route as primary causes. 

Study co-author William Snyder is a professor of entomology in UGA’s College of Agricultural and Environmental Sciences. 

“How do you say that the monarch butterfly is going extinct in the winter while they’re perfectly healthy in the summer?” said Snyder. “This paper fills in that gap by saying the problem is the fall migration.”

Monarch health during fall migration

Using data from Journey North, an organization that tracks wildlife migration patterns through citizen sightings, the researchers provided the most detailed picture to date of the monarchs’ fall migration health in eastern North America. 

The experts found that the timing of migration has not changed, and the route has become greener and warmer over time – conditions that should favor larger roost populations. Despite this, there is a consistent and dramatic decline in roost sizes independent of climate and landscape factors.

Potential factors contributing to migration losses

The researchers point to two main culprits behind the decline in migration success. The first is the increasing prevalence of a debilitating parasite, which has surged tenfold since the early 2000s. 

This rise corresponds with increased plantings of nonnative tropical milkweeds – the only plants on which monarchs can lay their eggs and their caterpillars can feed. 

Nonnative milkweeds may facilitate the buildup of infectious parasite spores on their leaves, leading to higher infection rates. Infected monarchs often cannot fly well and have shorter lifespans, contributing to the decrease in migrating butterflies.

The second possible factor is the release of captive-reared butterflies by well-intentioned individuals aiming to help the species. 

“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 explained.

Supporting monarch migration 

The researchers advise adopting a “less is more” approach when it comes to assisting monarch butterflies. While actions like planting nonnative milkweeds or raising monarchs in captivity are meant to help, they may actually hinder the butterflies’ ability to migrate long distances. 

Planting native flowering plants along the migration path can provide nectar sources for traveling butterflies without causing harm.

“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 suggested. 

“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.”

According to the researchers, conservation efforts that support fall migration success are most needed, such as providing high quality nectar plants along the migration route or limiting the planting of nonnative milkweeds that enhance monarch parasite loads. 

“Overall, it appears the fall migration of monarch butterflies is under imminent threat, even if the species’ overall survival is not,” the authors concluded.

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