Monarch Butterflies: Migration Challenges Revealed

Recent research from the University of Georgia addresses a crucial question about the monarch butterfly: why are winter populations declining while breeding populations remain stable? The study, published in the Proceedings of the National Academy of Sciences, suggests that many monarchs are dying during their fall migration to Mexico.

Monarchs roost in groups on trees or shrubs at night. This study analyzed 17 years of data from over 2,600 observations by citizen scientists along their migratory route. The findings revealed that roost sizes have dropped by as much as 80%, with losses becoming more pronounced from north to south along the migration path.

Lead author Andy Davis, an assistant research scientist at UGA's Odum School of Ecology, noted that monarchs are increasingly struggling to reach their winter destinations. The research ruled out habitat loss or food scarcity as primary causes of this decline. Co-author William Snyder emphasized that the study clarifies the issue lies in the fall migration.

The researchers relied on data from Journey North, which tracks wildlife migration through citizen sightings. Their comprehensive analysis highlights that while the timing of migration has not changed, the size of roosts has declined consistently, independent of climate or landscape factors.

Some scientists speculate that climate change might enable monarchs to skip migration and winter in warmer U.S. areas. However, the study suggests this is unlikely, as declines in roost sizes occur uniformly along the entire route.

Two key factors may be contributing to the migration decline: the rising prevalence of a debilitating monarch parasite and the release of captive-bred butterflies. The parasite has increased tenfold since the early 2000s, which corresponds with more nonnative milkweeds being planted along the migration path.

To help monarchs, the researchers recommend a cautious approach. While good intentions often lead to actions like planting nonnative milkweeds, these can harm populations by disrupting migration. Planting native flowering plants along the migration route can provide essential nectar without negative effects. Davis encourages people to allow caterpillars to thrive in their backyards, as nature is often better at producing healthy migrators than human efforts.