With intense discussions about the status of the monarch butterfly, recent findings from the University of Georgia may have uncovered a key issue troubling researchers. They are trying to understand why winter populations are decreasing while breeding groups remain stable.
Amidst lively discussions about the health of the monarch butterfly, new findings from the University of Georgia may have revealed the core issue that has been challenging butterfly researchers. Why do we see a drop in winter populations even though breeding populations appear stable?
According to a study published in the Proceedings of the National Academy of Sciences, it appears that monarch butterflies are perishing during their autumn migration to Mexico.
While migrating, monarchs do not fly at night; instead, they gather in groups on trees or shrubs for rest, a behavior known as roosting. The research utilized 17 years of data collected from more than 2,600 observations reported by citizen scientists along the butterflies’ migration path.
The findings reveal that the size of these roosts has decreased by up to 80%, with a noticeable decline as one moves from the northern to southern parts of the migration route.
“The monarchs are struggling to reach their wintering spots,” explained Andy Davis, the study’s lead author and an assistant research scientist at UGA’s Odum School of Ecology. “They might be losing their ability or their motivation to migrate.”
It’s essential to note that this decline does not seem to stem from a lack of habitat or food along their journey; the study largely dismissed this concern.
“How can you claim that the monarch butterfly is on the brink of extinction in the winter while remaining healthy during the summer?” queried William Snyder, a co-author and professor of entomology at UGA’s College of Agricultural and Environmental Sciences. “This study clarifies that the issue lies within the fall migration.”
What factors are causing the decline in migratory populations?
The researchers utilized data from Journey North, an organization focused on tracking animal migration through citizen sightings. The work presents 17 years of reported observations regarding migratory roosts and their sizes, in addition to assessments of climate conditions and landscape features.
This research provides the most thorough and insightful representation of the state of monarch fall migrations in eastern North America to date.
Interestingly, the timing of the migration hasn’t changed, and if anything, the route has become greener and warmer over the years. These more favorable conditions should ideally lead to larger roost populations.
However, the study documented consistent and dramatic reductions in roost sizes along the migration route, which are independent of climate and landscape variables.
Some experts propose that climate change could be leading some monarchs to skip their migration, opting instead to spend the winter in warmer regions of the U.S. This trend, amplified by warmer climates and the increased planting of nonnative tropical milkweed—essential for monarch reproduction—allows these monarchs to remain within the U.S. rather than travel to Mexico.
Yet, the current study indicates that this is possibly not the reason for the observed decline along the migration path, as the diminishing roost sizes appear consistent all along the route before reaching those areas.
Possible causes: parasite increase and captive breeding intervention
The authors point to previous studies that indicate two primary factors contributing to the decrease in migratory populations of monarchs.
The first is the rising occurrence of a fatal parasite impacting monarchs, which has seen a tenfold surge since the early 2000s, correlating with an increase in nonnative milkweed planting throughout the migration corridor.
A significant number of afflicted monarch caterpillars do not survive to adulthood. Those that do typically struggle with flight and have shorter lifespans, potentially contributing further to the drop in populations along their migratory route. The longer growing seasons of nonnative milkweeds may result in more caterpillars becoming exposed to the infection due to the accumulation of parasite spores on the leaves.
The second factor could be the release of captive-bred butterflies by well-intentioned individuals trying to support the monarch population.
“All our evidence indicates that monarchs raised in captivity, whether indoors or outdoors, do not perform as well during migration,” Davis explained.
How can individuals assist in ‘saving the monarchs’?
The researchers suggest a “less is more” strategy regarding interventions for monarchs.
Even with good intentions, various actions that individuals take to help—like planting nonnative milkweeds or breeding monarchs for release—could inadvertently harm the population by disrupting their capability for long-distance migration.
Planting flowering plants (not tropical milkweed) along the migration route can be beneficial, as it provides nectar sources for the migrating butterflies.
“One of the best ways for the public to support healthy monarch populations is simply to leave the caterpillars alone in your yard,” Davis remarked. “Resist the urge to bring them inside for protection since it seems nature is more adept at producing strong migrators than we are.”
This study, published in PNAS, was co-authored by Jordan Croy, a postdoctoral researcher at UGA’s College of Agricultural and Environmental Sciences.
