Butterfly populations in the United States have declined by more than a fifth over the past two decades, according to a new study.

Research conducted by Binghamton University in New York found that butterfly numbers fell by 22% between 2000 and 2020. Alarmingly, a third of the species studied experienced significant declines, with some, such as Julia’s Skipper, losing over 90% of their populations.

Despite these troubling figures, researchers believe butterfly populations could recover if urgent conservation actions are taken.

The study, published in the journal Science, measured butterfly “abundance”—the number of individuals within a particular area. Researchers analysed 12.6 million butterfly sightings from 76,000 surveys across 35 monitoring programs, including data from citizen science initiatives like the North American Butterfly Association’s Fourth of July counts.

Using statistical models, the researchers estimated population trends for 342 species. The findings revealed that 33% of these species are in significant decline, with many experiencing severe losses. Specifically, 107 species declined by more than 50%.

“While the results aligned with global trends, seeing the extent of the decline at such a large spatial scale was sobering,” said Prof Eliza Grames, an assistant professor of biological sciences at Binghamton University.

Some of the most affected butterflies include the Florida white, Hermes copper, tailed orange, Mitchell’s satyr, and West Virginia white—all of which have lost over 98% of their populations in the US.

Even adaptable species like the West Coast lady have declined by 80%, raising concerns about the broader environmental impact.

“That’s alarming because it suggests even common butterflies aren’t safe,” Prof Grames said.

The primary causes of these declines are habitat loss, pesticide use, and climate change. In particular, the US Southwest—a region known for its hot and dry climate—has seen the most severe population drops, with drought cited as a major contributing factor.

“Drought is a double threat – it harms butterflies directly and also affects their food and host plants,” Prof Grames explained.

Butterflies play a critical role as pollinators, supporting plant and crop growth. Their decline could disrupt food production and broader ecosystems. They are also considered key indicators of environmental health; falling butterfly numbers often signal wider ecological issues.

The study’s findings may drive crucial conservation initiatives, such as identifying species for inclusion on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species and securing protection under the Endangered Species Act.

Although the decline is serious, researchers remain hopeful that targeted conservation efforts can help reverse these trends.

“Butterflies can recover quickly because they have short generation times. Small actions like planting wildflowers, reducing pesticide use, or even leaving part of a backyard unmowed can significantly improve their chances,” Prof Grames said.