Campus News

Female monarch butterflies becoming scarce in U.S.

Female monarch butterflies in eastern North America have significantly declined throughout the past 30 years, a new UGA study reveals.

Andy Davis, a Ph.D. candidate in the Warnell School of Forestry and Natural Resources, analyzed published overwintering and migratory data for the insect from 1976 to the present, discovering that the female-to-male ratio for the butterflies east of the Rockies has gradually been changing.

In the late 1970s, females made up about 53 percent of the monarch butterfly population that migrated to Mexico for winter. Today, that number has dropped to about 43 percent, which paints a dire picture for population recruitment. Davis outlines his findings in a new paper co-authored with Eduardo Rendón-Salinas of World Wildlife Fund-Mexico. The paper appears in Biology Letters, available online at

“I nearly fell over when I saw the trend,” said Davis. “It was an unintentional but extremely important finding.”

The monarch butterfly, one of the most well-known and widely recognized insects in the world, is a flagship species for conservation. North American monarchs can migrate more than
2,000 miles as they fly to Mexico from Canada and the U.S. for the winter.

“The implications of this decline are huge,” Davis said. “Female monarchs can lay as many as 400 eggs throughout their lifetime, which is why the species is so resilient.”

But Davis said that as the monarch population struggles because of breeding habitat loss, widespread pesticide use and deforestation of the overwintering sites, losing a significant number of females could hinder the population’s ability to rebound after crashes.

He said that news of the decline has gone unnoticed until now “because no one’s ever looked at the data like this. For years, scientists have been collecting male and female monarchs at the overwintering sites and during the fall migration. When we compiled the numbers from these collections, along with the year they were made, the trend was obvious.”