Female monarchs in the eastern part of North America have declined in number over the last three decades, according to recent research.

the female to male ratio for the butterflies east of the Rockies has gradually been changing. In the late 1970s, Davis said, females made up around 53 percent of the Monarch Butterfly population that migrated to Mexico for the winter. Today, that number has dropped to about 43 percent, he said, 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, which is now online at rsbl.royalsocietypublishing.org.

“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 over their lifetime, which is why the species is so resilient.”

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