Monarch Butterfly Numbers Soar in California After Dramatic Losses

The orange-winged insect’s population increased from 2,000 in 2020 to nearly 250,000 in 2021

Monarchs cluster together on a tree
Monarch butterflies cluster together to stay warm. Yuval Helfman / 500px via Getty Images

Monarch butterfly populations have increased a hundredfold in overwintering sites in California after historically low numbers in 2020, per the Xerces Society for Invertebrate Conservation. Volunteers observed less than 2,000 monarchs in the state at the society’s annual Thanksgiving count in 2020. Last year, they counted more than 247,000. 

“We’re ecstatic with the results and hope this trend continues,” says Emma Pelton, the Western Monarch Lead with the Xerces Society, in a statement. “There are so many environmental factors at play across their range that there’s no single cause or definitive answer for this year’s uptick, but hopefully it means we still have time to protect this species.”

Though monarch numbers increased, they are far from the millions that California saw in the 1980s. In 2020, this represented a 99.9 percent decline, write Pelton and Stephanie McKnight on Xerces’ blog. Scientists think threats including habitat loss and pesticide use caused population numbers to plummet.  

The United States is home to two populations of monarch butterflies that are separated by the Rockies. The eastern population flies south to Mexico for the winter, while the western one overwinters in California. 

Pacific Grove, California, also called "Butterfly Town USA,” has celebrated the arrival of the monarchs every October since 1939 in its Butterfly Parade. In 2020, the town saw no monarchs in its two-acre sanctuary, one of California’s main overwintering sites, reports Erika Mahoney for KAZU News

A  monarch butterfly sits on a purple flower
Scientists think threats including habitat loss and pesticide use caused monarch butterfly populations to decline in the western U.S. Steven Katovich,

“I cried in my car because I've seen it coming, but I did not think I would not be able to find one monarch,” Connie Masotti, the Monterey County regional coordinator for the Thanksgiving count, told KAZU in January 2021. 

Late last year, the sanctuary counted thousands.  

“I don’t recall having such a bad year before and I thought they were done,” Moe Ammar, president of Pacific Grove Chamber of Commerce, told the Associated Press’s Haven Daley and Olga R. Rodriguez last November. “They were gone. They’re not going to ever come back and sure enough, this year, boom, they landed.”

In 2014, the Xerces Society and other groups petitioned the U.S. Fish & Wildlife Service to protect monarchs as a threatened species. The USFW determined in 2020 that “listing the monarch under the Endangered Species Act is warranted but precluded at this time by higher priority listing actions.” It will review the status of monarch butterflies each year until they are no longer a candidate. 

Scientists don’t know exactly why the monarch count increased last year, but some hypotheses include ideal weather conditions, fewer pesticides used during the Covid-19 pandemic, wildfires preparing the ground for wildflower growth, new additions from the eastern population and less competition, reports Alissa Greenberg for NOVA Next. It’s most likely a combination, experts tell NOVA.

Monarch butterflies cluster together on a plant.
Western monarch butterflies cluster in Pismo State Beach Monarch Butterfly Grove. Lisa Damerel

David James, an entomologist at Washington State University, tells staff at the Guardian that fewer monarchs counted in 2020 could have been because the butterflies spread out instead of clustering. 

“When we only had 2,000 overwintering at the traditional sites, at the same time there were many reports inland in San Francisco and the LA area of monarch butterflies reproducing in people’s backyards and parks and gardens throughout the winter,” he tells the Guardian

Though more monarchs overwintering this year is a cause for celebration, the Xerces Society warns that the numbers are still low. 

“It's crucial to remember that the modest uptick we're seeing is not population recovery or even evidence of an upward trajectory,” researchers write in a Xerces’ blog. “The population is still dangerously close to collapse, and there remains an urgent need to address the threats that this butterfly faces.”

Get the latest stories in your inbox every weekday.