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Western Monarch Butterflies Continue to Decline

Feb 02, 2018


  • Population Trends

Read the full press release by the Xerces Society for Invertebrate Conservation.

PORTLAND, Ore.; February 2, 2018—An annual census of monarch butterflies overwintering along California’s coast reveals that populations in western North America are at their lowest point in five years, despite recovery efforts. Volunteers with the Xerces Society’s Western Monarch Thanksgiving Count visited more sites this past year than have ever been counted since the survey began in 1997, yet they tallied fewer than 200,000 monarchs.

“This year’s numbers indicate a continuing decline in the monarch population,” noted Sarina Jepsen, the Xerces Society’s endangered species program director. “Two decades ago, more than 1.2 million monarchs were recorded from far fewer coastal sites, and just last year nearly 300,000 monarchs were observed at almost the same number of sites.”

Population estimates at individual sites also suggest that the western monarch population has continued to shrink. Of the 15 sites which have been monitored annually for more than two decades, 11 had lower counts than last year.

“Counts at some of the state’s largest sites were dramatically lower,” said Emma Pelton, conservation biologist with the Xerces Society. “Pismo Beach State Park was down by 38%, a private site in Big Sur was down by 50%, and the Monarch Butterfly Sanctuary in Pacific Grove was down 57%, from 17,100 to just 7,350 butterflies.”

The few sites in which monarch numbers remained stable or increased compared to 2016, include Natural Bridges State Park, Moran Lake, and Lighthouse Field State Park, all in Santa Cruz County.

Monarch butterflies that spend the winter within forested groves along California’s central coast are born on milkweed throughout western states, traveling to the coast from as far away as Arizona, Utah, Idaho, and Washington. The western population of monarchs has undergone a long term decline that mirrors the trend observed at overwintering sites in the mountains of central Mexico, where monarchs from both the eastern and western U.S. also spend the winter.

In the late 1990s, a small group of monarch enthusiasts in California became concerned that there appeared to be fewer and fewer monarchs each year and started the Western Monarch Thanksgiving Count (WMTC). The WMTC is the longest running, most comprehensive effort to monitor overwintering monarchs in California. The count occurs during a three-week period centered on Thanksgiving. Biologists, land managers, and citizen scientists visit overwintering sites year after year to monitor the butterflies.

Growing awareness and willingness to volunteer to aid in monarch conservation, coupled with added Western Monarch Thanksgiving Count trainings for new volunteers, led to a successful monarch monitoring season. Nearly 150 volunteers covered a record 262 sites this year. Many participants also took part in the second annual New Year’s Count, the results of which are still being tallied and will be reported in late-February.

This fall and winter have been challenging seasons for the residents of California, with unseasonably warm temperatures, wildfires, smoke, and mudslides; we do not yet know the impacts of these events on late season migrating and breeding monarchs.

The Xerces Society received numerous reports of late season breeding, and fewer clusters this fall, suggesting that some monarchs may have arrived at overwintering sites later and stayed active longer. The low estimate of monarchs this year may be attributable, in part, to later than average clustering. Regardless, monarch butterflies are at risk of extinction and action is needed across their range to bring them back.

The count is made possible by returning volunteers, new recruits, and dedicated regional coordinators including Mia Monroe who co-coordinates the count with the Xerces Society, Bill Shepard, Christina Garcia, Martha Nitzberg, Nick Stong, Jessica Griffiths, Charis van der Heide, Rick Hansen, Saul Riatiga, and Rachel Williams. Financial support for the 2017 WMTC came from the U.S. Fish and Wildlife Service, the Hind Foundation, and the San Diego Zoo.


Graph of Western Monarch Thanksgiving Count data from 1997 to 2017.

Citation for graph & data: Xerces Society Western Monarch Thanksgiving Count. 2018. Western Monarch Thanksgiving Count Data, 1997–2017. Available at: (For a full list of contributors, see: Click Here for larger version.


For More Information

The full press release by the Xerces Society is available here.

The data from and information about the Western Monarch Thanksgiving Count is at

Read more about Xerces’ Monarch Conservation Campaign, including efforts to conserve overwintering sites in California and restore breeding habitat in key regions of the United States at

To report non-overwintering monarch and milkweed sightings in the West, visit:


The Monarch Joint Venture is a national partnership of federal and state agencies, non-governmental organizations, and academic programs working together to conserve the monarch butterfly migration. The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. Header photo taken by Karen Oberhauser in California in 2002.