Monarch Butterflies Wintering in California Are Down 30 Percent From Last Year

The number of western monarchs spending the winter along California’s coast dropped by 30 percent this year, according to a statement from the nonprofit Xerces Society for Invertebrate Conservation.

Each year, scientists and volunteers tally the iconic black-and-orange butterflies that hunker down at their temperate overwintering sites in November and early December, when their numbers usually peak.

In 2023, teams counted 233,394 western monarchs at 256 sites between November 11 and December 3, which is down from 335,479 butterflies during the same period of 2022.

The atmospheric rivers and other severe storms that pummeled California in December 2022 and January 2023 may be to blame for the year-to-year drop, experts say.

“Last year’s winter storms meant we entered the spring breeding season with fewer butterflies and saw lower numbers this summer, so it is not surprising that the overwintering population is down,” says Emma Pelton, a conservation biologist with the Xerces Society, which organizes the annual count, in the statement.

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The western monarch population typically fluctuates, but zooming out, their current totals are just a fraction of what they were in the 1980s and 1990s, when the insects regularly numbered in the low millions. Overall, western monarch populations have declined by more than 95 percent over the last four decades because of habitat fragmentation, pesticide and herbicide use, a decline in native milkweed and nectar plants, human-caused climate change and other factors.

In 2020, the overwintering population dipped to just 2,000 butterflies—a record low. And while numbers have rebounded since then, conservationists warn that the butterflies are still in crisis.

“We’ve bounced back [from historical lows], but we’re still not all the way back,” says Pelton to the San Luis Obispo Tribune’s Mackenzie Schuman. “We’re still bouncing around on really low numbers.”

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Western monarchs are a geographically distinct subpopulation that lives west of the Rocky Mountains. (Their counterparts, eastern monarchs, live east of the Rockies.) Each fall, western monarchs migrate from inland locales in the western U.S. and Canada toward the Pacific coast, where they wait out the colder months in California and northern Baja California, Mexico.

The butterflies usually begin arriving at overwintering sites in October and stay as late as March. They seek out specific microclimates with high humidity, dappled sunlight, mild temperatures, little wind and close proximity to water and nectar—often, they’re found clustered in groves of Monterey pine and cypress trees, as well as non-native eucalyptus trees.

The congregation of these striking black-and-orange butterflies is a stunning natural spectacle. Travelers can easily observe the butterflies by visiting their overwintering sites, many of which are now linked through a new conservation initiative called the Western Monarch Trail. One of the sites along the trail, the Pismo State Beach Monarch Butterfly Grove, hosted 16,038 butterflies during the annual count in 2023, according to the Xerces Society.

Educational signs at sites along the trail encourage visitors to help save the butterflies by planting native milkweed and nectar plants and reducing the use of insecticides and herbicides.

Worldwide, the International Union for Conservation of Nature lists migratory monarchs as “vulnerable.” In the United States, meanwhile, the Fish and Wildlife Service has listed monarchs as a “candidate species” for inclusion on the list of endangered or threatened wildlife.

Conservationists are optimistic the colorful insects will receive federal protection soon.

“There is still very little meaningful protection for migratory monarch butterflies and their overwintering habitat,” says Pelton in the statement. “Hopefully, the U.S. Fish and Wildlife Service’s upcoming listing decision under the federal Endangered Species Act will provide protection to the important places that monarchs rely on each winter.”

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.