Western Monarch Butterfly Population Declines to Near Record Low

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San Francisco, Calif. Jan. 30, 2025— Following efforts by hundreds of volunteers and partners to count overwintering monarch butterflies in California, the 28th annual Western Monarch Count has reported a peak population of just 9,119 butterflies this winter. This is the second lowest overwintering population ever recorded since tracking began in 1997 and coincides with monarch butterflies being proposed for protection under the U.S. Endangered Species Act. This number is a sharp decline from the past three years, when more than 200,000 overwintering western monarchs were observed each year. It is only slightly above the all-time low of less than 2,000 monarchs in 2020, and well below the millions of butterflies observed in the 1980s that scientists consider a stable population level. “The population’s size is extremely concerning,” said Emma Pelton, an endangered species biologist with the Xerces Society for Invertebrate Conservation. “We know small populations are especially vulnerable to environmental fluctuations, and we think that’s what happened this year. The record high late summer temperatures and drought in the West likely contributed to the significant drop-off we saw in the third and fourth breeding generations.” Pelton says monarchs encounter a variety of threats across their migratory range, including pesticides, habitat loss, and increasingly severe weather exacerbated by climate change. January’s fires in Los Angeles County burned tree groves where monarchs overwinter, including a site in Lower Topanga Canyon.

Santa Cruz hosts largest number of butterflies

The largest overwintering clusters this year were observed in Santa Cruz County, with top mid-season counts at Lighthouse Field State Park (1,406 monarchs), Natural Bridges State Park (1,400 monarchs) and Moran Lake (645 monarchs). The Pismo Beach Monarch Butterfly Grove in San Luis Obispo County had the fourth largest count with 556 monarchs observed in late November, and the Bay Area’s Skywest Golf Course took fifth with 477 monarchs. Other notable sites include the Pacific Grove Butterfly Sanctuary in Monterey County with 228 monarchs and the UC Gill Tract Community Farm with 196 monarchs in Alameda, one of few sites to host more monarchs this year compared to past seasons. Monarch clusters were notably absent from the Goleta Monarch Butterfly Grove at Ellwood Mesa, with only three individual monarchs observed across multiple sites within the park. A privately-owned site in Santa Barbara owned by The Nature Conservancy that hosted top counts in previous years, with 33,200 monarchs last winter, had only 198 monarchs. The western monarch population is geographically distinct from eastern monarchs that overwinter in central Mexico each year, separated by the Rocky Mountains. Like their eastern counterparts, the western monarch migration takes place over multiple generations of butterflies. The western population’s final generation primarily overwinters in clusters sheltered in tree groves along coastal California and northern Baja, Mexico.

Federal policy changes could help struggling monarchs

Proposed changes in federal policy could bring much-needed relief to the struggling monarchs. “A lot of people care about monarchs. Voluntary efforts like pollinator gardens and restoring habitat are probably a reason they aren’t in worse shape,” said Isis Howard, coordinator of the Western Monarch Count with the Xerces Society. “However, these actions are not enough. To help monarchs recover, we need to work at a larger scale and address widespread issues like pesticide contamination and climate change that are beyond what voluntary efforts have been able to achieve.” Some of these changes may be underway. In December, the U.S. Fish and Wildlife Service proposed to list the monarch butterfly as a threatened species under the U.S. Endangered Species Act. When finalized, the decision is expected to provide vital support for the species including improved protection for overwintering habitat in California, greater focus by federal agencies on the species, and more incentives for breeding habitat restoration. A public comment period on the listing is open until March 12, and Pelton says the Xerces Society is asking individuals to sign on to support the listing. Less certain is whether federal pesticide regulations will see meaningful changes under the new administration. Research in California’s Central Valley by the Xerces Society and University of Nevada-Reno found that milkweed leaves – the food source for monarch caterpillars – were contaminated by 64 different pesticides. One of these, called methoxyfenozide, is likely to be highly toxic to caterpillars and was in 96% of milkweed samples, but it is classified as “practically non-toxic” to adult honey bees – the only insect species currently included in testing by the Environmental Protection Agency before a new pesticide is approved for market. In December, Earthjustice and the Xerces Society formally petitioned the EPA to close critical gaps in how it assesses pesticide risks to pollinators and include important native wildlife like bumble bees, solitary bees, butterflies and moths.