California’s Monarch Butterfly Population Plummets to Near-Record Low

The Western monarch butterfly population in California has plummeted to a near-record low of less than 10,000 this winter.

The 28th annual Western Monarch Count — conducted by hundreds of partners and volunteers — reported a peak monarch population of 9,119 overwintering butterflies. It was the second lowest on record since the count started in 1997, according to a press release from nonprofit Xerces Society for Invertebrate Conservation.

The low number coincides with the proposed protection of monarchs under the U.S. Endangered Species Act by the Fish and Wildlife Service (FWS).

“The population’s size is extremely concerning,” said Emma Pelton, a Xerces Society endangered species biologist, in the press release. “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.”

The count represents a steep decline from the more than 200,000 overwintering Western monarchs observed during each of the past three years. The all-time low was fewer than 2,000 recorded in 2020.

Since the 1980s, the Western population of monarchs has declined by over 95 percent, reported the Los Angeles Times. Back then, it was estimated that as many as four million butterflies spent the winter in California, the state’s fish and wildlife department said.

Unless urgent conservation efforts are taken, FWS says Western monarchs face a 99 percent chance of becoming extinct by 2080.

Pelton said a number of factors threaten the species throughout their migratory range, including habitat loss, pesticides and increasingly extreme weather due to climate change.

The recent Los Angeles county wildfires burned tree groves that monarchs use as overwintering habitat, including one site in Lower Topanga Canyon.

Santa Cruz County hosted the largest overwintering clusters observed during the last count. The highest mid-season counts were found at Lighthouse Field State Park with 1,406 butterflies, Natural Bridges State Park with 1,400 monarchs and Moran Lake with 645, the press release said.

San Luis Obispo County’s Pismo Beach monarch Butterfly Grove had the fourth biggest count in late November with 556 Western monarchs, followed by the Skywest Golf Course in the Bay Area with 477 individuals.

A site owned by The Nature Conservancy in Santa Barbara that had top counts in the past — including 33,200 last winter — had only 198 monarchs this time around.

“The iconic monarch butterfly is cherished across North America, captivating children and adults throughout its fascinating lifecycle. Despite its fragility, it is remarkably resilient, like many things in nature when we just give them a chance,” said Martha Williams, former director of FWS, in a December statement, as the Los Angeles Times reported. “Science shows that the monarch needs that chance.”

Western monarchs are separated from their Eastern counterparts by the Rocky Mountains. Eastern monarchs spend their winters each year in central Mexico. Both geographically distinct monarch species make their migrations over multiple generations.

Western monarchs generally overwinter clustered in sheltered tree groves along the California Coast and northern Baja, Mexico.

“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, Western Monarch Count coordinator with the Xerces Society, in the press release. “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.”

When the proposed listing by FWS is finalized, it is expected to lead to better support for Western monarchs, including improved overwintering habitat protection in California and increased incentives for the restoration of breeding habitat.

An ongoing concern is the contamination of milkweed by pesticides. Research by University of Nevada-Reno and the Xerces Society in California’s Central Valley found that the leaves of the plant — the monarch caterpillars’ food source — were contaminated by 64 distinct pesticides.

One of the pesticides — methoxyfenozide — is likely highly toxic to monarch caterpillars and was found in 96 percent of tested milkweed samples.

Currently the only insect species taken into consideration by the U.S. Environmental Protection Agency (EPA)’s testing of pesticides is the adult honey bee, for which methoxyfenozide was classified as “practically non-toxic.”

The Xerces Society and Earthjustice in December formally petitioned the EPA to include native wildlife such as solitary bees, bumble bees, moths and butterflies in their assessment of pesticide risks to pollinators.

“We know pesticides are a key driver of monarch and other pollinator declines. Yet there are glaring gaps in the EPA’s oversight of pesticides: the vast majority of pesticides have never been tested for their impacts on butterflies,” said Rosemary Malfi, Xerces Society’s director of conservation policy, in a statement, as reported by the Los Angeles Times. “How can we protect these essential species if we’re missing the basic information needed to make better decisions?”