Western Monarch Butterfly Overwintering Population Falls 96 Percent, Second-Lowest Number On Record

“The population’s size is extremely concerning,” Emma Pelton – an endangered species biologist with the Xerces Society for Invertebrate Conservation, which manages the initiative – said in a statement.

The migration of monarch butterflies, famous for being a visual treat, involves multiple generations of the winged insects. For the butterflies west of the Rocky Mountains, the population starts to spread over western states in spring. The last generation returns to spend the winter along the Pacific coast from Mendocino County, California, to Northern Baja, Mexico.

The Western Monarch Count clarifies that “although we call this a ‘monarch count’, in reality it’s more of a ‘monarch estimate’,” depending on volunteers’ valuable contributions. Usually, pairs of volunteers scout out the overwintering sites early in the morning while the butterflies are still in clusters – counting each butterfly individually when possible, and counting the number in a small cluster and extrapolating based on the number of other clusters when there are way too many butterflies to be able to count individually.

The western monarch overwintering population reached 4.5 million in the 1980s, according to the US Fish and Wildlife Service (USFWS), with the inaugural Western Monarch Count in 1997 counting 1,235,490 of them and in the following year, 564,349.

The mid-season count happens when the overwintering population is usually at its peak: this most recent number is from November 16 to December 8, 2024. The results showed a nearly 96 percent decrease from the 233,394 counted at the 2023 peak.

“We know small populations are especially vulnerable to environmental fluctuations, and we think that’s what happened this year,” Pelton explained. “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 overwintering western monarch population has risen and fallen over the years, with 2020 having the lowest number to date at just 1,901, versus 29,429 in 2019.

“These sites normally host thousands of butterflies, and their absence this year was heartbreaking for volunteers and visitors flocking to these locales hoping to catch a glimpse of the awe-inspiring clusters of monarch butterflies,” Director of Endangered Species at the Xerces Society Sarina Jepsen said in a 2021 statement.

However, the next year saw a population boom, with numbers bouncing back to 247,246 in 2021 and reaching 335,479 in 2022.

So, what happened this year? The Xerces Society cites “pesticides, habitat loss, and increasingly severe weather exacerbated by climate change” as some of the numerous threats facing these butterflies. The Los Angeles fires this January also struck tree groves in Lower Topanga Canyon, where the monarchs overwinter.

“This is bad news,” Pelton told The Associated Press. “But we have seen incredible recovery. This doesn’t mean we’re not going to have western monarchs. It’s hopefully a wake-up call that a bad year can set them back pretty significantly.”

“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.”

The USFWS proposed in December 2024 that the monarch butterfly be listed as a threatened species under the US Endangered Species Act, with a 90-day comment period ending on March 12, 2025.

“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,” USFWS Director Martha Williams said in a press release announcing the proposal.

“Science shows that the monarch needs that chance, and this proposed listing invites and builds on unprecedented public participation in shaping monarch conservation efforts. Providing monarchs with enough milkweed and nectar plants, even in small areas, can help put them on the road to recovery. Working together, we can help make this extraordinary species a legacy for our children and generations to come.”