The number of monarch butterflies spending the winter in the western United States has dropped to its second-lowest mark in nearly three decades as pesticides, diminishing habitat and the climate crisis take their toll on the beloved pollinator.
The butterflies, known for their distinctive orange-and-black wings, are found across North America. Monarchs in the eastern US spend their winters in Mexico, while monarchs west of the Rocky Mountains typically overwinter along the California coast.
The Xerces Society for Invertebrate Conservation has been counting western overwinter populations along the California coast, northern Baja California and inland sites in California and Arizona for the last 28 years. On Friday, it announced that it counted just 9,119 monarchs in 2024, a decrease of 96% from 233,394 in 2023. The total was the second-lowest since the survey began in 1997. The record low was 1,901 monarchs in 2020.
The highest number recorded was 1.2 million in 1997.
The survey noted that a site owned by the Nature Conservancy in Santa Barbara that saw 33,200 monarchs last winter hosted only 198 butterflies this year.
Monarchs across the continent face mounting threats, chief among them vanishing milkweed, the host plant for the insect’s caterpillars. The plant has been disappearing before a combination of drought, wildfires, agriculture and urban development, according to Monarch Joint Venture, a group that works to protect monarchs. Pesticides have contaminated much of the remaining plants, according to the Xerces Society.
It is unclear what caused such a sharp drop-off in the western population in just one year, said Emma Pelton, an endangered species biologist with the Xerces Society. The monarch population is already small, she said, and triple-digit heat in the western states last year may have slowed breeding.
Monarchs suffer when the mercury gets up to 100F (37.7C) and any temperatures above 108F are lethal to the insects, Pelton said. The western states saw a heatwave in July that drove temperatures in some areas well past 100F. Palm Springs, for example, hit a record 124F on 5 July. Another heatwave cooked northern California in early October, with multiple cities breaking heat records.
Pelton said it was too early to tell what long-term impact the dramatic losses might have on the overall western monarch population. Insects do have the potential for exponential growth, Pelton said. After bottoming out at 1,901 butterflies in 2020, the population rebounded to 247,246 insects the following year, an increase of nearly 13,000%. The year after that the survey recorded 335,479 monarchs.
“This is bad news,” Pelton said of the 2024 population drop. “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.”
The US Fish and Wildlife Service announced in December 2024 that it was working to list monarchs as threatened, a move that would prohibit anyone from killing, transporting them or making changes that would render their property permanently unusable for the species, such as eradicating all milkweed from the land. The listing would also protect 4,395 acres (1,779 hectares) in seven coastal California counties that serve as overwinter sites for western monarchs.
A public comment period on the proposal is set to end in March. The agency has until December to officially list the monarch as threatened if officials decide to move forward.
Earthjustice, an environmental law firm, petitioned the Environmental Protection Agency in December 2024 to mandate testing pesticide effects on insects such as bees, moths and butterflies.
The World Wildlife Fund, which counts monarchs in the eastern US, has yet to release data for this year.
