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.
After bottoming out at 1,901 butterflies in 2020, the population rebounded to 247,246 insects the following year. The year after that the survey recorded 335,479 monarchs. 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.
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 100˚F (37.7˚C) and any temperatures above 108˚F are lethal to the insects, Pelton said. The western states saw a heatwave in July that drove temperatures in some areas well past 100˚F. Palm Springs, for example, hit a record 124˚F 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.
The World Wildlife Fund, which counts monarchs in the eastern US, has yet to release data for this year.