MADISON, Wis. — The winter count of monarch butterflies in the western United States has reached alarming levels, marking the second-lowest figure in nearly 30 years. This decline has been attributed to various factors, including pesticides, habitat loss, and the effects of climate change on these iconic pollinators.
Since the initiation of surveys in 1997, monarch butterflies, recognizable for their vibrant orange-and-black wings, inhabit regions across North America. Eastern monarchs migrate to Mexico for the winter, with their numbers monitored by the World Wildlife Fund. However, the organization has yet to disclose this year’s data. In contrast, monarchs west of the Rocky Mountains typically find refuge along the California coast during the colder months.
The Xerces Society for Invertebrate Conservation has been diligently tracking the western overwinter populations along the California coastline, as well as in northern Baja California and inland areas of California and Arizona, for nearly three decades. Their findings reveal that the peak count was 1.2 million in 1997, but the latest assessment shows only 9,119 monarchs recorded for 2024. This represents a staggering 96% decrease from the 233,394 counted in 2023 and stands as the second-lowest figure since the surveys began, just above the historic low of 1,901 monarchs noted in 2020.
Notably, a site managed by The Nature Conservancy in Santa Barbara witnessed a dramatic decline from 33,200 monarchs last winter to just 198 this year.
The factors contributing to this sharp decline are multifaceted. Monarchs nationwide are grappling with a shrinking supply of milkweed, the essential host plant for caterpillars. This plant is disappearing due to a mix of drought conditions, wildfires, agricultural expansion, and urbanization, according to the Monarch Joint Venture, an organization committed to their preservation. The Xerces Society also pointed out that remaining milkweed plants have been compromised by pesticide usage.
Emma Pelton, an endangered species biologist with the Xerces Society, indicated that the precise reasons for the swift drop in the western monarch population are still unclear. The already reduced population may have been particularly vulnerable to extreme heat last year, which potentially hindered breeding activities. Monarchs can suffer in temperatures above 100 degrees Fahrenheit (37.7 Celsius), and any readings above 108 degrees Fahrenheit (42.2 degrees Celsius) can be lethal. Last summer, several regions in the western states experienced a heat wave, with areas like Palm Springs setting a record high of 124 degrees Fahrenheit (51.1 degrees Celsius) on July 5. Another heat event in early October further broke records throughout northern California.
Looking ahead, Pelton remarked that while it is premature to assess the long-term consequences of such a significant population decline, insects possess the ability for remarkable population rebounds. Following the low of 1,901 monarchs in 2020, the population surged to 247,246 in the subsequent year—an almost 13,000% increase. The following year saw another rise, with 335,479 butterflies recorded.
“This is concerning news,” Pelton stated regarding the 2024 figures. “However, we have witnessed extraordinary recoveries in the past. This does not spell the end for western monarchs, but it certainly serves as a wake-up call that a single poor year can greatly impact their numbers.”
In response to these conservation challenges, officials at the U.S. Fish and Wildlife Service announced in December 2024 that they are considering listing monarch butterflies as a threatened species. This designation would legally protect them from harm and restrict practices that could eliminate essential habitats, such as the removal of milkweed. The proposed listing would encompass 4,395 acres (1,779 hectares) in seven coastal counties of California vital for the monarchs’ overwintering.
A public comment period regarding this proposal is expected to conclude by March. Officials are tasked with making a decision by December on whether to classify the species as threatened. Additionally, Earthjustice, an environmental advocacy group, has urged the Environmental Protection Agency to enforce regulations requiring assessments of pesticide impacts on beneficial insects like bees, moths, and butterflies.
