MADISON, Wis. — The population of monarch butterflies in the western United States has plummeted to its second-lowest level in almost thirty years, largely due to adverse effects from pesticides, habitat loss, and climate change.
The monitoring of these butterflies commenced in 1997. Monarchs, recognized for their iconic orange-and-black wings, can be found throughout North America. Those located in the eastern U.S. migrate to Mexico for the winter, where their numbers are tallied by the World Wildlife Fund, which has yet to release its current data. In contrast, monarchs west of the Rocky Mountains typically overwinter along the California coastline.
For the past 28 years, the Xerces Society for Invertebrate Conservation has been counting the overwintering populations of monarchs along the California coast, northern Baja California, and various inland locations in California and Arizona. The highest recorded population was 1.2 million in 1997. Recently, they reported that only 9,119 monarchs were counted in 2024, representing a staggering 96% decline from 233,394 in 2023. This figure marks the second-lowest count since the monitoring began, with the lowest being just 1,901 butterflies in 2020. Notably, a location managed by The Nature Conservancy in Santa Barbara, which had hosted 33,200 monarchs last winter, saw a mere 198 butterflies this year.
One of the significant factors contributing to the decline of western monarchs is the disappearance of milkweed, the critical host plant for their caterpillars. As outlined by the Monarch Joint Venture, drought conditions, wildfires, agricultural practices, and urban expansion have severely impacted these plants. Additionally, pesticides have tainted much of the remaining milkweed stocks, as highlighted by the Xerces Society.
There is some uncertainty regarding why the population fell so dramatically in just a year, as noted by Emma Pelton, an endangered species biologist at Xerces. She emphasized that the monarchs’ already low numbers, coupled with record-breaking heat in the western states last year, could have hampered breeding efforts. Monarchs are particularly sensitive to high temperatures, with mortality rates escalating significantly beyond 108 degrees Fahrenheit (42.2 degrees Celsius). For instance, a heat wave in July caused temperatures in some regions to soar past 100 degrees Fahrenheit, with Palm Springs recording a staggering 124 degrees Fahrenheit on July 5. Another intense heat event subsequently struck northern California in early October, breaking multiple heat records.
Looking ahead, the future of western monarchs remains uncertain. Pelton indicated that while the immediate losses are troubling, insects have a remarkable potential for rapid population recovery. After plummeting to just 1,901 butterflies in 2020, the western monarch population surged to 247,246 the following year — an increase of nearly 13,000%. The count rose further to 335,479 in the subsequent survey.
“This is indeed concerning,” Pelton remarked regarding the recent drop in population, “but we’ve observed astonishing recoveries in the past. This situation doesn’t signify the end for western monarchs, but it should serve as a crucial reminder that a single adverse year can have significant repercussions for their population.”
In response to the declining numbers, federal authorities are taking action. The U.S. Fish and Wildlife Service revealed in December 2024 that efforts were underway to classify monarchs as a threatened species. Such a designation would make it illegal to harm or transport these butterflies or to modify habitats in a way that would eliminate milkweed from those areas. The proposed protections would also encompass 4,395 acres across seven coastal California counties that are essential wintering grounds for western monarchs.
A public commentary period regarding the proposal is expected to conclude in March, with the agency having until December to formally designate the monarchs as threatened, provided the decision moves forward. Additionally, Earthjustice, an environmental advocacy group, submitted a petition to the Environmental Protection Agency in December 2024, urging the necessity for testing the impacts of pesticides on crucial pollinators like bees, moths, and butterflies.
