A new peer-reviewed study has linked pesticides as a likely cause to a mass die-off of Western monarch butterflies that occurred in 2024.
In January 2024, researchers found hundreds of dead or dying monarch butterflies near the Pacific Grove Monarch Sanctuary in California, where Western monarch butterflies typically overwinter.
As The Guardian reported, researchers found the butterflies showing signs of neurotoxic pesticide poisoning, leading to further testing and analysis that has now been published in the journal Environmental Toxicology and Chemistry.
The researchers tested the dead butterflies using liquid and gas chromatography and mass spectrometry, which led the team to find a mix of 15 insecticides, herbicides and fungicides present on the butterflies.
“We found an average of seven different pesticides per butterfly, including multiple insecticides that are highly toxic to insects,” Staci Cibotti, lead author of the study and pesticide risk prevention specialist at Xerces Society for Invertebrate Conservation, said in a statement. “Although a review by Monterey County could not determine the source of the chemicals, the high levels detected suggest that insecticides were likely responsible for the monarch deaths.”
According to the study, three human-made pyrethroid insecticides, including bifenthrin, cypermethrin and permethrin, were found at or near their lethal doses. Further, every sample included bifenthrin and cypermethrin, and all but two samples contained permethrin.
Western monarch butterflies overwinter along the Pacific coast, but they are vulnerable to pesticide residue and drift from nearby farms and urban areas, Cibotti explained.
According to the Xerces Society for Invertebrate Conservation’s annual Western Monarch Count, monarch populations dropped to the second-lowest number ever recorded in 2024, and by 2025, overwintering Western monarchs totaled just 9,119 individuals.
There were already reasons to suspect pesticides for the death of hundreds of monarchs at an overwintering site in 2024, but our new research provides clear evidence of what happened. Each monarch had, on average, 7 different pesticides, many at lethal doses. ➡️ xerces.org/press/study-…
— The Xerces Society for Invertebrate Conservation (@xercessociety.bsky.social) July 23, 2025 at 3:31 PM
The Western monarch butterfly population has declined by almost 95% since the 1980s, Xerces Society reported. Migratory monarch butterflies are listed as endangered by the International Union for Conservation of Nature (IUCN), and the U.S. Fish and Wildlife Service estimated that Western monarch butterflies have a 99% chance of becoming extinct by 2080, the Los Angeles Times reported.
As such, preventing the deadly effects of pesticides is a priority for monarch butterfly conservation. Following the study results, Xerces Society has recommended several actions, including increased education about pesticide risks and safer alternatives, establishment of pesticide-free zones around overwintering sites, greater pesticide exposure protections in conservation and recovery plans for butterflies, and stronger coordination and tracking for pesticide risks by public officials.
“Protecting monarchs from pesticides will require both public education and policy change,” Emily May, co-author of the study and agricultural conservation lead at Xerces Society, said in a statement. “We are committed to working with communities and decision-makers to ensure that overwintering sites are healthy refuges for these butterflies.”
Update: On Aug. 4 EcoWatch received the following statement from Staci Cibotti, lead author of the study and pesticide risk prevention specialist at Xerces Society for Invertebrate Conservation:
“Although this incident occurred in California, pesticide exposure is a widespread threat to monarchs across their range. These chemicals are commonly used not only in agriculture, but also across developed landscapes for structural pest control, home gardening, landscaping, turf management, mosquito abatement and more. Pesticides are even applied in some natural areas to manage things like forest pests and disease vectors. All of these uses can pose risks to monarchs and other pollinators.
“Another key takeaway from this incident is just how vulnerable monarchs are to these types of incidents during migration and overwintering. These are times when butterflies gather in large numbers, meaning even a single pesticide application has the potential to kill hundreds of individuals at once. This was seen not only in Pacific Grove in 2024, but also in a similar event in North Dakota in September 2020, when hundreds of monarchs were found dead following a mosquito control spray that occurred during their annual migration. Because migration and overwintering are particularly sensitive phases of the monarch’s migratory cycle, reducing pesticide exposure during these windows is critical. Not only to prevent immediate losses, but also to support the population’s ability to rebound in the spring and recover over the long term.”