New Study Links Deadly Pesticide Exposure to Mass Western Monarch Butterfly Die-Off

A study published in the journal Environmental Toxicology and Chemistry has identified pesticides as the likely cause of a mass die-off of Western monarch butterflies in early 2024.

In January 2024, researchers discovered hundreds of dead or dying butterflies near the Pacific Grove Monarch Sanctuary in California, a site where Western monarchs typically overwinter. Subsequent analysis revealed high levels of pesticide exposure among the affected insects. The findings were detailed in a study led by Staci Cibotti, pesticide risk prevention specialist at the Xerces Society for Invertebrate Conservation.

Using advanced chemical testing methods, including liquid and gas chromatography and mass spectrometry, researchers detected an average of seven different pesticides on each monarch butterfly. Among these were three highly toxic pyrethroid insecticides—bifenthrin, cypermethrin and permethrin—found at or near lethal doses. Every sample contained bifenthrin and cypermethrin, while all but two samples had permethrin.

“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,” Cibotti stated.

Western monarch butterflies, which overwinter along the Pacific coast, face growing threats from pesticide residue and drift originating from agricultural and urban areas. The incident coincides with alarming population declines: the Xerces Society’s annual Western Monarch Count recorded the second-lowest population ever in 2024, with overwintering counts dropping to just 9,119 individuals by 2025.

The Western monarch population has plummeted by nearly 95% since the 1980s. The migratory monarch subspecies is listed as endangered by the International Union for Conservation of Nature (IUCN), while U.S. Fish and Wildlife Service projections indicate a 99% risk of extinction for Western monarchs by 2080.

In response to the findings, the Xerces Society has called for stronger protective measures, including public education on pesticide risks, the creation of pesticide-free buffer zones around overwintering sites, and enhanced coordination among officials to track and mitigate pesticide exposure.

“Protecting monarchs from pesticides will require both public education and policy change,” said Emily May, co-author of the study and agricultural conservation lead at Xerces. “We are committed to working with communities and decision-makers to ensure that overwintering sites are healthy refuges for these butterflies.”

Cibotti emphasized that pesticide exposure is a widespread threat affecting monarchs across their entire range. “These chemicals are commonly used not only in agriculture, but also in urban and residential settings, turf management, mosquito control, and even in some natural areas,” she noted. “Monarchs are particularly vulnerable during migration and overwintering when large numbers congregate, making even a single pesticide application potentially catastrophic.”

The incident mirrors a similar event in North Dakota in September 2020, when hundreds of monarchs were found dead following a mosquito control spray during their migration.

Experts stress that reducing pesticide use during critical migration and overwintering periods is essential to prevent immediate losses and support long-term population recovery.