In a new peer-reviewed study, researchers blame pesticides for the mass die-off of Western monarch butterflies in 2024. The study was a result of researchers discovering the mass die-off of hundreds of monarch butterflies near the Pacific Grove Monarch Sanctuary in California, where the butterflies overwinter.
This prompted researchers to further test and analyze the link between butterflies and pesticides after they found signs of neurotoxic pesticide poisoning.
“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. “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.”
Published in the journal “Environmental Toxicology and Chemistry,” the study’s research team found a mixture of 15 insecticides, herbicides and fungicides present on the dead butterflies. Every sample included human-made pyrethroid insecticides, bifenthrin and cypermethrin, and many others contained permethrin. Cibotti said that the new research proves that pesticide residue and drift from nearby farms, many at lethal doses, are to blame for the mass die-off.
“Another key takeaway from this incident is just how vulnerable monarchs are to these types of incidents during migration and overwintering,” Cibotti said. “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.”
In 2024, monarch populations were the second-lowest number ever recorded, according to the Xerces Society for Invertebrate Conservation’s annual Western Monarch Count. And the U.S. Fish and Wildlife Service predicted that Western monarch butterflies have a 99 percent chance of becoming extinct by 2080. Xerces Society is urging public officials to better educate people on pesticide risks, create a pesticide-free area surrounding overwintering sites, and a recovery plan for butterflies.
“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.”
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