A new peer-reviewed study has identified pesticides as the likely culprit behind a devastating mass die-off of Western monarch butterflies that shook conservationists and scientists in early 2024.
The findings, published in the journal Environmental Toxicology and Chemistry, offer definitive chemical evidence regarding an incident in January 2024, when researchers stumbled upon hundreds of dead and dying monarch butterflies near the Pacific Grove Monarch Sanctuary in California. This coastal sanctuary serves as a vital winter refuge where Western monarch butterflies typically gather to overwinter, seeking shelter from the harsher inland climates before beginning their spring migrations.
Initial field observations by researchers found the vulnerable insects displaying clear signs of neurotoxic pesticide poisoning, prompting an intensive round of laboratory testing and chemical analysis. The recently published study outlines how researchers utilized advanced analytical techniques, including liquid and gas chromatography paired with mass spectrometry, to examine the tissues of the deceased butterflies.
The results of these rigorous tests revealed a complex, hazardous cocktail of chemicals. Scientists detected a total of 15 different insecticides, herbicides, and fungicides present on the sampled 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 a pesticide risk prevention specialist at the Xerces Society for Invertebrate Conservation, said in a public 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 details of the study, the chemical burden carried by the insects was not merely trace contamination. Three synthetic pyrethroid insecticides—specifically bifenthrin, cypermethrin, and permethrin—were found at concentrations meeting or exceeding their lethal doses for insects. The data showed that every single butterfly sample tested positive for bifenthrin and cypermethrin, while all but two samples contained residues of permethrin.
Cibotti explained that while Western monarch butterflies rely on coastal groves along the Pacific for winter survival, they remain dangerously vulnerable to pesticide residue and chemical drift originating from nearby agricultural operations, suburban spaces, and urban developments.
The discovery of the chemical poisoning comes at a particularly precarious time for the species. According to the Xerces Society’s annual Western Monarch Count, monarch populations plummeted to the second-lowest number ever recorded in 2024. By the winter of 2025, the overwintering Western monarch population had rebounded only slightly on paper, totaling just 9,119 individuals across their traditional California sanctuaries—a tiny fraction of historical abundance.
The broader trajectory of the Western monarch population highlights an ongoing extinction crisis. Conservation data compiled by the Xerces Society indicates that the Western monarch butterfly population has declined by nearly 95 percent since the 1980s. Consequently, migratory monarch butterflies are classified as endangered by the International Union for Conservation of Nature (IUCN). Furthermore, the U.S. Fish and Wildlife Service has previously estimated that Western monarch populations face an alarming 99 percent chance of functional extinction by the year 2080, underscoring the urgency of identifying and mitigating localized mortality events.
Given these grim projections, mitigating the deadly impacts of chemical pesticides has become an absolute priority for wildlife conservationists, land managers, and policymakers. In response to the findings of the new study, the Xerces Society has outlined several proactive recommendations to safeguard the remaining populations. These proposed measures include ramping up public education initiatives regarding pesticide risks, promoting safer alternative pest management strategies, establishing dedicated pesticide-free buffer zones around critical overwintering sites, integrating stronger pesticide exposure protections into formal conservation and recovery frameworks, and fostering better coordination and tracking of pesticide application 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 the Xerces Society, noted in a statement. “Estamos committed to working with communities and decision-makers to ensure that overwintering sites are healthy refuges for these butterflies.”
Elaborating on the broader implications of the research, Cibotti emphasized that while the Pacific Grove incident took place in California, pesticide exposure represents a pervasive, nationwide threat to monarchs across their entire migratory range. These chemicals are heavily utilized not only in large-scale agricultural production, but also extensively across developed landscapes for structural pest management, residential gardening, landscaping, municipal turf management, mosquito abatement programs, and more. In some instances, pesticides are even introduced into natural forest ecosystems to combat localized tree pests and disease vectors, creating hidden hazards for non-target species like pollinators.
Cibotti further pointed out that a critical takeaway from the Pacific Grove tragedy is the extreme vulnerability of monarchs during specific phases of their life cycle, particularly migration and overwintering. Because these butterflies cluster tightly together by the thousands in localized microclimates, a single localized pesticide application can spell disaster, wiping out hundreds or even thousands of individuals in one catastrophic event.
A similar mass mortality event occurred in September 2020 in North Dakota, where hundreds of monarch butterflies were discovered dead following aerial mosquito control spraying executed precisely during their annual southward migration. Because migration and overwintering demand immense physiological endurance and require tight clustering, minimizing pesticide exposure during these sensitive windows is vital not only to prevent immediate, localized losses, but also to preserve the population’s capacity to rebound during spring breeding and achieve long-term recovery.
