The findings, published in the journal Environmental Toxicology and Chemistry, center on an alarming discovery made in January 2024. At that time, researchers came across hundreds of dead and dying monarch butterflies scattered near the Pacific Grove Monarch Sanctuary in California. The coastal sanctuary serves as a vital winter habitat where Western monarch butterflies typically gather to shelter during the colder months.
Initial field observations revealed that the distressed insects were displaying classic signs of neurotoxic pesticide poisoning. This prompted a deeper scientific investigation involving advanced laboratory testing and chemical analysis to determine the exact cause of the mortality event.
To uncover what happened, researchers utilized advanced analytical chemistry techniques, including liquid and gas chromatography paired with mass spectrometry. These sophisticated methods allowed the scientific team to screen the biological tissues of the dead butterflies for a wide variety of chemical residues. The results were startling: the analysis detected a complex chemical cocktail consisting of 15 different insecticides, herbicides, and fungicides present on the butterflies.
Staci Cibotti, lead author of the study and a pesticide risk prevention specialist at the Xerces Society for Invertebrate Conservation, highlighted the severity of the contamination in a public statement.
"We found an average of seven different pesticides per butterfly, including multiple insecticides that are highly toxic to insects," Cibotti 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."
According to the study’s detailed breakdown, three human-made pyrethroid insecticides—specifically bifenthrin, cypermethrin, and permethrin—were detected at concentrations reaching or exceeding their established lethal doses. Furthermore, the chemical footprint was remarkably consistent across the tested specimens: every single butterfly sample tested positive for bifenthrin and cypermethrin, while all but two samples contained permethrin.
Cibotti explained that while Western monarch butterflies rely on coastal groves along the Pacific for overwintering, they remain dangerously exposed to pesticide residue and chemical drift originating from nearby agricultural operations and urban environments.
The timing of the Pacific Grove incident coincided with an already perilous period for the species. According to data from the Xerces Society for Invertebrate Conservation’s annual Western Monarch Count, monarch populations plummeted to the second-lowest number ever recorded in 2024. The downward trend continued into the following year, with overwintering Western monarchs totaling a mere 9,119 individuals by 2025.
Reflecting on the study’s implications, the Xerces Society emphasized on social media that the new research moves beyond mere suspicion. While conservationists had strong reasons to suspect chemical contamination following the 2024 die-off, the latest findings deliver clear, empirical evidence of the chemical burden carried by the insects, with individual butterflies bearing an average of seven different pesticides, many present at lethal concentrations.
The broader outlook for the Western monarch population remains deeply concerning. Conservation groups report that the 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). Meanwhile, assessments by the U.S. Fish and Wildlife Service and reports by regional media have underscored the severe extinction risk facing the subspecies, with projections indicating a staggering probability of functional extinction by 2080 if current trends persist.
In response to the study’s conclusive findings, the Xerces Society has outlined several vital conservation priorities aimed at mitigating the lethal impacts of chemical exposure on monarchs. Their recommendations call for enhanced public education regarding the inherent risks of pesticide use alongside the promotion of safer pest-management alternatives. They also advocate for the establishment of designated pesticide-free buffer zones surrounding known overwintering habitats, stronger regulatory protections within official species recovery plans, and improved cross-agency coordination and tracking of pesticide risks by public officials.
Emily May, co-author of the study and agricultural conservation lead at the Xerces Society, emphasized that safeguarding the species requires a dual approach combining grassroots awareness with structural policy adjustments. She noted that the organization remains dedicated to collaborating with local communities and decision-makers to ensure that crucial overwintering sanctuaries function as safe, healthy refuges for the butterflies.
Expanding on the broader scope of the issue, lead author Staci Cibotti noted that while the Pacific Grove incident took place in California, pesticide exposure represents a pervasive, nationwide threat to monarchs throughout their entire migratory range. These chemical formulations are deployed extensively not only within commercial agriculture but also across heavily developed landscapes for structural pest management, residential gardening, landscaping, municipal turf maintenance, and mosquito abatement programs. In some instances, chemical treatments are even applied directly within natural areas to manage forest pests and disease vectors, creating unexpected hazards for monarchs and a wide array of other essential pollinators.
Cibotti further underscored the acute vulnerability of monarchs during specific phases of their life cycle, particularly migration and overwintering. Because these migratory butterflies congregate in dense, massive clusters during these periods, even a localized single pesticide application has the catastrophic potential to eliminate hundreds or thousands of individuals simultaneously.
This specific vulnerability is not unprecedented. A remarkably similar mass mortality event occurred in North Dakota in September 2020, when hundreds of monarch butterflies were discovered dead following aerial mosquito control spraying conducted during their autumn migration. Because migration and overwintering represent exceptionally sensitive windows in the monarch’s biological cycle, researchers stress that minimizing chemical exposure during these critical phases is paramount. Such measures are essential not only to prevent immediate, localized losses but also to preserve the population’s capacity to rebound during the spring breeding season and achieve long-term recovery.
