A newly published, peer-reviewed study has provided definitive scientific evidence connecting toxic pesticides to a devastating mass die-off of Western monarch butterflies that shook conservationists and scientists in early 2024.
In January 2024, researchers in California uncovered hundreds of monarch butterflies either dead or in the final stages of dying near the Pacific Grove Monarch Sanctuary. The coastal sanctuary is a critical habitat where Western monarch butterflies traditionally gather to overwinter, seeking shelter from the colder months before migrating inland during the spring.
Initial observations by field researchers revealed that the fallen insects were exhibiting unmistakable signs of neurotoxic pesticide poisoning. This alarming discovery prompted rigorous scientific testing and chemical analysis, the results of which have now been officially published in the academic journal Environmental Toxicology and Chemistry.
To determine the exact cause of death, the research team employed advanced laboratory techniques, including liquid chromatography, gas chromatography, and mass spectrometry. These sophisticated analytical methods allowed scientists to screen the tissues of the dead butterflies for a wide variety of agricultural and commercial chemicals. The results were startling: the testing revealed a complex and toxic mix of 15 different insecticides, herbicides, and fungicides present on the bodies of the butterflies.
“We found an average of seven different pesticides per butterfly, including multiple insecticides that are highly toxic to insects,” said Staci Cibotti, the lead author of the study and a pesticide risk prevention specialist at the Xerces Society for Invertebrate Conservation, 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 comprehensive findings of the study, three distinct human-made pyrethroid insecticides—specifically bifenthrin, cypermethrin, and permethrin—were detected at or near their known lethal doses. Furthermore, chemical traces of bifenthrin and cypermethrin were discovered in every single butterfly sample tested by the research team, while permethrin was present in all but two of the samples.
Western monarch butterflies naturally depend on the Pacific coast to overwinter safely, but their biology makes them uniquely susceptible to environmental hazards. During this vulnerable period, the butterflies cluster tightly together in specific groves, where they remain exposed to pesticide residue and drift originating from nearby agricultural lands, suburban neighborhoods, and urban developments, as Cibotti explained.
The timing of this poisoning event compounded an already dire ecological situation 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 crisis deepened into the following year, when the official overwintering Western monarch population dropped to an alarming total of just 9,119 individuals.
The Xerces Society emphasized the gravity of the situation on social media, noting that while environmentalists had long suspected chemicals played a role in the 2024 Pacific Grove incident, the new research provides undeniable proof. Each analyzed butterfly carried an average of seven distinct pesticides, many of which were present in concentrations high enough to prove fatal.
Conservation data underscores the broader existential crisis facing the species. The Western monarch butterfly population has experienced an astonishing decline of nearly 95 percent since the 1980s, according to figures compiled by the Xerces Society. Recognizing this catastrophic downward spiral, the International Union for Conservation of Nature has classified migratory monarch butterflies as endangered. Furthermore, assessments by the U.S. Fish and Wildlife Service cited by the Los Angeles Times indicate that Western monarch butterflies face a staggering 99 percent probability of becoming extinct by the year 2080 if current trends and environmental pressures continue unchecked.
Consequently, mitigating the lethal impacts of chemical exposure has emerged as an absolute priority for wildlife conservationists and environmental regulators. In the wake of the study’s publication, the Xerces Society has outlined a series of recommended corrective actions aimed at shielding the fragile insects from further harm. These recommendations include expanding public education initiatives regarding the risks associated with chemical pest control and promoting safer alternatives. Conservation advocates are also pushing for the establishment of designated pesticide-free buffer zones around known overwintering habitats, the integration of stronger pesticide exposure safeguards into official species recovery plans, and improved coordination and risk-tracking mechanisms among public health and agricultural officials.
“Protecting monarchs from pesticides will require both public education and policy change,” said Emily May, a co-author of the study and the agricultural conservation lead at the Xerces Society, in a statement. “We are committed to working with communities and decision-makers to ensure that overwintering sites are healthy refuges for these butterflies.”
Expanding on the broader implications of the research, lead author Staci Cibotti noted that while the Pacific Grove incident took place in California, chemical exposure represents a pervasive, widespread threat to monarch populations across their entire geographic range. These potent chemical compounds are utilized not only in large-scale commercial agriculture but also extensively across developed residential and commercial landscapes for structural pest management, domestic home gardening, professional landscaping, turf maintenance, and mosquito abatement programs. In certain instances, pesticides are even deliberately applied in natural, wild areas to suppress forest pests and manage disease vectors. All of these diverse applications present distinct, ongoing risks to monarchs and other vital pollinators.
Cibotti further highlighted that migratory species face heightened vulnerability during specific phases of their life cycle. Because migratory and overwintering butterflies gather in massive, dense colonies numbering in the thousands, a single localized pesticide application possesses the devastating capacity to kill hundreds or even thousands of individuals simultaneously. A comparable tragedy occurred in September 2020 in North Dakota, where hundreds of monarch butterflies were found dead following aerial mosquito control spraying conducted directly during their annual autumn migration.
Because migration and overwintering represent exceptionally delicate phases in the biological cycle of the monarch, minimizing chemical exposure during these critical windows is vital. Protecting butterflies from sudden, mass-exposure events not only prevents immediate, catastrophic losses but also preserves the resilience of the surviving population, supporting its capacity to rebound during the spring breeding season and recover over the long term.
