Scientific Research Links Mass Die-Off of Western Monarch Butterflies to Lethal Pesticide Exposure at California Overwintering Sites

A peer-reviewed study published in the journal Environmental Toxicology and Chemistry has identified a direct link between a devastating mass die-off of Western monarch butterflies and the presence of high concentrations of neurotoxic pesticides. The research, which follows a catastrophic event in early 2024 near the Pacific Grove Monarch Sanctuary in California, provides empirical evidence that chemical contamination is a primary driver in the precipitous decline of this iconic migratory species. Scientists discovered a complex mixture of 15 different insecticides, herbicides, and fungicides within the tissues of dead and dying butterflies, suggesting that even protected sanctuaries are no longer safe from the reach of modern agricultural and urban chemical drift.

The investigation began in January 2024 when researchers and local volunteers discovered hundreds of monarch butterflies displaying distressing symptoms of neurological distress. The butterflies, found at one of California’s most famous overwintering sites, were unable to fly, twitching uncontrollably, or already dead on the forest floor. The subsequent laboratory analysis, led by the Xerces Society for Invertebrate Conservation, utilized advanced liquid and gas chromatography alongside mass spectrometry to isolate the substances responsible for the mortality event. The results revealed an average of seven different pesticides per butterfly, with several samples containing concentrations that met or exceeded known lethal doses for lepidoptera.

The Pacific Grove Incident: A Timeline of Discovery

The crisis first came to light during the peak of the 2023–2024 overwintering season. Pacific Grove, long celebrated as "Butterfly Town, USA," serves as a critical refuge for Western monarchs, which migrate from across the Pacific Northwest and the Rocky Mountains to cluster in the town’s microclimate-controlled eucalyptus and pine groves. In mid-January, observers noted a sudden and sharp increase in mortality that did not align with natural winter attrition or predatory patterns.

By late January 2024, the Xerces Society and local biologists began collecting specimens for forensic analysis. While initial suspicions pointed toward a localized weather event or disease, the physical symptoms of the butterflies—specifically the tremors and paralysis—strongly indicated neurotoxicity. Throughout the spring of 2024, these samples underwent rigorous testing. The findings, which were finalized and peer-reviewed by August 2025, confirmed that the butterflies had been exposed to a "lethal cocktail" of chemicals shortly before or during their stay at the sanctuary.

The timing of the die-off is particularly significant. Western monarchs are most vulnerable during their overwintering phase, a period when they remain largely sedentary to conserve energy for their spring migration and subsequent breeding. Any chemical exposure during this window can have an outsized impact on the population, as thousands of individuals are concentrated in a very small geographic area.

A Lethal Cocktail: The Chemical Analysis

The research team, led by Staci Cibotti, a pesticide risk prevention specialist at the Xerces Society, identified 15 distinct chemical compounds on the collected monarchs. Among the most concerning were three synthetic pyrethroid insecticides: bifenthrin, cypermethrin, and permethrin. These chemicals are designed to disrupt the nervous systems of insects by keeping sodium channels in the nerve cells open, leading to continuous firing of the nerves, paralysis, and eventual death.

The data showed that every single butterfly tested contained bifenthrin and cypermethrin. Furthermore, all but two of the samples contained permethrin. The concentrations of these pyrethroids were found at levels near or above the median lethal dose (LD50), the standard measurement used to determine the amount of a substance required to kill half of a tested population.

Beyond the primary insecticides, the study documented a variety of herbicides and fungicides. While these substances are often marketed as non-toxic to insects, the "synergistic effect" of multiple chemicals can be far more deadly than a single toxin. The presence of fungicides, for instance, can sometimes inhibit an insect’s ability to metabolize and detoxify insecticides, making a sub-lethal dose of a chemical suddenly fatal. Cibotti noted that the high levels detected strongly suggest that insecticides were the direct cause of death, even if the specific source of the application could not be pinpointed by Monterey County officials.

The Precarity of the Western Monarch Population

The 2024 die-off is a major setback for a population that is already teetering on the edge of biological collapse. Historically, the Western monarch population numbered in the millions. However, since the 1980s, the population has plummeted by approximately 95%. This decline is attributed to a combination of habitat loss, climate change, and the ubiquitous use of pesticides across the American West.

The annual Western Monarch Count, conducted by the Xerces Society, provides a grim statistical backdrop to the recent study. In 2024, populations dropped to the second-lowest number ever recorded. By the 2025 count, the number of overwintering Western monarchs had dwindled to a mere 9,119 individuals. For context, scientists have previously suggested that the population needs to maintain a minimum of 30,000 individuals to avoid an "extinction vortex," a cycle where low numbers lead to a lack of genetic diversity and increased vulnerability to random environmental events.

The International Union for Conservation of Nature (IUCN) currently lists the migratory monarch butterfly as endangered. Furthermore, data cited by the U.S. Fish and Wildlife Service and reported by the Los Angeles Times suggests that the Western monarch has a 99% probability of reaching quasi-extinction by the year 2080 if current trends are not reversed.

Mechanisms of Toxicity and Environmental Drift

One of the most challenging aspects of monarch conservation is the "drift" of pesticides from their intended targets. Monarchs do not only encounter chemicals on farms; they are exposed in urban gardens, along roadsides, and in public parks. Because many of the chemicals found—such as bifenthrin—are widely used in residential pest control for ants and spiders, the butterflies are at risk even in suburban neighborhoods.

Western monarchs migrate through diverse landscapes, including vast stretches of industrial agriculture in California’s Central Valley and heavily landscaped coastal cities. During migration, they stop to feed on nectar-producing flowers, which may be treated with systemic insecticides. Once they reach overwintering sites like Pacific Grove, they are susceptible to aerial drift from nearby mosquito abatement programs or structural pest control applications in the surrounding community.

Staci Cibotti emphasized that the vulnerability of monarchs is heightened during migration and overwintering because of their tendency to cluster. A single poorly timed pesticide application near a roosting site can wipe out a significant percentage of the remaining population in a matter of hours. This was previously observed in 2020 in North Dakota, where hundreds of monarchs were killed following an aerial mosquito control spray during their southward migration.

Recommendations for Policy and Conservation

In light of the study’s findings, the Xerces Society and other conservation groups are calling for immediate and robust policy changes to protect the remaining monarch population. The study highlights that voluntary measures are insufficient to prevent mass mortality events at critical habitat sites.

Key recommendations from the research team include:

• Establishment of Pesticide-Free Buffer Zones: Creating mandatory chemical-free zones around known overwintering sites along the California coast to prevent drift from reaching sensitive habitats.
• Enhanced Education and Outreach: Increasing public awareness regarding the risks that common home-and-garden pesticides pose to pollinators, and promoting the use of non-chemical alternatives for pest management.
• Policy and Regulatory Reform: Strengthening state and federal protections for butterflies within conservation and recovery plans, specifically focusing on the regulation of pyrethroids.
• Improved Tracking and Reporting: Implementing more rigorous systems for tracking pesticide applications near sensitive environmental areas to help officials identify the sources of contamination when die-offs occur.

Emily May, a co-author of the study and agricultural conservation lead at the Xerces Society, stated that protecting these insects requires a shift in how society manages both public and private lands. "We are committed to working with communities and decision-makers to ensure that overwintering sites are healthy refuges for these butterflies," May said.

Broader Impact and the Path Forward

The findings of the Environmental Toxicology and Chemistry study serve as a warning for the broader health of the ecosystem. Monarchs are often considered an "indicator species," meaning their health reflects the overall condition of the environment. If the chemicals in the environment are at levels high enough to cause mass mortality in monarchs, it is highly likely that other, less visible pollinators—such as native bees, moths, and beetles—are suffering similar fates.

The loss of the Western monarch would represent more than just the disappearance of a beautiful insect; it would signify a failure in the management of the natural resources and biological diversity of the Western United States. As the U.S. Fish and Wildlife Service continues to evaluate the monarch for potential listing under the Endangered Species Act, studies like this provide the necessary scientific foundation for more aggressive federal intervention.

The recovery of the species will depend on a multi-faceted approach that includes the restoration of milkweed (the only plant monarch larvae eat), the protection of nectar-rich stopover sites, and, most urgently, the mitigation of chemical threats. Without a significant reduction in the pesticide load found at overwintering sites, the "butterfly trees" of the California coast may soon become a memory of the past rather than a living sanctuary for the future.