16
Jul
Mass Kill of Monarch Butterflies in California Linked to Pesticide Residues in Their Bodies
(Beyond Pesticides, July 16, 2025) A study following a mass mortality event of approximately 200 monarch butterflies (Danaus plexippus plexippus) in Pacific Grove, California, highlights the role of pesticides, synthetic pyrethroids in particular, in causing lethal and sublethal effects to nontarget organisms. The research, published in Environmental Toxicology and Chemistry, detects residues of 15 pesticides and associated metabolites in the bodies of 10 deceased butterflies collected from the January 2024 event that occurred near an overwintering site frequented by monarchs.
“On average, each monarch butterfly contained 7 pesticides,†the authors report. They continue, “Notably, three pyrethroid insecticides—bifenthrin, cypermethrin, and permethrin—were consistently detected at or near each chemical’s lethal dose (LD50).†LC50, or Lethal Concentration 50, values represent the concentrations of chemicals lethal to 50% of a test population.
To assess pesticide residues within ten of the deceased butterflies, the researchers use liquid and gas chromatography with tandem mass spectrometry (LC-MS/MS and GC-MS/MS) and find “a total of 15 pesticides and associated metabolites in the butterflies, including 8 insecticides (plus 1 associated metabolite), 2 herbicides (plus 2 associated metabolites), and 2 fungicides.â€
The study “present[s] evidence that the mortality incident at the Pacific Grove Monarch overwintering site was likely caused by nontarget pesticide poisoning,†the authors state. “These findings demonstrate pesticide contamination in monarch butterflies, including within urban areas, and highlight the risks pesticides, especially insecticides, pose to monarch populations.â€
Background
Monarch butterflies are an important pollinator species and encounter pesticides through multiple exposure routes while performing ecosystem services. Eggs, larvae, and adults of the species may directly contact pesticides. Monarchs can also experience dietary exposure when feeding on contaminated host plants or flowers. “Pesticides have been detected in milkweed plants across agricultural, urban, and open areas in California, and throughout other regions of North America, highlighting exposure risks for monarch larvae,†the researchers note. Contaminated soil and water also present a risk. (See research here, here, and here.)
Many studies (see here, here, here, and here) show that pesticide use in private gardens, on turfgrass, and for mosquito and vector control also presents a risk to butterflies. The use of pesticides near overwintering sites, such as the one in the current study, presents a heightened risk to the species as well.
The authors share: “Monarchs aggregated at roosting locations along migratory corridors or overwintering groves may be particularly vulnerable to pesticide exposure events, as a single application could potentially impact hundreds or thousands of individuals. Because these clustering events occur during especially sensitive phases of the migratory cycle, pesticide exposure during these stages could have significant consequences for subsequent generations. Furthermore, monarchs return to the same overwintering locations annually, relying on specific microclimatic conditions for survival, making relocation to avoid pesticide exposure unfeasible.â€
Study Methodology and Results
The current study is authored by staff of The Xerces Society for Invertebrate Conservation, U.S. Geological Survey, and the Pacific Grove Museum of Natural History. Approximately 200 monarch butterflies were discovered dead and dying near the large overwintering site, where “[m]any of the moribund butterflies were exhibiting signs of pesticide poisoning.â€
The Pacific Grove Monarch Sanctuary is one of approximately 400 overwintering sites along the California coast where monarchs have been documented as visiting for over 100 years. “The site regularly supports up to tens of thousands of butterflies each fall and winter during their reproductive diapause [dormancy],†the researchers say. As an example, in November of 2023 alone, The Xerces Society for Invertebrate Conservation observed approximately 6,600 butterflies overwintering at the site.
As the study describes: “Some of the butterflies were scattered across a turf lawn, while others were grouped in multiple piles parallel to a structural perimeter. Many of the butterflies were already dead, while others were twitching, a symptom commonly observed in response to neurotoxic pesticide poisoning. The dead butterflies’ wings were flipped forward and their abdomens curled, suggesting similar involuntary movements prior to death… Staff and volunteers continued to observe small numbers of butterflies displaying these symptoms on the lawn over the next two weeks.â€
This discovery led to the researchers analyzing “pesticide residues in several deceased individuals to assess pesticide exposure and determine whether it may have contributed to the mortality event.†The analyses show residues of 15 pesticides and associated metabolites, with the three neurotoxic pyrethroid insecticides (bifenthrin, cypermethrin, and permethrin) at the highest concentrations across samples. Residues of bifenthrin and cypermethrin are noted in every sample, while permethrin is present in all but two of the samples.
For the three pyrethroid insecticides identified, there is only a published LC50 value for bifenthrin relating specifically to monarchs. In comparing the reported LC50 to the study results, four out of the ten butterflies contained bifenthrin concentrations exceeding this value. For permethrin and cypermethrin, the only comparison that can be made is to other butterfly species. This, however, does not paint an accurate picture of how these chemicals can impact monarchs, as variations in sensitivity between species can be quite drastic.
For permethrin, an LD50 range is established for other adult nymphalid butterflies. The authors state, “In our study, six of the sampled butterflies had concentrations falling within or exceeding this range.†For cypermethrin, there is only an LC50 available for the butterfly pest species of Pieris brassicae, with the study data “indicating that six of the ten monarchs we sampled were exposed to a potentially lethal dose.â€
Also of note is that LD50 values represent the dose required to kill 50% of a test population, while some individuals can experience debilitating symptoms, and still die, from lower doses. One study finds that “monarch larvae exposed to bifenthrin at less than half of its lethal dose exhibited severe poisoning symptoms, including bleeding and spasming.†In the current study, eight of the ten monarchs show bifenthrin levels at which such symptoms could occur.
As the researchers point out, there are additional issues in assessing risks to species since “available toxicity values are based on exposure to a single active ingredient, whereas all the sampled monarchs contained residues of multiple pesticides.†Exposure to multiple pesticides can result in additive or synergistic effects, which then enhance toxicity, as has been demonstrated in many studies of pollinator species.
The researchers further note: “The toxicological effects of multiple pyrethroids are likely additive due to their shared mode of action. Interactions between different modes of action groups can lead to synergistic effects, where the combined toxicological effect exceeds the sum of the individual effects. Therefore, it is possible that combined exposure to various pesticides could have enhanced pyrethroids’ toxicities in the sampled monarchs.†(See research here, here, and here.)
History of Monarchs and Pesticides
Adverse impacts to monarch butterflies have been documented since the 1980s, with studies finding populations across North America declining by approximately 80% in the east and 95% in the west. (See studies here and here.) These drastic results have led the U.S. Fish and Wildlife Service to propose listing the monarch as a threatened species under the Endangered Species Act of 1973 (ESA). The most recent proposal, posted for public comment on December 12, 2024, and covered in the Daily News, also included adding designated critical habitats for the species that would span approximately 4,395 acres throughout overwintering sites in coastal California. The comment period, in which Beyond Pesticides submitted comments, closed on March 12, 2025.
As Beyond Pesticides continues to report, the biodiversity crisis is one of multiple crises that are compounding one another. While human actions are contributing to an ongoing Holocene or sixth mass extinction, the globe is also facing crises in human disease and climate change. Declining monarch populations are symptomatic of the ongoing insect apocalypse, which threatens worldwide ecology and agriculture—and exemplifies the regulatory process that fails to protect biodiversity. (See Daily News here.)Â
Preserving biodiversity is imperative for all life and starts by considering all the factors that contribute to the crisis in the first place. Pesticide use is a major cause of declining biodiversity, which is manifested in extinctions, endangered species, and species vulnerable to environmental disturbances—including climate change, habitat fragmentation, and toxic chemicals. As emphasized in the public comments, Beyond Pesticides urges the protection of monarch butterflies as critically necessary—rooted in science and given the foundational role that pollinators play in the ecosystem and the knowledge that their extinction would have cascading negative impacts.
Several studies also show the contamination from pesticides on milkweed plants, which monarch caterpillars rely on as their only food source. (See studies here, here, and here as well as related Daily News coverage here, here, and here.)
The mass mortality event in Pacific Grove is not the first to occur. A 2020 Daily News article reports on a “Monarch Massacre†where hundreds of monarch butterflies were found dead after the Vector Control Department of Cass County, North Dakota, aerially sprayed the county for mosquito control. This case also links the use of synthetic pyrethroids to the deaths of monarchs.
Organic Solution
The more recent mass mortality incident in California adds to the wide body of science that highlights the ongoing risks pesticides pose to monarchs and other pollinator species. The alternative to toxic chemicals that do not threaten health and biodiversity? Organic land management.
The elimination of petrochemical pesticides and synthetic fertilizers is the only solution moving forward that can protect species, like the monarch butterfly, from population effects. Mitigation measures have consistently and repeatedly failed. In adopting organic practices, the harmful effects seen with chemical-intensive, conventional methods are prevented, and the health of the environment, including all organisms it supports, is prioritized.
Learn more about the benefits of organic here and here, as well as in the Daily News Biodiversity Threatened by Pesticide Drift, Study Finds; Organic Agriculture Cited as a Holistic Solution and Pesticides and You article, Thinking Holistically When Making Land Management Decisions.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Cibotti, S. et al. (2025) Pyrethroid insecticides implicated in mass mortality of monarch butterflies at an overwintering site in California, Environmental Toxicology and Chemistry. Available at: https://academic.oup.com/etc/advance-article-abstract/doi/10.1093/etojnl/vgaf163/8177160.
