In January 2024, researchers arrived at the Pacific Grove Monarch Sanctuary in California to find hundreds of dead or dying butterflies scattered across the site where Western monarchs spend their winters. What they discovered in the lab was sobering: each butterfly carried an average of seven different pesticides in its body.
Using liquid and gas chromatography and mass spectrometry, the research team—led by Staci Cibotti, a pesticide risk prevention specialist at the Xerces Society for Invertebrate Conservation—found a cocktail of 15 insecticides, herbicides, and fungicides. Three pyrethroid insecticides stood out: bifenthrin, cypermethrin, and permethrin were present at or near lethal doses in every sample tested. The findings, now published in Environmental Toxicology and Chemistry, point to neurotoxic poisoning as the likely cause of death.
"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," Cibotti said. Western monarchs overwinter along the Pacific coast in a state of profound vulnerability—gathered in massive numbers, exhausted from migration, exposed to pesticide drift from farms and urban areas nearby.
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Start Your News DetoxThe Urgency Behind the Numbers
The 2024 die-off arrives at a critical moment. The Western monarch population has collapsed by nearly 95% since the 1980s. By 2025, overwintering Western monarchs numbered just 9,119 individuals. The International Union for Conservation of Nature lists them as endangered. The U.S. Fish and Wildlife Service estimates a 99% chance of extinction by 2080.
Yet this study also reveals something actionable: the threat isn't invisible or inevitable. Pesticides are everywhere—in agriculture, yes, but also in home gardens, landscaping, mosquito control, and even some natural areas managing forest pests. They're a choice, which means they can be unmade.
What Happens Now
The Xerces Society has outlined a path forward. Pesticide-free zones around overwintering sites. Stronger tracking and coordination from public officials. Greater protections written into conservation plans. Education campaigns that reach farmers, homeowners, and landscapers about safer alternatives.
Emily May, co-author of the study and agricultural conservation lead at Xerces Society, frames it plainly: "Protecting monarchs from pesticides will require both public education and policy change. We are committed to working with communities and decision-makers to ensure that overwintering sites are healthy refuges for these butterflies."
The vulnerability of monarchs during migration and overwintering—when they gather in the thousands—means a single pesticide application can wipe out hundreds of individuals in one moment. It happened in Pacific Grove in 2024. It happened in North Dakota in 2020. But it also means that preventing those applications, during those weeks, in those places, could be the difference between extinction and recovery.
