"The Silent Crisis: Pesticides Linked to Monarch Butterfly Mass Die-Off in California"

Pesticides Linked to Mass Die-Off of Western Monarch Butterflies in 2024

Los Angeles, CA — A recent peer-reviewed study has established a concerning link between pesticide exposure and a significant die-off of Western monarch butterflies, which occurred in January 2024. Researchers discovered hundreds of dead or dying butterflies near the Pacific Grove Monarch Sanctuary in California, a critical overwintering site for these iconic insects.

The study, published in the journal Environmental Toxicology and Chemistry, revealed that the butterflies exhibited signs of neurotoxic pesticide poisoning. Advanced testing methods, including liquid and gas chromatography and mass spectrometry, detected a cocktail of 15 different pesticides, including insecticides, herbicides, and fungicides, on the deceased butterflies.

Staci Cibotti, the lead author of the study and a pesticide risk prevention specialist at the Xerces Society for Invertebrate Conservation, stated, “We found an average of seven different pesticides per butterfly, including multiple insecticides that are highly toxic to insects.” Notably, three pyrethroid insecticides—bifenthrin, cypermethrin, and permethrin—were found at or near lethal doses in the samples tested.

Declining Monarch Populations

The Western monarch butterfly population has seen a staggering decline of nearly 95% since the 1980s, according to the Xerces Society. The annual Western Monarch Count reported that in 2024, the population reached its second-lowest number on record, with only 9,119 individuals counted during the overwintering period in 2025. This alarming trend has led the International Union for Conservation of Nature (IUCN) to classify migratory monarch butterflies as endangered, with predictions indicating a 99% chance of extinction by 2080, as reported by the Los Angeles Times.

Pesticide Exposure Risks

The study highlights the vulnerability of Western monarchs to pesticide residues and drift from agricultural and urban areas. Cibotti emphasized that the high levels of pesticides detected suggest a direct link to the deaths of the monarchs, although a review by Monterey County could not pinpoint the exact source of the chemicals.

“Protecting monarchs from pesticides will require both public education and policy change,” said Emily May, co-author of the study and agricultural conservation lead at the Xerces Society. The organization has proposed several recommendations, including:

• Increased education about pesticide risks and safer alternatives.
• Establishment of pesticide-free zones around overwintering sites.
• Enhanced pesticide exposure protections in conservation and recovery plans for butterflies.
• Improved coordination and tracking of pesticide risks by public officials.

Broader Implications

The implications of this study extend beyond California. Cibotti noted that pesticide exposure is a widespread threat to monarchs across their migratory range. These chemicals are commonly used not only in agriculture but also in urban landscaping, home gardening, and even in natural areas for pest management.

The vulnerability of monarchs during migration and overwintering periods is particularly concerning, as these times see large congregations of butterflies, making them susceptible to mass die-offs from single pesticide applications. This was previously observed in a similar incident in North Dakota in 2020, where hundreds of monarchs died following a mosquito control spray during migration.

Conclusion

As the Western monarch butterfly faces unprecedented challenges, the findings of this study underscore the urgent need for protective measures against pesticide exposure. The Xerces Society remains committed to collaborating with communities and policymakers to ensure that overwintering sites remain safe havens for these vital pollinators.

For more information on how to support monarch conservation efforts, visit the Xerces Society’s website.

About the Author: Paige is a Los Angeles-based writer passionate about sustainability. She holds a Bachelor’s degree in Journalism from Ohio University and specializes in sustainable agriculture.