A new study published in Environmental Toxicology and Chemistry documents a mass mortality event of monarch butterflies near one of California’s most iconic overwintering sites and links the deaths to pesticide contamination. In late January 2024, hundreds of monarchs were found dead or dying on a private property near the trees of Pacific Grove Monarch Sanctuary, where they typically cluster during the winter.The study was conducted by scientists from the Xerces Society for Invertebrate Conservation, the U.S. Geological Survey, and the Pacific Grove Museum of Natural History. Tissue analysis revealed that dead butterflies collected at the site in January 2024 contained residues from multiple pesticides, including several at levels known to be lethal to insects.“We found an average of seven different pesticides per butterfly, including multiple insecticides that are highly toxic to insects,” said Staci Cibotti, lead author and pesticide risk prevention specialist with the Xerces Society. “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.”The western monarch population has declined by nearly 95% since the 1980s, with fewer than 10,000 individuals counted in 2024, making losses of this magnitude particularly concerning. Approximately 200 of the 2,000 butterflies present at the Pacific Grove sanctuary in January 2024 were affected.