Researchers discovered hundreds of butterflies that had died or were dying in January 2024 near an overwintering site, where the insects spend the winter months. The butterflies were found twitching or dead in piles, which are common signs of neurotoxic pesticide poisoning, researchers wrote.
The testing 10 insects revealed an average of 7 pesticides in each, and at levels that researchers suspect were lethal. Researchers identified 15 pesticides and associated metabolites in the butterflies, including 8 insecticides (plus one associated metabolite), 2 herbicides (plus 2 associated metabolites), and 2 fungicides. On average, each monarch butterfly contained seven pesticides.
Notably, 3 pyrethroid insecticides—bifenthrin, cypermethrin, and permethrin—were consistently detected at or near each chemical’s lethal dose. Bifenthrin and cypermethrin were found in every sample, whereas permethrin was present in all but two samples.
Proving that pesticides kill butterflies in the wild is a challenge because it is difficult to find and test them soon after they die. Though the sample size is limited, the authors wrote, the findings provide “meaningful insight” into the die-off and broader population decline.
“The incident gave us a rare opportunity to directly document pesticide exposure and its impacts on monarchs in the real world,” said Staci Cibotti, the study’s lead author, and an entomologist and pesticide program specialist with the Xerces Society for Invertebrate Conservation.
She added: “Even though laboratory studies and population models have shown that pesticides are harmful to monarchs, it can be difficult to capture the impacts of pesticides in the field on wild populations. This study helps to fill that gap.”
Source:
Staci Cibotti, Michelle L Hladik, Emily May, Emma Pelton, Timothy A Bargar, Natalie Johnston, Aimee Code, Pyrethroid insecticides implicated in mass mortality of monarch butterflies at an overwintering site in California, Environmental Toxicology and Chemistry, 2025;, vgaf163, https://doi.org/10.1093/etojnl/vgaf163
