Study: 2024 California Monarch Dieoff Likely Caused By Exposure To Multiple Pesticides

A 2024 mass monarch butterfly die-off in California was probably caused by pesticide exposure, new peer-reviewed research finds, adding difficult-to-obtain evidence to the theory that pesticides are partly behind dramatic declines in monarchs’ numbers in recent decades. Researchers discovered hundreds of butterflies that had died or were dying in January 2024 near an overwintering site, where insects spend winter months. The butterflies were found twitching or dead in piles, which are common signs of neurotoxic pesticide poisoning, researchers wrote.

Testing of 10 of the insects revealed an average of seven pesticides in each, and at levels that researchers suspect were lethal. 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.”

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https://www.theguardian.com/us-news/2025/aug/01/monarch-butterflies-mass-die-off-pesticides