A 2024 mass monarch butterfly die off in southern 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