Study Suggests Pesticides Caused Butterfly Die-Off in California

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 big declines in monarchs' numbers in recent decades.

- Tom Perkins

Researchers discovered hundreds of butterflies that had died or were dying in January 2024 near an over-wintering site. They were found twitching or dead in piles, 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's difficult to find and test them soon after they die. Although the sample size was limited, the authors wrote, the findings provided "meaningful insight" into the die-off and broader population decline.