Lab Tests Point to Culprit in Monarch Butterfly Die-Off

Hundreds of monarch butterflies mysteriously died in southern California last year, and new research points to a likely culprit lurking in the landscape: common pesticides. Scientists say these peer-reviewed findings published in Environmental Toxicology and Chemistry offer rare, direct evidence linking chemical exposure to the monarchs' dramatic decline, reports the Guardian.

• Scientists examining hundreds of monarchs found dead or dying near the Pacific Grove Monarch Sanctuary in January 2024 say the symptoms—twitching and clustered fatalities—pointed to neurotoxic poisoning. Lab analysis of 10 specimens confirmed each had traces of seven different pesticides at levels believed to be lethal to the insects.

• All tested butterflies carried high concentrations of three pyrethroids, a pesticide class broadly used in both agriculture and residential settings across California. Notably, researchers suspect the source may have been local residential applications, which are not closely monitored.
• "The incident gave us a rare opportunity to directly document pesticide exposure and its impacts on monarchs in the real world," said lead author Staci Cibotti of the Xerces Society for Invertebrate Conservation.
• Monarch populations in North America have crashed by an estimated 90% since the 1980s, per Environmental Health News. The butterfly may soon get protection under the Endangered Species Act.