Mass Kill of Monarch Butterflies in California Linked to Pesticide Residues in Their Bodies

(Beyond Pesticides, July 16, 2025) A study following a mass mortality event of approximately 200 monarch butterflies (Danaus plexippus plexippus) in Pacific Grove, California, highlights the role of pesticides, synthetic pyrethroids in particular, in causing lethal and sublethal effects to nontarget organisms. The research, published in Environmental Toxicology and Chemistry, detects residues of 15 pesticides and associated metabolites in the bodies of 10 deceased butterflies collected from the January 2024 event that occurred near an overwintering site frequented by monarchs.

“On average, each monarch butterfly contained 7 pesticides,” the authors report. They continue, “Notably, three pyrethroid insecticides—bifenthrin, cypermethrin, and permethrin—were consistently detected at or near each chemical’s lethal dose (LD50).” LC50, or Lethal Concentration 50, values represent the concentrations of chemicals lethal to 50% of a test population.

To assess pesticide residues within ten of the deceased butterflies, the researchers use liquid and gas chromatography with tandem mass spectrometry (LC-MS/MS and GC-MS/MS) and find “a total of 15 pesticides and associated metabolites in the butterflies, including 8 insecticides (plus 1 associated metabolite), 2 herbicides (plus 2 associated metabolites), and 2 fungicides.”

The study “present[s] evidence that the mortality incident at the Pacific Grove Monarch overwintering site was likely caused by nontarget pesticide poisoning,” the authors state. “These findings demonstrate pesticide contamination in monarch butterflies, including within urban areas, and highlight the risks pesticides, especially insecticides, pose to monarch populations.”