Pyrethroid insecticides implicated in mass mortality of monarch butterflies at an overwintering site in California.

Since the 1980s, monarch butterfly (Danaus plexippus plexippus) populations across North America have declined by 80%-95%. Although several studies have implicated pesticides as a contributing factor to their population declines, our understanding of monarch exposure levels in nature remains limited. In January 2024, a mass mortality event near an overwintering site in Pacific Grove, California, USA, provided an opportunity to analyze dead overwintering monarch butterflies for pesticide residues. Ten recently deceased butterflies were collected and analyzed using liquid and gas chromatography with tandem mass spectrometry. We identified 15 pesticides and associated metabolites in the butterflies, including eight insecticides (plus one associated metabolite), two herbicides (plus two associated metabolites), and two fungicides. On average, each monarch butterfly contained seven pesticides, excluding transformation products if the parent compound was also detected. Notably, three pyrethroid insecticides-bifenthrin, cypermethrin, and permethrin-were consistently detected at or near each chemical's lethal dose. Bifenthrin and cypermethrin were found in every sample, whereas permethrin was present in all but two samples. The average concentrations of these insecticides were 451.9 ng/g dry weight for bifenthrin, 646.9 ng/g dry weight for cypermethrin, and 337.1 ng/g dry weight for permethrin. These findings demonstrate pesticide contamination in monarch butterflies, including within urban areas, and highlight the risks pesticides, especially insecticides, pose to monarch populations. Additional measures may be required to safeguard this species from pesticide exposure, particularly near aggregation locations, such as overwintering sites in coastal California.