Vulnerability in numbers: Pyrethroid exposure implicated in a mass mortality of overwintering monarchs in California

Monarch butterfly (Danaus plexippus plexippus) populations across North America have declined by 80–95% over the past 40 years. Although several studies have identified pesticides as a contributing factor in monarch declines, data on their exposure levels in natural settings remains limited. In January 2024, a mass mortality event adjacent to an overwintering site in Pacific Grove, California, USA, provided an opportunity to analyze deceased overwintering monarch butterflies for pesticide residues. Ten recently expired butterflies were collected and analyzed using liquid and gas chromatography with tandem mass spectrometry (LC-MS/MS and GC-MS/MS). In total, 15 pesticides and metabolites were detected in the butterflies, including 8 insecticides (and 1 associated metabolite), 2 herbicides (and 2 associated metabolites), and 2 fungicides. On average, each butterfly contained 7 pesticides, excluding transformation products if the parent compound was co-detected. Notably, concentrations of 3 pyrethroid insecticides (bifenthrin, cypermethrin, and permethrin) were consistently detected at or near a known or presumed lethal dose (LD50). Bifenthrin and cypermethrin were detected in every sample, while permethrin was found in all but two. The average concentrations of these insecticides were 452 nanograms per gram (ng/g) for bifenthrin, 647 ng/g for cypermethrin, and 337 ng/g for permethrin. These findings demonstrate pesticide contamination in monarch butterflies overwintering within an urban area, and underscore the risks these compounds pose to monarch populations. Additional measures to protect this species from pesticide exposure, particularly near key aggregation sites such as overwintering areas along the California coast, may be valuable to support their conservation.