Pesticide residues in monarch butterflies collected from Pacific Grove, California, USA in January 2024

In January 2024 adult monarch butterflies (Danaus plexippus (Linnaeus, 1758)) were found dead near their overwintering site in Pacific Grove, Monterey County, California, USA. Ten of the deceased monarchs (six females and four males) were collected from three sites by staff at the Pacific Grove Museum of Natural History and sent to the U.S. Geological Survey Organic Chemistry Research Laboratory for analysis. Each individual adult monarch was analyzed following methods in Black et al., 2023 and Gross et al., 2024a and 2024b for 181 pesticides and pesticide transformation products; 15 pesticides were detected and included 8 insecticides/insecticide transformation products, 2 fungicides, and 4 herbicides/herbicide transformation products. Concentrations of individual pesticides ranged from 1.0 to 2,645 nanograms per gram (ng/g) dry weight. Concentrations were highest for three pyrethroid insecticides; bifenthrin ranged from 18.8 to 855.3 ng/g, cypermethrin 6.7 to 2,645.1 ng/g and permethrin from not detected (less than 1.0 ng/g) to 1,349.3 ng/g, all other pesticides detected were less than 25 ng/g.