Mass Kill of Monarch Butterflies in CA linked to Pesticides
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.