A mass die-off of monarchs gives researchers a rare look at poison use in urban settings.

It was a disturbing scene that Natalie Johnston and her team from the Pacific Grove Museum of Natural History came across on Jan. 25, 2024, on a property adjacent to the P.G. Monarch Butterfly Sanctuary. There on a lawn lay about 200 monarchs either dead or in the throes of death. The insects were spasming, their bodies curled, their wings flipped forward from the force of the spasms, all classic signs of pesticide poisoning.

“They’d come so far, they’d lasted so long,” says Johnston, fighting back tears, recalling how the monarchs traveled thousands of miles from the north to overwinter along the California coast. “All of that just for them to die in a place that should have been safe.”

Johnston, the interpretative programs manager for the museum, was there that day with staff and volunteers for a weekly count of the overwintering population inside the sanctuary and on private properties surrounding it. They found piles of monarchs on the lawn of a private residence, as well as dead or dying monarchs around the perimeter of the house.

That day they counted 2,000 monarchs, meaning 10 percent of the population was wiped out in a mass mortality. The die-off was significant in light of the fact that the Western monarch population has been fighting for existence, with numbers dwindling by 95 percent since the 1990s. Last December the U.S. Fish and Wildlife Service proposed listing monarchs as threatened under the Endangered Species Act.

The team contacted city officials who advised them to collect specimens. Officials talked to the property owner who denied using any pesticides. (The owner’s name and property location have not been disclosed.) They also contacted the Monterey County Agricultural Commissioner’s Office, as well as the Xerces Society, which tracks overwintering populations each year.

Eventually they connected with the U.S. Geological Survey, which agreed to determine which poisons were involved in the monarchs’ deaths. The results are now part of a recently published study, one of only a few that documents the impact of urban pesticide use on monarchs.

Researchers identified five pesticides and associated metabolites including eight insecticides, two herbicides and two fungicides. Each butterfly contained, on average, seven pesticides. Three insecticides were detected at or near their lethal dose. And while the source remains unidentified, the report suggests that the culprit could have been pyrethroid products used for outdoor ant control.

Staci Cibotti, pesticide risk prevention specialist for Xerces, says the incident highlights a point often overlooked: It’s not just in agriculture where monarchs and other pollinators face danger from pesticide exposure.

“This incident is a demonstration of how vulnerable monarchs are as they migrate and are overwintering,” she says. Cibotti suggests people rethink using pesticides in their yards. Even if products are sprayed on nonpermeable surfaces like walls, walkways and driveways, they can still wash off onto the surrounding soil.