A 2024 mass monarch butterfly die-off in California was probably caused by pesticide exposure, new peer-reviewed research finds, adding difficult-to-obtain evidence to the theory that pesticides are partly behind dramatic declines in monarchs’ numbers in recent decades.
Researchers discovered hundreds of butterflies that had died or were dying in January 2024 near an overwintering site, where insects spend winter months. The butterflies were found twitching or dead in piles, which are common signs of neurotoxic pesticide poisoning, researchers wrote.
Testing of 10 of the insects revealed an average of seven pesticides in each, and at levels that researchers suspect were lethal. Proving that pesticides kill butterflies in the wild is a challenge because it is difficult to find and test them soon after they die. Though the sample size is limited, the authors wrote, the findings provide “meaningful insight” into the die-off and broader population decline.
“The incident gave us a rare opportunity to directly document pesticide exposure and its impacts on monarchs in the real world,” said Staci Cibotti, the study’s lead author, and an entomologist and pesticide program specialist with the Xerces Society for Invertebrate Conservation.
She added: “Even though laboratory studies and population models have shown that pesticides are harmful to monarchs, it can be difficult to capture the impacts of pesticides in the field on wild populations. This study helps to fill that gap.”
As much as 90% of the monarch butterfly population in some US regions has been wiped out in recent decades, and evidence has pointed to pesticides, climate crisis and habitat loss as the drivers.
The butterflies were found adjacent to the Pacific Grove Monarch Sanctuary, one of about 400 wintering sites along California’s coast that are crucial points in the monarchs’ migratory and reproductive cycles. Xerces counted about 6,600 butterflies overwintering at Pacific Grove in November 2023.
Though an investigation by a state agricultural official did not determine a source of the die-off, pesticide residues on crops, lawns and ornamental flowers are a poisoning risk for monarch larvae, and for adults eating nectar. Pesticide run off in shallow water sources near where high numbers of butterflies collectively drink present a risk for the type of mass die-off at Pacific Grove.
All the butterflies showed high levels of the same three pyrethroids, a pesticide class widely used in California on agricultural and residential land. The residential applications are not reported, and Cibotti said the presence of the same types of pyrethroids in all the samples points to a nearby residential exposure as the source.
Some of the pyrethroid levels found in the monarchs are known to be lethal, and researchers suspect the combined levels of multiple pesticides also probably contributed. Combinations of pesticides can have a synergistic effect that makes them even more toxic.
The solution, Cibotti said, is less pesticides.
“These clustering events occur during especially sensitive phases of the migratory cycle, so reducing pesticide exposure during these times is essential not only to prevent immediate losses, but also to improve the population’s chances of rebounding the following spring and support its long-term recovery,” Cibotti said.
