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. 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. Though an investigation by a state agricultural official did not determine a source of the die-off, 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.