In June 2013, in a Target parking lot in Wilsonville, Ore., an estimated 50,000 bumblebees dropped dead. Shoppers reported bees falling from branches and crawling on the ground. Piles of carcasses scattered beneath dozens of linden trees marked the largest mass bee kill ever recorded.

The Oregon Department of Agriculture later determined that a pesticide used against aphids had poisoned the bees. They banned its use on lindens. But still more reports surfaced of mass bumblebee deaths around Oregon. Explaining them wasn’t as simple.

“It’s not your classic story of, oh, the pesticides kill the bees,” said Sujaya Rao, an entomologist who studied the bee deaths while at Oregon State University.

“Not every dead bee could be because of a pesticide,” she added. “There are strange phenomena in nature that lead to things like this.”