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.”
To examine the other alleged death-by-linden cases in Oregon, Dr. Rao, now at the University of Minnesota, assembled a team of bee detectives who followed leads to various sites for observation and collection of actively foraging and dying bumblebees, and then drew connections between the cases.
After many dead ends, a pattern emerged: Deaths seemed to occur on cooler mornings, late in the blooming season, when trees start running out of nectar. So what did it mean?
Bumblebees are warm-blooded. To fly and forage, their thoraxes — where their wings and flight muscles are — must reach a temperature of 86 degrees Fahrenheit. To generate that, they shiver. But the cooler it is outside, the more energy the bee needs to do it.
In crawling bees near death, the team found compounds related to energy processing that were different from those in active and foraging bees, suggesting the dying bees didn’t have enough energy.
As the floral blooming season progresses, the linden tree runs out of nectar. Why, then, would the bees keep returning if they are able to detect nectar levels?
Alkaloids, like caffeine, can boost how rewarding bees find linden nectar. The team found an alkaloid called trigonelline that they think may attract bees.
They haven’t tested the idea, but they speculate that trigonelline could boost bees’ loyalty to linden even as nectar runs out. But when they visit on cool mornings without enough energy to heat up for a return flight to their nests, they fall to the ground, crawl around and die.
“It’s like you have a favorite Starbucks, and you keep going there,” said Dr. Rao. “And one time they’re out of coffee, and then you don’t have enough gas to go to the next Starbucks. And so you’re stranded.”
Hauke Koch, a biologist who was not involved in the new study, said the story is plausible. He previously led a team at Royal Botanic Gardens, Kew, in England, that identified a few ways these mass deaths can occur, and also suggested that bumblebees may become so loyal to linden trees that they would endure starvation.
But Dr. Koch wonders what direct tests of the new study’s hypotheses will reveal, or what would happen to the bees if other flowering plants were available.
The authors stress their findings don’t mean we should stop planting linden trees. Instead, they call for planting flowers alongside to supplement bees throughout the season. Even when these bees die, many more survive and carry on their colonies’ legacy.