But then some of them die.
In the 1970s, scientists fed eight bees nectar from Tilia flowers, and they died too. It was determined that nectar from the tree contained a toxic sugar, called mannose, that poisoned and killed the bees. The theory pervaded public opinion and scientific literature for years.
Not so fast. In the 1990s, scientists used more advanced techniques to search for the sugar again in the nectar and in the bodies of dying bees that fed on the trees. There wasn’t much there. And when they fed the bees the tree’s nectar, they survived. In the earlier experiment, the bees had most likely died of starvation.
Let’s move on, to Wilsonville, Ore., in 2013: An estimated 50,000 bumblebees dropped dead in a Target parking lot. The Tilia trees in the lot had been sprayed with dinotefuran, one of a class of pesticides called neonicotinoids that scientists and environmentalists have accused (with not-so-clear cut data) of being harmful to bees.
Six additional incidents were reported across the state, according to Aimée Code, a pesticide program director at the Xerces Society for Invertebrate Conservation. In 2015, Oregon banned the use of four pesticides on Linden trees, and the mass bee deaths seemed to stop.
So the pesticides may have contributed in these cases. But what about historical accounts — could it just be nature taking its course?
“There are so many bees on Tilia trees in the height of summer, some of them are going to die and just drop to the ground because they’re old,” Dr. Stevenson said. Or predators, like wasps, may kill them. Indeed, all bumblebees die at the end of the season, except for the queen, who stores up food and hibernates to start a new hive the next spring.
But Dr. Stevenson and his colleague found dead bees even in the absence of predator attacks. And an informal autopsy of bee carcasses revealed that age didn’t seem to make a difference.
Could it be that the bees simply starved? Most deaths occur at the end of the blooming season, when Tilia nectar supply becomes limited, and the bees have less energy stored up to keep them going.
But there’s a problem with this hypothesis: “We get dead bees at Kew even when there’s other stuff around,” Dr. Stevenson said.
This case isn’t closed, but here’s what the researchers believe following their review: The bumblebees end up relying too much on the Tilia as a food source because they form strong associations with its odor, color or flower shape. It’s even possible that nicotine or caffeine, which some evidence suggests is in linden nectar, enhances these associations (as it has for honeybees with citrus and coffee plants). More experimentation is required, but the bees may continue visiting Tilia, perhaps instead of other flowers, even when there’s nothing left to eat.
“It takes them too long to realize that’s not a good source of nectar before they drop to the ground,” Dr. Stevenson said.
But Dr. Stevenson adds that this is only a partial explanation. Any combination of stressors — especially around weak, hungry bees — could be to blame.
“It’s a strange phenomenon,” he said. “The most exciting thing for me is we know what’s happening, but we can’t explain why.”