Bumblebees aren’t merely bumbling around our gardens. They’re actively assessing the plants, determining which flowers have the most nectar and pollen, and leaving behind scent marks that tell them which blooms they’ve already visited.
Now, a new study reveals that bumblebees force plants to flower by making tiny incisions in their leaves—a discovery that has stunned bee scientists.
“Wow! was my first reaction,” says Neal Williams, a bee biologist at the University of California, Davis. “Then I wondered, how did we miss this? How could no one have seen it before?”
Consuelo De Moraes, a chemical ecologist at the Swiss Federal Institute of Technology in Zurich, had the same reaction when one of her students, Foteini Pashalidou, noticed buff-tailed bumblebees making tiny incisions in the leaves of their greenhouse plants. The insects didn’t seem to be carrying off the bits of leaves to their nests or ingesting them.
Suspecting the bees were inducing the plants to flower, the team set up a series of experiments. The results show that when pollen sources are scarce, such as in a greenhouse or during early spring, bumblebees can force plants to bloom up to a month earlier than usual.
The research is promising for two reasons. For one, it strongly suggests bumblebees manipulate flowers, a particularly useful skill as warming temperatures worldwide are causing the pollinators to emerge before plants have bloomed. The insects depend nearly exclusively on pollen for food for themselves and their larvae in the early spring. (Read how bumblebees are going extinct in a time of climate chaos.)
It’s also a potential boost for the human food supply: If agriculturalists can coax their crops to flower early, it could increase food production of some plants.
Master gardeners
For the study, De Moraes, Pashalidou—the study’s lead author—and colleagues placed flowerless tomato and black mustard plants in mesh cages with pollen-deprived buff-tailed bumblebee colonies. They then removed the plants after worker bees made five to 10 holes in their leaves.
The small punctures caused the black mustard plants to flower two weeks earlier than usual, and the tomato plants a month sooner than normal, according to the study, which was published May 21 in Science.
The scientists also placed pollen-fed and pollen-deprived bumblebee colonies in mesh cages with the flowerless plants to compare their behaviors. Worker bees from the pollen-fed colonies rarely damaged the plants, while those from the pollen-deprived colonies busily did so.
To ensure that their results weren’t due to the lab’s artificial conditions, the scientists placed bumblebee colonies and a variety of flowerless plant species on their Zurich rooftop in late March 2018.
The bees—a very common European species—were free to forage as far afield as they liked. Yet they set to work damaging the leaves on all the nonflowering plants nearest to their hives. The bees’ interest in this activity tapered off toward the end of April as more local flowers came into bloom—again, establishing that the bees’ leaf-biting behavior is driven by the availability of pollen, the scientists say. (See seven intimate pictures that reveal the beauty of bees.)
They continued their rooftop experiment through July and found that wild workers from two other bumblebee species (B. lapidgrius and B. lucorum) came to their nonflowering patch of plants to puncture the leaves.
It remains to be seen how widespread the behavior is in other bumblebees, over 250 species of which are found around the world, the authors say.
Cracking the code
The mutually beneficial relationship between insect pollinators and flowers extends back some 130 million years. Plants provide the pollinators with food; in exchange, the pollinators fertilize their flowers. (Read more about nature’s “gold dusters.”)
But neither benefits if they’re out of synch with each other, so they’ve found ways to communicate.
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“That’s what this study shows,” says Lars Chittka, a behavioral ecologist at Queen Mary University of London, who wrote an essay accompanying the Science paper. “In a sense, the bees are signaling, Hey, we need food. Please speed up your flowering, and we’ll pollinate you.”
“It’s a very sophisticated type of communication,” adds Santiago Ramirez, a chemical ecologist at the University of California, Davis, who wasn’t involved in the study. “It seems bees have cracked the code that causes plants to flower.”
But many questions remain. Why do the incisions cause the plants to flower?
And, asks Chittka, “Does flowering early lead to higher fitness for the plants—meaning, do they have a larger number of offspring?”
Boost for agriculture?
When the study authors used metal forceps and a razor to mimic the holes the bees made, the plants bloomed earlier than normal, but not as soon as they did in response to the bees’ bites.
“They do something we haven’t quite captured,” says study co-author Mark Mescher, an evolutionary ecologist also at the Swiss institute. “It could be they introduce a biochemical or odor cue” from a saliva gland. “We hope to figure this out.”
Doing so could create a whole new way for humans to cultivate plants, a potentially major boon for agriculture. (Here are nine ways to support bees and other pollinators at home.)
For bee experts, one of the greatest marvels of the study is that it started with simple, old-fashioned observation.
“Charles Darwin followed bumblebees around,” says Williams. “Anyone interested in bumblebees has likely spent hours watching them on flowers. But probably not on plants that aren’t in bloom.”
Then Pashalidou did just that—and opened an entirely new phenomenon to our eyes.