Pollen-deprived bumblebees may speed up plant blooming by biting leaves

In a pollen shortage, bees can make tomatoes bloom early by nipping foliage

bumblebee

A bumblebee sinks its mouthparts into a plant leaf to nip a small hole. Little cuts like this seem to jolt plants into starting to flower early — good news for a bee deprived of pollen.

Hannier Pulido, C. De Moraes, Mescher Laboratories

Here’s a bumblebee tip that might get a slowpoke plant to bloom early. Just bite its leaves.

At least three species of bumblebees use their mouthparts to snip little confetti bits out of plant foliage, researchers report in the May 22 Science. This foliage biting gets more common when there’s a pollen shortage, says Consuelo De Moraes, a chemical ecologist and entomologist at ETH Zurich.

Experiments show that mustard and tomato plants nibbled by Bombus terrestris bees bloomed earlier than unbitten plants by days, or even weeks, say De Moraes and her colleagues. So for the bumblebees, accelerating bloom times could be a lifesaver. When trying to found colonies in early spring, the bees rely on flower pollen as a protein source for raising their young.

Foteini Paschalidou, an ecologist now at France’s National Institute for Agricultural Research in Versailles-Grignon, was the first team member to call attention to the behavior. She was working on a different project with caged B. terrestris bees indoors. At first, De Moraes worried. “Is it something wrong with them?”

The bees’ supplier and some farmers who used them to pollinate crops assured the researchers that nipping happens elsewhere, although the team hasn’t found any accounts in the scientific literature.

To test a link between leaf biting and pollen shortages, the researchers did a caged-bee test. After three days without pollen, bumblebees trapped with nonblooming plants were more likely to poke holes in foliage than a bee group buzzing among plentiful flowers. When researchers swapped the bees’ situations, the insects now trapped without blooms started nibbling leaves.

Tests done on the roof of the lab building with bees free to seek flowers in rooftop planters and elsewhere also found a link between pollen shortage and increased leaf biting, the researchers report.

Bee on leaf, leaf with holes
During a pollen shortage, a bumblebee cruising a leaf (left) becomes more likely to make quick punctures and cuts that leave small holes (right).F. Pashalidou et al/Science 2020

The notion that bee damage to a leaf could jump-start flowering originally struck coauthor Mark Mescher of ETH Zurich as a long shot. Yet in lab tests, tomato plants punctured five to 10 times by pollen-deprived bees bloomed 30 days earlier on average than undamaged plants. But the speed-up time varied by plant species. For instance, bee-nipped black mustard (Brassica nigra) bloomed only about 16 days early.

The bee-pestered plants’ acceleration is not entirely unprecedented. Some other forms of stress, including drought, skimpy nutrients and assault by leaf-eating insect pests, also have triggered early blooming, Mescher points out. But just what’s going on with the bee bites and how they might tap into the internal clock that triggers a plant to switch from leafing to flowering remain big questions.

So far, the best efforts of human scientists waiting with forceps and a razor on a lab rooftop to mimic bee activity in real time, bite by bite, on comparison plants have produced only modest acceleration in the black mustard, and no meaningful change in the tomatoes. So there might be something special in a bee bite.

In a happy accident, the outdoor trials attracted visits from two other Bombus species that checked out the plants on offer and also nicked holes in leaves. That confirms that leaf nibbling is not just some quirk of a commercial lineage of bees, although two long-time bumblebee watchers — Dave Goulson at the University of Sussex in England and Lynn Adler at the University of Massachusetts Amherst — say they’ve never noticed it.

Goulson says he’s fascinated by the idea. B. terrestris commonly cuts holes in plant parts, but in a slightly different context. Instead of groping for nectar through the natural openings of flowers, these and other bumblebees often just bite little holes through the outer wall of a flower for a sip. “I can imagine that hungry bees unable to find flowers might try biting leaves in desperation,” Goulson says. Flower biting might thus have evolved into leaf biting, though, as Mescher points out, it could have happened the opposite way, too.

With those intriguing ideas buzzing around, clearly now is a great time to go watch bees.

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