No Pollen? Bite me! Bumble bees make plants hurry up and grow flowers

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I recently discovered even more evidence that bumble bees are awesome.

A team of scientists in Switzerland and France saw bumble bees biting holes in the leaves of some black mustard, eggplant, and silverleaf nightshade plants. No one had ever reported bumble bees biting leaves before. From what the scientists could see, the bees were not eating the leaves. They were also not bringing pieces of leaves back to their nest. So why the heck were they biting them?

A bumble bee biting a leaf. Source: Scientific American.

The scientists knew that when some plants experience stress they tend to grow flowers. The plants that the bumble bees were biting did not have any flowers yet. Were the bees trying to speed up the plants’ flowering process?

Also, the scientists suspected that the bumble bees were after pollen. The bees were starting new colonies, and larvae (baby bees) need pollen as a protein source to grow. The bees might have been saying to the flowers, “Hurry up and flower! We need some pollen!”

This was a perfect circumstance for an experiment! And they way the scientists investigated this leaf-biting behaviour was very clever. They used laboratory and field experiments.

First, the laboratory experiments. The scientists let buff-tailed bumble bees (Bombus terrestris, widely found in Europe) fly around tomato and black mustard plants that only had leaves but no flowers. After the bumble bees had bitten 5 to 10 holes in the plants’ leaves, the scientists removed the plants. Then they did something really smart: They paired each bee-damaged plant with a plant that they had damaged themselves using forceps and a razor, to copy as closely as possible the way the bees had bitten the leaves. The result? The bee-damaged plants sprouted flowers up to 30 days earlier than normal, undamaged plants or plants that were damaged by the scientists. By biting the leaves, the bumble bees were speeding up the plants’ flowering process! Amazing!

A: A bumble bee using its proboscis (tongue) to pierce a hole in an eggplant leaf. B: A bumble bee biting a leaf with its mandibles (mouthparts). C: What bee bite-holes look like in the leaves. Source: Science journal.

For the next laboratory experiment, they gave some colonies of bumble bees lots of pollen in their nest, whereas for other colonies they gave none: the bees had to go out and get it themselves. The scientists then let bees from both types of colonies (pollen-rich or pollen-deprived) fly around young black mustard plants that had no flowers yet. They found that bees from pollen-deprived colonies bit holes in the plants’ leaves way more often than the bees from pollen-rich colonies. This is a convincing sign that the bees were biting the leaves in order to “tell” the plants that they needed pollen.

And finally, the field experiment. The scientists wanted to make sure that this leaf-biting behaviour happens in natural environments and not just in laboratories. So, they put some young colonies of bumble bees on the rooftop of a university building, along with some plants that had not flowered yet. They did this from March to May. This timing was important. Their bees were free to fly wherever they wanted to get food, but in March there wouldn’t be any flowers anywhere yet. But come May, there would be plenty of flowers to be found beyond the rooftop garden. Sure enough, the scientists discovered that the bees damaged many more plants in the early part of spring when no flowers could be found, but that damage decreased as spring progressed and flowers started to appear elsewhere.

The field experiment was also run between June and July, when plants are in full bloom. The scientists offered their rooftop colonies of bees some plants that had not yet flowered. The result? There was way less leaf damage to the flowerless plants they had placed near the colonies, presumably because the bees were flying farther to get pollen elsewhere. But the really cool part? They saw different species wild bumble bees biting holes in the leaves, too–not just bees from their own colonies! Wild bees, then, bite leaves as well.

What I think is so cool about all of this is that bumble bees have somehow figured out a way to make plants bloom faster when they are just establishing their families and are in need of pollen. This could be an adaptation to cope with climate change: cooler springs make it less likely that flowers will bloom on time, so bees help them speed up. And what’s even extra cool is that it is not just leaf damage alone that causes the flowers to come out early: something about bee bites in particular makes the plants sprout flowers. Are they injecting some kind of chemical into the plant? Do bumble bees have magic spit?

Sigh…bumble bees are amazing.

References

Pashalidou, F. G., Lambert, H., Peybernes, T., Mescher, M. C., & De Moraes, C. M. (2020). Bumble bees damage plant leaves and accelerate flower production when pollen is scarce. Science, 368, 881-884. DOI: 10.1126/science.aay0496

Chittka, L. (2020). The secret lives of bees as horticulturists? Pollen-starved bumble bees may manipulate plants to fast-forward flowering. Science, 368, 824-825. DOI: 10.1126/science.abc2451