Bumble bees are plant bullies
Bumble bees need pollen and nectar to survive. When plants don’t flower and pollen becomes scarce, their survival is at stake. Luckily, bumble bees have their own strategy to make plants flower and thus produce pollen.
Why do bumble bees need pollen in the first place?
While nectar is their source energy due to its wide range of complex sugars, pollen is essential for bumble bees as it provides them with proteins and fats. Moreover, it’s the only food source for their larvae.
Did you know that the pollination technique that bumble bees are using is called “buzz pollination”? In contrast to other bees, bumble bees don’t climb into flowers to collect pollen, but hang onto the petals and vibrate the muscles they usually use to fly, which produces the famous “buzz” sound. Pollen then falls out of the flower and onto the bumble bee and is collected with the help of their spit and stored in specialized pollen baskets (corbicula). Only female (workers) bumble bees have a corbicula.
What do bumble bees do when there are not enough flowering plants?
During winter, the bumble bee queen remains in hibernation until next spring. The moment temperatures begin to rise, the queen emerges from her sleep and feeds on nectar to regain energy. After she found a suitable nest for the next colony, she proceeds to lay the first eggs. These will be her first worker bees, which are all female. It's not uncommon to have a mismatch between the first early flowers and the awakening of the bumble bee queen or the hatching of the first worker bees in early spring. The resulting pollen scarcity could be fatal.
Luckily, researchers discovered that bumble bees have their own strategy to deal with pollen scarcity: they damage non flowering plants to make them flower. It has been shown that bumble bees use the tools attached to their mouth (proboscises and mandibles) to cut small holes in plant leaves (see the picture below). The scientists excluded that bumble bees cut the leaves to feed on them or to transport them back to the hive to use them as some sort of material.
Interestingly, when scientist imitated the damage by the bumble bees with metal forceps and razors, plants harmed by the bumble bees still flowered faster. The effect is impressive: Tomato plants damaged by bumble bees flowered 30 days earlier compared to undamaged plants. But they also flowered 25 days earlier than mechanically damaged plants. Similar results were seen in black mustard. Here, plants flowered 16 days earlier if damaged by bees or 8 days earlier if damaged mechanically. It is not yet clear why bumble bee damage shows a different effect than mechanical damage. Maybe it's a specific molecule in their spit that enters the plant through the holes and induces a particular response? Hopefully scientists are able to answer this question soon.
Last but no least, have a look at the video of a bumble bee "bullying" a plant by Hannier Pulido, courtesy of the De Moraes and Mescher Laboratories ;)
I hope you enjoyed today's rather short but highly interesting article.
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(6493), 881-884.
Video by Hannier Pulido, courtesy of the De Moraes and Mescher Laboratories