Tiny gardeners: bumblebees bite plants to make them flower early - By Olivia Pool

Bumblebees are seasoned professionals when it comes to pollination. They buzz around our gardens detecting which plants will provide the most nectar. They even leave oily footprints on petals to signal which flowers they have visited, saving wasted trips to flowers whose nectar has already been harvested. As well as their acute ability to detect nectar, bumblebees are perfectly adapted to transport pollen between plants. Their fuzzy bodies are covered in forked hairs which trap pollen, and they have mastered ‘buzz pollination,’ contracting their flight muscles to vibrate the anthers and release the pollen packed inside. Adaptations like these enable bumblebees to fertilise a broad swathe of wild flora, as well as crucial agricultural crops such as tomatoes, blueberries and squash. Without bumblebees, our landscapes would be far less diverse and our diets less varied.

Scientists have long marvelled at the bumblebee’s fine-tuned adaptations for pollination and it seems there is always something more to uncover. In 2013, scientists discovered that bumblebees use the tiny hairs covering their bodies to sense electric fields around flowers. While sharks and rays are known to have an electric sense, this was the first time the ability had been detected in an insect. Now, new research has revealed bumblebee’s can act as tiny gardeners, forcing plants to flower by nibbling their leaves. This has baffled bee biologists, like Neal Williams, who exclaimed “how did we miss this? How could no one have seen it before?” on hearing the news.

A baffling breakthrough

The study, published in Science last month, showed that bumblebees actively damage plants when faced with a shortage of pollen, forcing them to flower up to a month earlier than usual. This unusual behaviour was first noted by a team of ecologists in Zurich, who observed buff-tailed bumblebees biting the leaves of their greenhouse plants. The bees weren’t taking the leaf cuttings to their nests or ingesting them. So, they set up a series of experiments to test whether the biting could be stimulating flowering.

A colony of pollen-deprived buff-tailed bumblebees was placed in a mesh cage with flowerless tomato and black mustard plants. They let the worker bees make between 5-10 incisions on each plant, then removed the plants from the cage. As a result, the black mustard plants bloomed two weeks earlier, and the tomato plants a month earlier, than expected. The researchers also compared behaviours between pollen-fed and pollen-deprived bees. While workers from the pollen-deprived colony actively damaged leaves, the workers from the pollen-fed colony rarely did so, confirming that the bees elicit this behaviour when pollen is limited. To make sure this behaviour wasn’t influenced by artificial lab conditions, researchers placed the bumblebee colonies and a variety of flowerless plants on their rooftop. Straightaway, the bees set to work nibbling the plants closest to their hives. This activity tapered off at the end of spring as more local flowers came into bloom, providing further evidence that the behaviour is driven by pollen availability.

This discovery marks a huge step forward in our understanding of how bumblebees communicate with the flora they pollinate. Bumblebees and flowers mutually benefit from pollination, but neither benefit if they go out of sync. As behavioural ecologist, Lars Chittka explains, “in a sense, the bees are signalling, hey, we need food. Please speed up your flowering, and we’ll pollinate you.”

Climate change resilience?

Climate change is driving warmer temperatures in early spring, causing bumblebees to emerge before plants have bloomed in a process called trophic mismatch. This deprives them of vital food resources in early spring to feed themselves and their larvae. However, this research provides a glimmer of hope that bumblebees may be more resilient to the harmful effects of climate change than we anticipated. By stimulating flowering, bumblebees may be able to generate the resources they need to survive. During the rooftop experiment, the team noticed two other species from wild colonies also nibbling leaves, providing promising evidence that multiple species may be able to adapt to trophic mismatch.

Insights for agriculture?

Questions remain about how the bees’ nibbling stimulates flowering. When the team tried to recreate the incisions using forceps and razors, plants flowered earlier than normal but not as soon as they did in response to the bees’ bites. This suggests that an additional cue, such as a chemical compound in the bees’ saliva, may be involved. Identifying the mechanism behind this process could have huge implications for agriculture. If we uncovered the molecular pathway driving earlier flowering, crop production could be accelerated, allowing higher yields to be produced. Bumblebees have benefited farmers for millennia by pollinating their crops. Now, it seems we have even more to gain from these tiny gardeners. Identifying the molecular pathway inducing flowering could provide a whole new way to cultivate crops, generating a much-needed boon for agriculture as global food demand continues to rise.






Buff-tailed bumblebee image: Wikimedia Commons

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