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Bumblebees speed up flowering

Due to climate change, seasonal anomalies are becoming more common – this year, most plants were already in full bloom around three or four weeks earlier than normal. These issues threatens to disturb the timing of relationships between the plants and their insect pollinators.

A research team, led by ETH Professors Mescher and De Moraes, have discovered that bumblebees have a unique behaviour which may help to overcome such challenges. Bumblebee workers have been found to use their mouth parts to pinch the leaves of plants that are yet to flower – the damage this causes stimulates the plant to produce new flowers and bloom earlier.

“Previous work has shown that various kinds of stress can induce plants to flower, but the role of bee-inflicted damage in accelerating flower production was unexpected,” Mescher says.

Surprising behaviour from bumblebees

The team first discovered the behaviour while other experiments were being conducted by one of the authors, when the bees were noted biting the leaves of test plants in the greenhouse.

“On further investigation, we found that others had also observed such behaviours, but no one had explored what the bees were doing to the plants or reported an effect on flower production,” Mescher explains.

The observations led to several new experiments being set up by the team, both in the laboratory and outdoors, using commercially available bumblebee colonies and various plant species.

The lab studies showed that the bees’ tendency to damage the leaves strongly correlated with how much pollen they could obtain – the leaves were damaged much more frequently when the bees had little or no pollen available. They also found that the damage had a dramatic effect on flowering time in two different plant species. Tomato plants flowered up to 30 days earlier when targeted by the bees, while mustard plants flowered around 14 days earlier when damaged.

“The bee damage had a dramatic influence on the flowering of the plants — one that has never been described before,” De Moraes says. The evidence also suggests that the developmental stage of the plant may influence how much the flowering is accelerated, and the researchers plan to look closer at this in future.

The team also attempted to manually replicate the damage caused by the bees in order to examine whether it was possible to reproduce the effects on flowering time. Although this led to slightly earlier flowering in both plant species, the effect was not as significant as that caused by the bees. For this reason, De Moraes suggests that a chemical or other cue could also be involved. “Either that or our manual imitation of the damage wasn’t accurate enough,” she says. The team is currently attempting to identify the exact cues responsible, and characterising the molecular mechanisms involved in the plant response.

Phenomenon also observed in the field

The research team also observed the bees’ behaviour under more natural conditions. The team conducted follow-up studies on the rooftops of two buildings in central Zurich. The observations of these experiments showed that hungry bees with insufficient pollen supplies regularly damaged the leaves of non-flowering plants. In line with the lab studies, this behaviour was reduced when the team made more flowers available to the bees.

Additionally, it was not only captive-bred bumblebees from the researchers’ colonies that damaged the leaves – wild bees from at least two different bumblebee species were observed biting the leaves in the experimental plots. Other pollinating insects, such as honeybees, did not exhibit the biting behaviour – although they frequently visited flowering plants within the plots, they ignored the non-flowering plants altogether.

Delicate balance starting to tip

“Bumblebees may have found an effective method of mitigating local shortages of pollen,” De Moraes says. “Our open fields are abuzz with other pollinators, too, which may also benefit from the bumblebees’ efforts.” However, this mechanism may not be sufficient to overcome the difficulties of climate change. Flowering plants and their pollinators have evolved together, and so the balance between efflorescence and pollinator development is a delicate one.

Climate change and other environmental changes brought about by humans may disrupt the timing of these interactions and other important beneficial interactions between species further. Rapid environmental change may cause plants and insects to become even further out of sync in their development. “And that’s something from which both sides stand to lose,” Mescher says.

Foteini G. Pashalidou, Harriet Lambert, Thomas Peybernes, Mark C. Mescher, Consuelo M. De Moraes. Bumble bees damage plant leaves and accelerate flower production when pollen is scarceScience, 2020; 368 (6493): 881 DOI: 10.1126/science.aay0496

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