Bumblebees Bite Leaves of Flowerless Plants to Stimulate Earlier Flowering

Bumblebees rely heavily on pollen resources for essential nutrients as they build their summer colonies. Therefore, annual differences in the availability of these resources must simply be tolerated, but a team of researchers from ETH Zürich and the Universite Paris-Saclay made observations suggesting that bumblebees have strategies to cope with irregular seasonal flowering: when faced with a shortage of pollen, bumblebees actively damaged plant leaves and this behavior resulted in earlier flowering by as much as 30 days.

“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,” said co-lead author Professor Mark Mescher, a researcher in the Department of Environmental Systems Sciences at ETH Zürich.

Initial observations with four plant species revealed that bumblebee workers use their proboscises and mandibles to cut holes in plant leaves.

However, Professor Mescher and his colleagues saw no clear evidence that bumblebees were actively feeding on leaves or transporting leaf material back to the hive.

The scientists then conducted several lab experiments and outdoor studies using commercially available bumblebee colonies and a variety of plant species.

They were able to show that the bumblebees’ propensity to damage leaves has a strong correlation with the amount of pollen they can obtain — the bees damage leaves much more frequently when there is little or no pollen available to them,

They also found that damage inflicted on plant leaves had dramatic effects on flowering time in two different plant species: tomato plants subjected to bumblebee biting flowered up to 30 days earlier than those that hadn’t been targeted, while mustard plants flowered about 14 days earlier when damaged by the bees.

“The bee damage had a dramatic influence on the flowering of the plants — one that has never been described before,” said co-lead author Professor Consuelo De Moraes, also from the Department of Environmental Systems Sciences at ETH Zürich.

“The developmental stage of the plant when it is bitten by bumblebees may influence the degree to which flowering is accelerated.”

The authors also tried to manually replicate the damage patterns caused by bumblebees to see if they could reproduce the effect on flowering time. But, while this manipulation did lead to somewhat earlier flowering in both plant species, the effect was not nearly as strong as that caused by the bees themselves.

“Chemical or other cue may also be involved. Either that or our manual imitation of the damage wasn’t accurate enough,” Professor De Moraes said.

“Bumblebees may have found an effective method of mitigating local shortages of pollen,” she added.

“Our open fields are abuzz with other pollinators, too, which may also benefit from the bumblebees’ efforts.”

The findings were published in the journal Science.