Spring has sprung earlier than ever before this year, accompanied by temperatures more typical of early summertime. Many plants were already in full bloom by mid-April, about three to four weeks earlier than normal. These types of seasonal anomalies are becoming increasingly frequent due to climate change, and the resulting uncertainty threatens to disrupt the timing of mutualistic relationships between plants and their insect pollinators.

A research team led by ETH Professors Consuelo De Moraes and Mark Mescher has now discovered that one peculiar bumblebee behaviour may help to overcome such challenges by facilitating coordination between the bees and the plants they pollinate. The group has found that bumblebee workers use their mouth parts to pinch into the leaves of plants that haven’t flowered yet, and that the resulting damage stimulates the production of new flowers that bloom earlier than those on plants that haven’t been given this “nudge”.

Their study has just been published in the journal Science . “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 researchers first noticed the behaviour during other experiments being undertaken by one of the authors, Foteini Pashalidou: pollinators were 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.

Phenomenon also observed in the field

The ETH research team was also able to observe the bees’ damaging behaviour under more natural conditions, with doctoral student Harriet Lambert leading follow-up studies on the rooftops of two ETH buildings in central Zurich. In these experiments, the researchers again observed that hungry bumblebees with insufficient pollen supplies frequently damaged the leaves of non-blooming plants. But the damaging behaviour was consistently reduced when the researchers made more flowers available to the bees.

Furthermore, it was not only captive-bred bumblebees from the researchers’ experimental colonies that damaged plant leaves. The investigators also observed wild bees from at least two additional bumblebee species biting the leaves of plants in their experimental plots. Other pollinating insects, such as honeybees, did not exhibit such behaviour, however: they seemed to ignore the non-flowering plants entirely, despite being frequent visitors to nearby patches of flowering plants.

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.” But it remains to be seen whether this mechanism is sufficient to overcome the challenges of changing climate. Insects and flowering plants have evolved together, sharing a long history that strikes a delicate balance between efflorescence and pollinator development. However, global warming and other anthropogenic environmental changes have the potential to disrupt the timing of these and other ecologically important interactions among species. Such rapid environmental change could result in insects and plants becoming increasingly out of sync in their development, for example. “And that’s something from which both sides stand to lose,” Mescher says.