Play Video

When pollen’s scarce, bees stab plants to speed up flowering

If bumble bees find too little pollen, they pierce the leaves of non-​flowering plants in order to force them to produce flowers more quickly. (Credit: Hannier Pulido/ETH Zurich)

When pollen is in short supply, bumble bees damage plant leaves in a way that accelerates flower production, researchers report.

In some areas of the world, 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.

Researchers have now discovered that one peculiar bumble bee behavior may help to overcome such challenges by facilitating coordination between the bees and the plants they pollinate. The researchers found that bumble bee 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 received this “nudge.”

“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,” says Mark Mescher, a professor in the environmental system sciences department at ETH Zurich.

A bee on the underside of a green leaf stabs through it
A bumble bee pierces a leaf with its tongue. (Credit: Hannier Pulid /ETH Zurich)

Bees getting more pollen

The researchers first noticed the behavior during other experiments 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 behaviors, but no one had explored what the bees were doing to the plants or reported an effect on flower production,” Mescher explains.

Following up on their observations, the researchers devised several new laboratory experiments, and also conducted outdoor studies using commercially available bumble bee colonies—typically sold for the pollination of agricultural crops—and a variety of plant species.

Based on their lab studies, the researchers were able to show that the bumble bees’ propensity to damage leaves has a strong correlation with the amount of pollen they can obtain: 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 bumble bee biting flowered up to 30 days earlier than those not targeted, while mustard plants flowered about 14 days earlier when bees damaged them.

“The bee damage had a dramatic influence on the flowering of the plants—one that has never been described before,” says Consuelo De Moraes, also a professor in the environmental system sciences department. She also suggests that the developmental stage of the plant when bumble bees bite it may influence the degree to which flowering accelerates, a factor the investigators plan to explore in future work.

The researchers tried to manually replicate the damage patterns caused by bees 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. This leads De Moraes to suggest that some chemical or other cue may also be involved.

“Either that or our manual imitation of the damage wasn’t accurate enough,” she says. Her team is currently trying to identify the precise cues responsible for inducing flowering and characterizing the molecular mechanisms involved in the plant response to bee damage.

In natural settings, too

The researchers were also able to observe the bees’ damaging behavior under more natural conditions, with doctoral student Harriet Lambert leading follow-up studies on the rooftops of two buildings in central Zurich.

In these experiments, the researchers again observed that hungry bumble bees with insufficient pollen supplies frequently damaged the leaves of non-blooming plants. But the damaging behavior was consistently reduced when the researchers made more flowers available to the bees.

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

“Bumble bees 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 bumble bees’ 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.

The study appears in the journal Science.

Source: ETH Zurich