Zumwalt Pollinator

A bumble bee searches a gumweed wildflower on Zumwalt for the day’s last pollen.

As seasonal anomalies become increasingly frequent due to climate change, the resulting uncertainty threatens to disrupt the timing of mutual relationships between plants and their insect pollinators.

But bumblebees may have a way to partly solve that problem. 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.” Researchers led by Professors Consuelo De Moraes and Mark Mescher of ETH University in Zurich, Switzerland made the discovery.

The researchers first noticed the behavior during other experiments being undertaken by one of the authors, Foteini Pashalidou: pollinators were biting the leaves of test plants in the greenhouse.

They found that bumblebees’ tendency 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 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,” De Moraes says. She also suggests that the developmental stage of the plant when it is bitten by bumblebees may influence the degree to which flowering is accelerated, 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.

The ETH research team also observed the bees’ plant-damaging behavior under more natural conditions. Hungry bumblebees 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.

Other pollinating insects, such as honeybees, 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.

“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.”

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