|HANNIER PULIDO/DE MORAES AND MESCHER LABORATORIES|
When bumble bee queens emerge from hibernation, they need to gather pollen and nectar to start their new colonies. If they wake up too soon, there may not be enough flowers in bloom. Now, researchers have discovered the bees have a way to order some fast food: They nibble holes in leaves, spurring plants to blossom weeks ahead of schedule. Many questions remain about the details of this strategy and how it evolved.
“It’s certainly surprising,” says Lars Chittka, a behavioral ecologist at the Queen Mary University of London, who was not involved. “We’re only at the beginning of exploring this phenomenon.”
Researchers at ETH Zürich chanced upon the discovery when they noticed curious bite marks on leaves while studying how bees respond to plant odors. They had added bumble bees to a research greenhouse and observed them cutting holes in the shape of half-moons. What was going on? At first, the researchers thought the insects might be feeding on fluid from the leaves, but the bees didn’t stay long enough to get much. Nor did they appear to be taking any part of the leaves back to their colonies.
A key observation—that bumble bees from colonies with less food were more avidly damaging the leaves—suggested another goal. The researchers wondered whether the damage triggered the plants to flower sooner, providing pollen to the hungry pollinators. After all, some plants speed up their flowering when they are stressed by disease or drought because these threats provide an incentive to reproduce sooner. But no one had ever shown that a pollinator can stimulate flowering. “I thought it was a long shot,” recalls Mark Mescher, an evolutionary biologist at ETH Zürich who co-led the research.
The researchers set up a greenhouse experiment with black mustard (Brassica nigra), a crop they had been studying. Ten plants were put in mesh bags with bumble bees that hadn’t eaten any pollen for 3 days; they proceeded to nibble five to 10 holes in each plant. On average, those plants flowered after 17 days; undamaged plants that had not been exposed to bumble bees took an average of 33 days, the researchers report today in Science. In a similar experiment, tomato plants sped up their flowering by 30 days. “The magnitude of the effects is huge,” Mescher says.