Now is the time of year when bees buzz from flower to flower. And for
many plants, the very survival of their species depends on that buzz.
The flowers and the insects are joined together in a partnership of
sound.
Bumblebees and other insects use buzzing to shake pollen out of flowers
for food — and they fertilize flowers along the way. Scientists are
exploring this acoustic feat to figure out how it has evolved, and how
it helps sustain our own food supply.
Flowering plants typically reproduce by delivering pollen to each other
to fertilize seeds. Some flowers, like corn and ragweed, cast their
pollen to the wind. Others depend on animals like bees, bats or birds to
do the job.
In many cases, these flowers lure an animal with the reward of nectar.
As the pollinator sips the plant’s sugary liquid, it gets covered in
pollen. It then travels to another flower in search of nectar and
delivers the grains.
But 20,000 plant species — including familiar ones like tomatoes,
potatoes and cranberries — strike a different deal. They offer pollen
itself as food. These flowers don’t simply put the protein-rich pollen
out for any animal to eat, however. They keep it tucked deep inside
special tubes.
Only bumblebees and certain other insects can get this pollen out. In
every case, the method is the same: the pollinator grabs the tube with
its jaws and starts vibrating hundreds of times a second.
“It has to hold on, because the vibrations are so strong that otherwise
it could come flying off the flower,” says Mario Vallejo-Marín of the
University of Stirling in Scotland, who recently co-authored a review of this behavior in the journal Current Opinion in Plant Biology.
The animals produce a peculiar buzz with this technique. “It sounds like
a bee is giving you a raspberry,” said Stephen Buchmann of the
University of Arizona. In fact, they’re creating resonating vibrations
to loosen the pollen grains inside the tubes. “The bees are turning
themselves into living tuning forks,” said Dr. Buchmann.
The bees cause the pollen grains to bounce up and down in the tube and
then gain so much energy they blast out in a cloud that coats the bee.
As it flies off, the insect gathers the grains out of its fur and tucks
most of them in wet clumps on its legs. It can later feed the clumps to
larvae back at its hive.
“Fortunately, they’re a little bit messy,” said Dr. Buchmann. When the
bee visits another flower for a buzz, the extra pollen still on its body
can fertilize the plant.
“It seems so unlikely — it would never work if you were designing a
plant,” said Anne Leonard, an ecologist at the University of Nevada at
Reno. “But it’s evolved many times.”
Dr. Leonard belongs to a small coterie of scientists who are trying to
understand the evolution of so-called buzz pollination.
The transformation probably starts when a plant species shifts to using
pollen as a lure instead of nectar. If the flowers simply put out the
pollen like cookies on a table, an assortment of insects may steal it
and never pass on any grains to another flower. Growing a tube to store
away the pollen gives flowers control over who can eat it — and the
power to dust visitors with it, as well.
Once flowers hide away their pollen this way, natural selection favors
bees that can give the flowers a harder shake. The harder the bees
shake, Dr. Vallejo-Marín and his colleagues have found,
the more food they can bring home. Bees experience forces 30 times
greater than gravity as they buzz for pollen — near the limit of human
endurance.
That can be too much of a good thing for the flower, though. If a bee
shakes out a plant’s entire pollen supply, it carries the flower’s whole
reproductive future with it. “The risk of a failure can be very high,”
said Dr. Vallejo-Marín.
Dr. Vallejo-Marín suspects that this risk of failure drove the continued
evolution of the flowers. The shape of the tubes changed so that even
harder buzzing could only free a fraction of the pollen, meaning a
single greedy bee couldn’t exhaust a plant’s supply. Instead, the
flowers have enough pollen to spread over several bees, raising the odds
that their genes will get into the next generation.
There’s a lot more that scientists have left to learn about buzz
pollination. Are flowers tuned to particular species of bees, for
example? Why can bumblebees buzz pollinate, but honeybees can’t?
Understanding buzz pollination is not just interesting in its own right,
but a matter of pressing importance. Bumblebees and other buzz
pollinators are suffering worrying declines,
probably due to a combination of threats like diseases, pesticides and
destruction of their habitat. Their decline threatens the many wild
plants that have evolved to depend on their buzz. And a lot of food on
our dining room table depends on the buzz as well.
“We could live just on wind-pollinated plants like wheat and barley and
millet,” said Dr. Buchmann, wearily listing each food, “but it would be a
pretty bland, nasty diet.”
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