Heavy Loads Of Pollen May Shift Flight Plans Of The Bumblebee

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Ready, set, fly! The ball bearings glued to this bumblebee's legs simulate the weight and placement of pollen loads. The tag on the insect's back is a lightweight sensor, designed to track its movements in flight. (Courtesy of Andrew Mountcastle)
Ready, set, fly! The ball bearings glued to this bumblebee's legs simulate the weight and placement of pollen loads. The tag on the insect's back is a lightweight sensor, designed to track its movements in flight. (Courtesy of Andrew Mountcastle)

Bumblebees are important pollinators of crops and wildflowers across the U.S., and they gather heavy loads of nectar and pollen from flowers. A study published Monday shows that the type of food they carry affects how they fly.

That's because they store nectar in a special pouch inside their abdomen, while pollen gets packed into little hollows in their hind legs. When bumblebees carry a pollen load rather than a nectar load, "they are more stable, but less maneuverable in flight," according to an online report in the Proceedings of the National Academy of Sciences.

Despite what you may have heard, bumblebees do not defy the laws of physics when they fly.

Decades ago, this idea entered popular culture, maybe because engineers could not explain bumblebee flight using models developed for fixed wing aircraft, like airplanes, explains Andrew Mountcastle, a biologist at Harvard University.

"Of course, bumblebees fly in a very different way than airplanes do," he notes. "They flap their wings, and their wings bend and twist as they flap them."

Bumblebees are very good at flying in all kinds of weather, says Mountcastle, and they do it while loaded down with cargo.

"What might be surprising to many people is just how much load they're able to carry," he says. "Bumblebees are basically aerial tankers." They can store enough nectar to roughly double their body weight; and they can carry nearly half their own weight in pollen.

Mountcastle and his colleagues wanted to understand how a bumblebee's flight is affected by what it's carrying. After all, they note, many insects carry substantial loads, but not too many studies have assessed how this affects their flight performance.

So, first the scientists ordered a hive of bumblebees online. "It comes packaged in a nice box," says Mountcastle. "And it always excites the FedEx guy when he drops off a buzzing hive at your door."

They trained the bumblebees to fly through a wind tunnel by rewarding them with nectar from an artificial flower at the other end.

Next, to start the experiment, they weighed the bumblebees down. "We use artificial weights, in the form of really, really small steel ball bearings," says Mountcastle. They either glued them on the bees' legs — to simulate a pollen load — or on their abdomens, to simulate a load of nectar.

They sent the bees flying through the wind tunnel again, under various wind conditions, and filmed them with high-speed video.

When they analyzed the video, they found that the type of cargo did make a difference. In turbulent wind conditions, bees were more stable when they carried weight on their legs — as they do when gathering pollen.

"Conversely though, carrying a pollen load carries a downside when the goal is maneuverability," says Mountcastle. In calm air, bees with heavy legs were less nimble than bees with fully loaded bellies.

All of this leads to an interesting possibility: Maybe bumblebees consider the wind conditions when deciding whether to gather pollen or gather nectar.

"This is something which would be the next question we ought to tackle," says Sridhar Ravi, of the Royal Melbourne Institute of Technology in Australia, who also worked on this study.

After all, bumblebees are very clever. "They can solve problems," Ravi notes. "We have them go through obstacle courses, mazes — and they just come up trumps. You just have to wonder, 'Oh, my God, they're so smart!' You know?"

He says experiments are now in the works to see if the weather really does influence whether bumblebees bring home more pollen or nectar.

Copyright 2015 NPR. To see more, visit http://www.npr.org/.

Copyright NPR. View this article on npr.org.

Transcript

MELISSA BLOCK, HOST:

If you've been outside today, maybe you heard this summertime sound.

(SOUNDBITE OF BUZZING)

BLOCK: A bumblebee gathering pollen from a flower. Bumblebees routinely haul around heavy loads of pollen or nectar. NPR's Nell Greenfieldboyce reports that scientists recently investigated how the flight of a bumblebee is affected by that.

NELL GREENFIELDBOYCE, BYLINE: Despite what you may have heard, bumblebees do not defy the laws of physics when they fly. Decades ago, this idea entered popular culture maybe because engineers could not explain bumblebee flight using models developed for fixed-wing aircraft like airplanes.

ANDREW MOUNTCASTLE: But of course, bumblebees fly in a very different way than airplanes do. They flap their wings, and their wings bend and twist as they flap them.

GREENFIELDBOYCE: Andrew Mountcastle is a biologist at Harvard University. He says bumblebees are very good at flying in all kinds of weather, and they do it while loaded with cargo.

MOUNTCASTLE: And what might be surprising to many people is just how much load they're able to carry. Bumblebees are basically aerial tankers.

GREENFIELDBOYCE: They have a specialized pouch in their abdomen to store nectar - a lot of nectar. They can double their body weight. They pack pollen into storage bins on their hind legs. Mountcastle and his colleagues wanted to understand how a bee's flight is affected by what it's carrying. So first, they went online and ordered a hive of bumblebees.

MOUNTCASTLE: And it comes packaged in a nice box, and it always excites the FedEx guy when he drops off a buzzing hive at your door.

GREENFIELDBOYCE: They trained the bumblebees to fly through a wind tunnel by rewarding them with nectar from an artificial flower at the other end. Next, to start the experiment, they weighed the bumblebees down.

MOUNTCASTLE: We use artificial weights in the form of really, really small steel ball bearings.

GREENFIELDBOYCE: They either glued them on the legs to simulate a pollen load or on the abdomen to simulate a load of nectar. They sent the bees flying through the wind tunnel again under various wind conditions and filmed them with high-speed video. When they analyzed the video, they found that the type of cargo did make a difference. In turbulent wind conditions, bees were more stable when they carried weight on their legs like they do when gathering pollen.

MOUNTCASTLE: Conversely, though, carrying a pollen load comes with a downside when the goal is maneuverability.

GREENFIELDBOYCE: In calm air, bees with heavy legs were less nimble than bees with full bellies. The researchers describe their work in the proceedings of the National Academy of Sciences. Now they want to know, do bumblebees decide to gather pollen or nectar depending on the wind conditions?

SRIDHAR RAVI: This is something which would be the sort of next question we ought to tackle.

GREENFIELDBOYCE: Sridhar Ravi of RMIT University in Australia also worked on this study. He says bumblebees are very clever.

RAVI: They can solve problems. We have had them fly through obstacle courses, mazes, and they just come out trumps. And you just have to sort of wonder, oh, my god, they are so smart, you know?

GREENFIELDBOYCE: He says experiments are in the works to see if the weather affects what bumblebees bring home. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.