Sonication: why honey bees cannot buzz pollinate

Roughly eighty-nine percent of flowering plants need animal assistance for ideal pollination. Oddly, about eight percent of those needy plants make the process as difficult as possible. This strange minority hides its pollen in protective capsules that require sonication to release the pollen. Eight percent may sound like a paltry number until you realize it’s about 24,000 species, give or take.¹

Why do plants do this? Biologists think these plants evolved to protect their precious pollen supply from being wasted on insects that don’t do a good job of pollination, perhaps some beetles, flower flies, or wasps. By saving the pollen for those insects hell-bent on finding it — the ones that will keep sampling flower after flower — a plant is more likely to become completely pollinated.²

Other botanists think the enclosed system may have developed to protect pollen from rainfall, excessive UV light, or some other environmental hazard. Regardless of the reason, it has appeared independently in many plant families, although it displays more frequently in some than others.

In most cases, encapsulated pollen is extremely high in protein compared to the pollen of other plants. The plants, it seems, reward the proper pollinators for a job well done.

Poricidal anthers require sonication

Most of these hard-to-access plants have poricidal anthers, formed when the anthers fuse into a tube or capsule.² Instead of being released directly to the outside, the pollen remains inside the capsule, safe and dry. However, for bees in the know, there is a small pore or slit — either at the end of the capsule or along the side — where the pollen can escape.

A bee with all the right equipment — and a yen for that particular pollen — can release it by grabbing the anther with her mandibles or feet, curling her abdomen around it, and vibrating for all she’s worth. The sound is audible, a distinct, high-pitched whine unlike the normal buzzing of wings. Sonication, also known as buzz pollination, comes in short, repeated bursts, ranging from 200 to 400 Hertz, depending on the species.³

The pollen held within poricidal anthers is small and smooth, completely lacking the typical sticky coating. When the bee vibrates fast enough, the pollen shoots from the pore and floats in the air like dust motes. It may land on a receptive stigma nearby and fertilize a plant directly, or it may land on the bee’s face, legs, thorax, or venter.⁴

Some sonicating bees even have special facial hairs that ensnare the pollen as it shoots from the pores. Once she’s dusted with pollen, the bee uses her legs to groom the pollen from miscellaneous body parts into her scopae. Then she’s off to the next flower, inadvertently pollinating as she goes.

Lots of pollen, little nectar

Most poricidal flowers do not produce nectar. This is likely an evolutionary adaptation to attract only the pollinators the plant needs and to discourage those simply looking for a free lunch. As with most things, exceptions occur. The nectar-rich flowers of blueberry and cranberry are good examples.

It seems unlikely, but poricidal anthers are abundant in our crop plants. Besides the berries mentioned above, crops such as tomatoes, eggplants, peppers, and potatoes all have poricidal anthers, as do kiwis. And all require sonication.

But even plants without poricidal anthers can benefit from a little shaking. The bees that can sonicate often jiggle many common crops, such as gourds, squashes, persimmons, and even almonds.⁵

Who are the sonication specialists?

Roughly 58 percent of known bee species can sonicate. Most sonicators are larger bees; the very tiny species do not have the body mass needed to dislodge the pollen from its capsule.

Sonicating species exist throughout all seven of the bee families, but the distribution is random.³ For example, the honey bee and the bumble bee are both in the Apidae family, as are carpenter bees and the cactus bees (Diadasia). But while carpenters and bumbles shimmy and shake, the honey bees and the cactus bees wouldn’t think of it. Many biologists have wondered why.⁶

Honey bees are almost perfect pollinators

Honey bees are nearly perfect pollinators. Among their many virtues, honey bees maintain year-round colonies ready to work the moment spring arrives, they practice floral fidelity, and they are broadly polylectic (meaning not picky about pollen sources). In addition, they can forage over enormous distances and communicate excellent finds to their nest mates. Best, there are lots and lots of them. The one thing that keeps honey bees from perfection is their inability to sonicate.

Although bumble bees are adept at sonication, they don’t overwinter, have relatively small colonies, and can’t be easily transported from place to place. Although they are extremely efficient pollinators, they show less floral fidelity than honey bees. The agricultural darling would be, perhaps, a cross between a bumble bee and a honey bee.

It’s a software problem

Like most bees, honey bees have all the hardware they need for sonication, but for an unknown reason, they lack the software. To understand the discrepancy, let’s look at how a fully functioning sonicator works, starting with the flight muscles.

The flight muscles of a bee reside in the thorax. A bee has both direct and indirect flight muscles, and both sets are necessary for soaring from bloom to bloom.

The direct flight muscles connect the four wings to the thorax, allowing the bee to position her wings in any situation. You can think of them as the steering muscles. A bee can send her wings out to the side, bring them in, twist them horizontally or vertically, or rest either set on top of the other. This allows her to navigate up or down, port or starboard, or even hover. The direct muscles also help join her wings together using the hook-shaped hamuli that keep the wings on each side moving as a unit.

In contrast, the indirect flight muscles are not connected to the wings at all. The two sets of indirect muscles attach to the insides of the flexible thorax, one set running from front to back (longitudinal) and the other set running from top to bottom (vertical). The sets take turns contracting, first one set, then the other.

When the bee contracts the front-to-back muscles, the thorax changes shape, becoming shorter and thicker. The change in thorax shape causes the extended wings to push down. Next, the front-to-back muscles relax and the top-to-bottom muscles contract, causing the thorax to become longer and flatter. As the thorax shape changes again, the wings are forced up.⁷

A soft assist to pump the thorax

The lightning-fast shape-shifting of the thorax would allow the bee to fly if the nervous system could work that fast — but it can’t. Instead, the bee has a software fix that translates each nervous impulse into multiple up-and-down strokes. We call this an asynchronous flight system.⁸

Asynchronous flight muscles are found in many phylogenetically advanced insects, including flies, mosquitoes, midges, beetles, bees, and wasps.

In contrast, synchronous flight muscles — where one nervous impulse yields one flap of the wings — are common in many of the more primitive insects. Often these are heavy-bodied species with large wings such as butterflies, moths, and locusts.

Stationary movement without flight

Worker bees use their flight muscles for many purposes besides flying. We’ve all seen honey bees lifting their abdomens and fanning the air to distribute pheromones. We’ve also seen bees ventilate a hive or dry nectar by fanning, setting up air currents, and sending the luscious scent of beehive into our backyards.

When bees are flying or fanning, they are using both their direct and indirect flight muscles. The bee’s wings flap up and down, but the angle of the wings changes depending on what the bee wants to accomplish, sort of like the ailerons on an airplane.

But bees use their wings for things other than flying or moving air. For example, we know honey bees use their flight muscles for temperature control within the brood nest. Those workers who press their abdomens to the surface of brood cells and vibrate their wing muscles are sometimes called heater bees, and the little hot spots they create glow on an infrared camera.

But heater bees have an extra step to take before they vibrate — each of them must disengage their flight muscles from their wings. If they didn’t, any heat they generated would leave, dispersing away from the brood like those heavenly odors. These bees remain stationary by “decoupling” their wings from their flight muscles.

Similarly, bees preparing for flight often vibrate in place, warming up like a jogger running in place. And, as you may have guessed by now, sonicating bees also vibrate in place.

Decoupling without a wrench

I always thought the term “decoupling” was misleading. And it is. Decoupling the wings is not like unhitching the hardware between the cars of a freight train or even unhooking the hamuli between two sets of wings. It’s more subtle than it sounds.

Think of it like this: When a bee lands after a flight, she pulls her wings close to her body, a function of the direct flight muscles. The shift of wing position changes the shape of her thorax, causing the scutal suture (or scutal fissure) on the back of the thorax to close. This closing has the effect of “choking up” on the wing connection, making it shorter. When the thorax vibrates in this “resting” position, the vibrations are smaller and faster than when the wings assume the flying position.

Closing the scutal suture can be compared to holding the string of a guitar against a fret. Shortening the string’s length, decreases its wavelength and increases its frequency. Although the wings still vibrate, the motion is at a higher frequency than if the bee were flying. The sound is distinct too, high-pitched and whiny.⁸

All the speed necessary for sonication

During flight, a honey bee’s thorax vibrates at about 220-250 Hertz, while a sonicating bumble bee’s thorax vibrates at about 400 Hertz. Does this difference explain why a honey bee can’t sonicate? Apparently not.

As mentioned above, because the thorax changes shape when the bee is stationary, the vibrations are shallower and faster. Based on sound measurements taken in beehives, a honey bee is completely capable of equally fast vibration. In fact, she vibrates her thorax much faster when she is heating brood than when she is flying because of this “decoupling” of her wings.

Researchers have recorded a wide range of in-hive frequencies, ranging from around 10 to around 1000 Hertz. However, most hive sounds group together at certain values, especially around 300, 400, and 500 Hertz. Since honey bees cannot sonicate, we can only speculate. However, it seems reasonable to assume they have all the proper hardware to sonicate at, say, 400 Hertz like a bumble bee, but their DNA doesn’t give them the go-ahead.⁹

Lost: the shake-it-loose gene

Morphologically speaking, then, honey bees should be able to sonicate. They are the right size, they require large amounts of high-quality pollen for maintaining a year-round colony, and they have strong indirect flight muscles they use for defense, communication, and thermoregulation. So why do they draw the line at sonication?

Some researchers speculate that since honey bees build wax combs instead of mud nests, they lost the ability to use their indirect flight muscles for some purposes. It turns out that many of the bee species that build nests with mud use their flight muscles to compact the mud. Some species even vibrate soil as they excavate it, jarring it loose from its surroundings by using the thorax like a jackhammer.

Perhaps honey bees lost the genetic instruction to excavate by vibration after they lost the need to excavate. When you think about it, excavating mud and extricating pollen from poricidal flowers are not very different. To get the stuff you need, just jiggle it. Yes? But, alas, no.

There’s no perfect bee

Oddly, the closest thing to the theoretically perfect bee might be one of the Melipona bees. These are stingless, honey-producing bees, that live in year-round colonies, pollinate a wide variety of plants, and sonicate, too. Although they make wax nests, they somehow retained their ability to buzz pollinate.

Ultimately, the question of who sonicates and who doesn’t — and why — may be one of nature’s best-kept secrets.

Rusty Burlew
Honey Bee Suite

Notes and References

Vallejo-Marín M. 2019. Buzz pollination: studying bee vibrations on flowers. New Phytol. 224: 1068–1074.

DeLuca PA, Mallefo-Marin M. 2013. What’s the ‘buzz’ about? The ecology and evolutionary significance of buzz-pollination. Current Opinion in Plant Biology 16: 1-7.

Danforth BN, Minckley RL, Neff JL. 2019. The Solitary Bees: Biology, Evolution, Conservation. Princeton NJ. Princeton University Press.

The venter is the underside of the abdomen.

Buchmann, SL 1985. Bees Use Vibration to Aid Pollen Collection from Non-Poricidal Flowers. Journal of the Kansas Entomological Society. 58: 517-525.

Cardinal S, Buchmann SL, Russell AL. 2018. The evolution of floral sonication, a pollen foraging behavior used by bees (Anthophila). Evolution 72: 590-600.

Snodgrass RE, 1956. Anatomy of the Honey Bee. Ithaca, NY. Comstock Publishing Associates.

King, Marcus & Buchmann, Stephen & Spangler, H. 1996. Activity of asynchronous flight muscle from two bee families during sonication (buzzing). The Journal of experimental biology. 199. 2317-21. 10.1242/jeb.199.10.2317.

Collison C. 2016. A closer look: sound generations and hearing. Bee Culture: Feb. 22, 2016.

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