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 keep most pollinators at bay. 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.
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.
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 sonicators?
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 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 to sonicate, 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
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.
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 ….