How bees transfer pollen between flowers

The pollen transfer problem: Over there, just beyond a tall hedgerow, grows a vast field of wildflowers. Many of those plants need insect pollinators to move their pollen from flower to flower. Over here, just outside your garage, is a thriving honey bee colony with lots of brood to feed. Pollen is an essential part of the honey bee diet, so the bees crave it.

What you have is a biological logistics problem. The flowers have the pollen and want to distribute it. The bees need the pollen and want to acquire it. The problem is how to effect a fair and equitable transfer of goods and services.

Bees and flowers are tight

During eons of evolution, bees and flowering plants evolved systems that complement each other. Since plants can’t walk, jump, hop, or swim, they developed ways of tricking insects into doing the work for them. Even though sweet nectar, alluring odors, and attractive flowers are biologically expensive for the plant to produce, pollination by bees is more reliable than simply relying on the wind. Furthermore, it requires far less pollen. Even accounting for the pollen the bees steal, flowering plants benefit greatly from insect pollination.

While we normally think of honey bees collecting nectar, an average-sized colony may accumulate 125 pounds of pollen in a season.¹ From the bee’s perspective, pollen is a rich source of protein and other nutrients including vitamins and minerals, lipids and fats. It’s also a relatively safe type of food to collect. Since pollen does not fight back, it is a lot easier to collect than insects or spiders. So while the plants became more attractive to bees, the bees developed methods of collecting, carrying, processing, and storing the pollen.

Honey bees are adaptable

Many species of bees have a one-on-one association with a particular species of plant, while other bees forage from a closely-related group of plants. But honey bees are polylectic, meaning they forage from a large variety of plant species. The European honey bee was introduced into North America, South America, Australia, New Zealand, and other regions without missing a beat. The flowers in each place were different—not at all what the honey bee evolved with—but it didn’t matter. Her adaptations for finding, collecting, and storing pollen and nectar worked well even for plants she had never seen.

That’s not to say that honey bees forage and pollinate all flowers with the same degree of success. They don’t. For example, flowers requiring buzz pollination, such as tomatoes, are not pollinated by honey bees. And some plants with complex flower parts, or barely sweet nectar, do not attract honey bees.

DNA wrapped in color

Pollen grains are designed to protect the plant’s genetic material as it is transferred from one flower to another. In order to assure the genetic message is not scrambled in transit or destroyed completely, the DNA is locked inside several secure layers.

At the very core of the pollen grain, the genetic package floats in a pool of cytoplasm, which is protected by a layer of cellulose called the intine. The intine is protected by another layer called the exine.² The exine is very tough, resistant to things like UV radiation, moisture, dryness, pressure, and changes in pH. As you can imagine, the exine is not easily breached.

Finally, the exine is wrapped in a super-sticky and colorful layer called pollenkitt.³ Pollenkitt keeps the pollen from blowing away from the flower, but it also allows the honey bee to clump pollen together into those hard pellets packed on her back legs.

One of the first things a new beekeeper notices is the rainbow of colors on a honey bee’s hind legs. The corbiculae, often called pollen baskets, are found on the outside of each hind tibia and comprise a wide spot surrounded by stiff hairs.

Depending on your area, pollen loads come in multiple shades of white, yellow, orange, pink, red, blue, green, gray, and purple. Because honey bees visit only one type of flower on any single foraging trip—a characteristic called floral fidelity—honey bee pollen pellets are nearly always a uniform color throughout. Other corbiculate bees with less floral fidelity, such as bumbles, often collect multicolored pellets.

Dusting the bees

For the most part, some members of a honey bee colony collect nectar and others collect pollen. To defeat this division of labor, flowers have devised ways to load pollen onto all bees, regardless of what they are collecting.

To collect nectar, bees must reach deep into the center of the flower. Since stamens containing pollen surround the nectaries, bees are forced to rub against the stamens to reach the nectar. The pollen sticks to their hairy bodies, aided by electrostatic charges and the stickiness of pollen grains.

As a result, even those bees that are not actively collecting pollen are pollinating the flowers they visit. As the forager collects nectar from each successive flower, the pollen grains adhering loosely to her body are easily rubbed onto the stigma of another flower. In contrast, the pollen grains packed tightly into a corbicula do not release easily. Instead, they are lost to pollination.

Many pollination biologists believe non-corbiculate bees are more efficient pollinators than honey bees or bumble bees. Instead of packing pollen into tight balls, non-corbiculate bees carry it loosely in tufts of hair called scopae. Depending on the species, scopae can occur on the legs, under the abdomen, or along the sides and back of the thorax. Pollen carried in scopae can easily rub off on the next flower, increasing the chance of pollination.

Packing the corbiculae

A honey bee uses all six legs to collect and pack pollen into her corbiculae. Using her forelegs, she may scrape pollen directly from the anthers of the flower. At other times she uses her front legs to clean the pollen that sticks to her body.

To begin the cleaning process the bee will often stroke her proboscis (a tongue of sorts) with her forelegs, a maneuver that covers her legs with a thin film of nectar. Then she swipes those sticky legs across her head, eyes, and the back of her thorax. If her antennae are messy with pollen, she may pull them through the antennae cleaners on her forelegs. Next, she uses her middle two legs to further clean the thorax and wipe the pollen from the forelegs.

The tricky part comes next. The honey bee swipes the left middle leg against the right hind leg and vice versa. Thick rows of hair called pollen combs cover the inner surface of each rear basitarsus. By rubbing each middle leg against the opposite pollen comb, she cleans the pollen from her middle legs. Once the pollen is clumped on the inside of her rear legs, she is ready to load the pollen baskets.

Putting the squeeze on pollen

The pollen press, also found on the rear legs, is made of two flat plates that are hinged together. One plate is on the distal end of the tibia, and the other is on the proximal end of the basitarsus. When the leg is bent, the plates pull apart and the bee can stuff the opening with pollen that she accumulated from her body. Stiff bristles of hair called a pollen rake surround the opening, so all she needs to do is scrape opposite legs against the pollen rakes. As she performs this second side-to-side transfer, the open press fills with pollen.

When she straightens her leg, the flat plates close against the pollen and force it up into the pollen basket on the tibia. It works very much like a tube of toothpaste: when you squeeze the two sides together, the paste comes out the top. When the bee squeezes the plates together, the pollen is forced into the pollen basket. Although it may seem backward, the pollen is always loaded from the bottom and squeezed up.⁴

The honey bee continues to fill the baskets on both sides of her body simultaneously, keeping the weight distributed evenly between the two. Since she can carry as much as a third of her weight in pollen,⁵ it’s important to keep the load balanced. Once filled to her liking, she takes off for home.

Technical difficulties

When bees are out and about collecting pollen, not everything goes perfectly well. Every spring, at least one beekeeper sends a photo of bees with “fungus” growing on their backs. Others wonder if their bees got into fresh paint. The thoracic patches may be nearly rectangular or have a distinct hourglass shape.

According to entomologist Rosanna Mattingly, honey bees with these markings have been in flowers that were shedding copious amounts of pollen that completely covered the bee. Even though the honey bee uses her two middle legs to clean the back of her thorax, there’s a place right in the middle that she can’t quite reach from either side.  When she swipes each side of her thorax, the pollen is removed in two arcs. So her legs, much like a pair of windshield wipers, carve a concave design in the pollen.⁴

More than just pollen

At other times, pollen pellets may have odd-looking strings poking out of them. These are actually the filaments that support the anthers of a flower. Instead of scraping just the pollen, the bee snagged the whole stamen. Unable to separate the sticky parts, she shrugs and packs them into her basket—something for the home crew to fix.

Other problems can occur at the landing board. A bee may tumble heavily near the entrance and lose her load. Workers coming and going, guards defending the hive, or drones lounging in the sun can accidentally knock a pellet from a bee’s corbicula. Although it seems wasteful, bees typically ignore these pellets.

The problem seems to be one of lifting and carrying. Even though honey bees move all sorts of objects, including dead bees, deformed brood, small parasites, pieces of cardboard, and even woodchips, moving a pollen pellet seems to be the impossible task. According to Dr. Norm Gary, “To collect a full load of pollen a bee may spend as little as 6-10 minutes.”⁶ Perhaps it is easier to collect more pollen than wrestle the wayward pellet into a storage cell.

Unloading the loot

Unlike nectar carrying bees that transfer their load to another bee, pollen-laden bees must off-load their own freight. Once inside the hive, she must travel to a pollen collection area, locate a cell with extra space, and deposit her pellets inside. She does this by placing her rear legs in the cell and pushing against the pellets with her middle legs until the balls release. Once free of the load, she may spend a few moments cleaning pollen from her body and depositing that as well.⁷

Once the forager leaves the pollen storage area, young house bees press the fresh pellets into the bottom of the cell with their head, mandibles, or forelegs.⁶ In addition, the house bees add regurgitated honey to the pellets along with enzyme-rich saliva that helps to preserve the quality. Fresh pollen doesn’t keep well, so adequate preparation is important, even for short-term storage.

Floral fidelity run amok

Since pollination requires loose and fluffy pollen, you might think that a honey bee hive is the last stop for a flower’s pollen. But not quite. A flower has one more chance to get its DNA distributed.

As mentioned above, a honey bee’s floral fidelity keeps her returning to the same type of flower during an entire foraging trip. For the most part, floral fidelity is great for the plant. If a single bee carried many different types of pollen, she would be less likely to deliver the right type to the next flower.

However, even floral fidelity has its drawbacks, as we sometimes see in fruit trees. Varieties of fruit are often self-incompatible, which means one variety needs to be cross-pollinated by a different variety of the same species. As a result, apple, sweet cherry, and almond varieties are often inter-planted in an orchard so they can easily cross-pollinate.

But these trees often have such dense flower clusters that a honey bee can actually walk from flower to flower, collecting all the pollen she needs from a single small branch. She doesn’t need to fly from tree to tree because all the pollen she can carry is in one place. Piece of cake! But if she doesn’t move from tree to tree, no pollination takes place.

In-hive pollen transfer

In spite of this problem, the trees still get cross-pollinated because of a phenomenon known as in-hive pollen transfer. Inside the densely populated hive, bees rub against each other. As they jostle around, the loose and fluffy pollen stuck to their bodies transfers from bee to bee. So much pollen is transferred in this way that a bee that has never before left the hive may pollinate the very first flower she visits.⁸

The amount of pollination due to in-hive transfer is greatest when bees are pollinating large monocultures where other pollen types are scarce, such as in fruit tree orchards. Conversely, in-hive pollen transfer is less effective in situations where the bees are foraging on several different crops at once.

The lady is a tramp

The pollen trail is long and complex and requires bees of many predilections. So the next time you see a pollen tramp—my name for those messy ladies with pollen from head to tarsus—remember that they too, in spite of bad grooming, are busy pollinating the crops and keeping our world fertile and green.

Rusty
Honey Bee Suite

References

1. Sammataro D, Avitabile A. 1998. The Beekeeper’s Handbook. Ithaca NY. Cornell University Press.
2. Raven PH, Evert RF, Eichhorn SE. 2005. Biology of Plants, Seventh Edition. New York, NY. WH Freeman and Company.
3. Simpson MG. 2006. Plant Systematics. London, UK. Elsevier Academic Press.
4. Mattingly RL. 2012. Honey-Maker: How the Honey Bee Worker Does What She Does. Portland OR. Beargrass Press.
5. Caron DM, Connor LJ. 2013. Honey Bee Biology and Beekeeping. Kalamazoo MI. Wicwas Press.
6. Gary N. 2015. Activities and Behavior in Honey Bees in JM Graham (Ed.) The Hive and the Honey Bee (pp 271-3108). Hamilton IL. Dadant & Sons, Inc.
7. Winston ML. 1987. The Biology of the Honey Bee. Cambridge MA. Harvard University Press.
8. Degrandi-Hoffman G. 2015. Crop Pollination in JM Graham (Ed.) The Hive and the Honey Bee (pp 803-830). Hamilton IL. Dadant & Sons, Inc.