Pollen, the miraculous germ cell of plants and the primary protein nutrient for bees, is the fertility key to every flowering plant and tree in the world.
Honored by the Native American Navajo as a path to the source of spiritual life, and perplexing to the likes of Darwin, who referred to the origin of flowering plants as the “abominable mystery,” pollen, in some form, has been on our planet for about 247 million years.1 While its primary function is to fertilize plants and supply nutrition for its insect partners, pollen may also play a role in determining the development of queens in a colony. Recent work examined how the genome of honey bees may be regulated by the genome of the pollen they collect in what’s known as cross-kingdom genome interactions.2 The fascinating part is the possibility that plant genes and insect genes are linked in a way that allows them to co-evolve as cooperative species. There is a lot to learn about pollen, so let’s start at the beginning with some of its fascinating characteristics.
At the origin, some form of beetle likely lumbered up the stalk of an ancient angiosperm (flowering plant) to gather pollen and probably fertilize the plant. That arrangement is theorized to have lasted about 100 million years until flying pollinators like bees came on the scene. Since then, bees and flowering plants have joined in a fertility covenant that still exists little changed.
For plants, pollen is the rough equivalent of the male sperm in animals.3 Still, unlike animal sperm, protected inside an organism until fertilization is complete, pollen must survive a potentially destructive environment on its aerial or insect journey from plant to plant. Pollen’s almost impenetrable protective layers make it well-equipped to survive the trip. However, as you will see, those same protective layers present an obstacle for bees seeking the rewards of using it as a protein-rich food.
The mystery and beauty of how a pollen grain can yield freely to a plant and, at other times, stay intact and unyielding, tell a story of unprecedented natural adaptation and wonder. What follows details a small part of how plants and bees have evolved so that pollen can serve its dual role as a fertility agent and the primary protein source for bees.
Figure 1 These pollen grains are a testimony to durability. They have survived for 55 million years and still carry their decay-resistant skeleton, making them identifiable as pollen.
The Pollen Grain
A pollen grain’s armor is the biological equivalent of a medieval castle wall. Layers of defense defy digestion and decay to the extent that angio-sperm-like pollen grains have survived from the Middle Triassic period (Figure 1).
One distinctive characteristic of a pollen grain comes from its unique surface, the pollenkitt. The pollenkitt is a coating made of lipids (fats) and other waterproof chemicals that provide a scent and add color to the grain.4 The pollenkitt is not a defensive layer but rather an attractant. The lipids give the grains a semi-adhesive quality which helps hold the pollen on the plant’s anthers until dispersal. When combed by a bee using its legs to wipe its body clean, the sticky pollenkitt helps adhere the grains as they are moved from leg to leg and finally get pressed into the bright-colored pollen pellets we see arriving at our hive’s entrance.
Figure 2 Note that the germ pores are part of the exine layer and provide the openings needed for use during grain germination.
The first actual layer of defense is called the exine (ek-seen) (Figure 2). You can think of the exine as a resilient plastic or polymer similar to those used for everyday products like water bottles or plastic bee frames. Organic polymers are almost indestructible.5 Despite all the hype, pollen can resist enzymatic digestion by animals and humans to the extent that eating bee pollen as a human nutrient is not very useful.6
The exine gives pollen species their distinctive shape (Figure 3). Unique pollen grains form the basis of a field of science called palynology, which identifies plant species by pollen. A branch of palynology called melissopalynology uses the pollen grains found in honey to determine the ori- gin of that honey’s plant nectars.7
Figure 3 Notice the irregular surface of the exine on dandelion pollen, which makes it easy to identify but less digestible than other species.
The exine’s hardened defense may have evolved in early land plants as protection against desiccation or UV radiation. The exine, being a polymer, is somewhat elastic and expands with different moisture levels both outside and inside the grain, which plays an important role when the grain germinates. The exine wall also has microscopic openings called apertures or germ pores that allow the delivery of its interior proteins during fertilization (Figure 2).8 When a pollen grain fertilizes a plant, it attaches to the stigma, which is the female part of the plant. The reaction between the pollen grain and the stigma gets very interesting. The stigma hydrates the pollen grain, and the grain begins to germinate. During germination, the germ pores on the exine begin to exude enzymes. If the enzymes match those needed by the stigma, fertilization begins.
Figure 4 The pollen tube’s growth follows the plant’s style into the ovaries and to the egg. Once the tube is fully grown, the sperm or male gamete part of the pollen grain follows the tube and fertilizes the egg.
Fertilization begins with growing a pollen tube down through the style of the plant and into the ovaries.9 When the tube develops, the sperm part of the pollen grain follows down the tube to fertilize the egg (Figure 4).10 The process involves a complex and wondrous protein exchange between the pollen and the plant. The result can be anything from an edible mouthwatering fruit or a simple maple copter seed.
That’s the plant side; now, what do the bees get?
How Bees Collect Pollen
Balanced rewards make it all sustainable in any functional natural system where cross-species are engaged in mutual survival. In the bee and plant world, we have known for centuries that pollen is the reciprocal reward given back to bees. As an aside, this invariably leads to the perplexing question of how the exchange began. No one knows for sure, and there are few hints from the fossil record, so the age-old question of which came first, the pollen or the pollinator, remains a mystery.11
Either way, the first task is to collect pollen from a plant’s anthers, and bees have a few tricks to get that done. One is an interesting electrical phenomenon using static electricity. As a honey bee flies through the air, it picks up a positive static change, much like an article of clothing sometimes does when it comes out of the dryer. Static charge makes materials attach, and a bee’s static charge acts the same way with pollen. Knowing that everything on a plant touching the earth has a negative charge, including its pollen grains, helps us understand how it all works. When a positively charged bee nears the negatively changed pollen, the grains have a magnetic-like attraction much the same as animal hair does to clothing. The attraction is strong enough that, at times, pollen will leap from the plant’s anthers onto the bee before the bee even lands. Of course, after the bee lands on the plant, the process of pollen transfer continues as the bee physically contacts the anthers of the plant.
Once on the bee, the sticky pollenkitt keeps pollen in place.12 A honey bee has hundreds of hairs covering every part of its body, even the eyes (Figure 5). The pollen coverage on a bee’s body depends on the location of the plant’s anthers, and some plants will cover a bee with loads of grains. But having those grains all over its body would make flight and navigation difficult, so the second task is to collect all those grains and put them safely in their pollen baskets. Like everything else about bees, using their legs to collect the pollen grains off all the hairs on their body and moving them between their legs on the journey to the pollen baskets has some not-so-obvious and fascinating peculiarities.
Figure 5 There’s hair on every part of a bee’s body, and the eyes and antennae are no exception when collecting pollen. A bee’s front legs have antenna cleaners they use right before a flight to clean off anything interfering with their sense of smell on the trip home.
First, each set of legs uses a special brush on their tibia designated to comb the hairs on a particular segment of the bee’s body (Figure 6). The front legs brush pollen from the head and eyes, the middle legs clean parts of the thorax, and the hind legs clean the abdominal area.
It’s interesting to note that not all body hairs on a bee are the same. Their length, diameter, and spacing vary, allowing for the precise suspension of pollen grains above the particular contours of the bee’s body where they grow. Even with all that specialization, completing the collection process takes time and effort. For example, it can take a bee twenty strokes to clean its eyes, and even then, some grains remain in place.11
Figure 6 This bee brush has special hairs that comb and collect pollen, and specialized hairs that can sense the movement of pollen.
Once a leg’s brush has collected pollen, moving the pollen from leg to leg, ending on the back legs’ baskets, begins. Bees know when to do this because each leg has specialized hairs that act like mechanical sensors signaling the need for pollen movement. Those same hairs also signal that movement has occurred. Bees also use sensor hairs on their rear legs to tell when their pollen baskets are full, signaling that it’s time to return to the colony.13
Movement of pollen from leg to leg is another one of those not-so-obvious behaviors because even as you watch, it’s not apparent what’s happening. Bees don’t move pollen straight back from their legs on one side of their body to the basket on that same side. The pollen grains cross the bee’s body from left to right and vice versa. The exchange happens at the middle legs, and I’ve watched bees in the process of packing pollen several times and have even filmed it, but I still can’t say I saw it happening because the packing is amazingly fast. The process ends on the rear leg, where the pollen is pressed into a pellet using a specialized adaptation of the hind leg appropriately named the pollen press (Figure 7). The pellet is formed around a single coarse hair, called the pollen pin, which holds it securely during the flight back home.
Figure 7 You can see all the specialized hair on this pollen press, which bees use to form the pollen pellet. The action of the press depends on those rake-like hairs in the center called the rastellum.
Bees mix a little nectar with the collected pollen to adjust its viscosity, ensuring it’s sticky enough to make the return flight without detaching. Bees will also gauge the type of pollen collected and change the added nectar accordingly since pollens vary in viscosity naturally. Adding nectar to pollen slightly changes its color, so it’s best to look at the source in the flower and compare its color to the collected pellet for accurate identification.
How Bees Digest Pollen
Once back at the colony, pollen is either consumed by nurse bees for the immediate production of brood food or placed in cells for later use. Storing pollen in cells begins a fermentation process fostered by the growth of bacteria and yeasts that naturally occur on the grains. The fermentation provides the grains with a more suitable environment for long-term storage, in a form which we call beebread. Although beebread involves fermentation, the individual grains remain unchanged. So whether a bee eats fresh pollen, which they prefer, or beebread, they’re up against the same task of extracting the protein from grains.
It would be simple if a bee’s mandibles were strong enough to break a grain open, but they’re not. Instead, pollen consumed by bees is, in theory, subject to a form of pre-digestion that mimics natural plant germination. When bees consume pollen, the grains end up in their honey crop, mixed with a sugar and acid solution. In that solution, most pollen species will start to pseudo-germinate and grow pollen tubes, mimicking the process of natural tube growth down a plant’s style.14 The pollen grains pass to the midgut, where the nutrients gradually empty from the grain.15 Studies of pollen digestion in other bee species have included another process that involves pressure when the grains pass from the honey crop to the midgut. That theory suggests that the grains may burst due to a change in osmotic pressure as they move into the midgut. The argument has some support, but there’s still skepticism about the lack of observable evidence in honey bees.
The last part of the process is how pollen passes through the midgut during digestion. Pollen grains don’t merely travel from the honey crop to the midgut; they are packed together and encapsulated in a porous membrane just as they leave the honey crop. The membrane package or bolus then travels through the midgut while digestive enzymes and possibly osmotic pressure work to ex-tract the protein. The time needed for the bolus package to move through the midgut depends on the pollen type. If you look at pollen grains in the hindgut after digestion, you will discover that approximately 30% of consumed pollen goes undigested, and that number can vary by species. Some species, like dandelion, resist digestion while others, like clover with a thin-walled exine, are more digestible.16 In the hindgut, digested grains look like a collapsed balloon, while those undigested look as good as new.
Bees need ten amino acids for larval growth and continued adult health. It’s important to note that few pollen species provide all ten, so bees must forage on various sources (Figure 8). Other factors determining the protein content are soil type, regional climate, and the season. Spring pollens are more nutritious, which works favorably with the spring build.16 Influenced by soil type and environment, some pollen will yield protein in the 9% range while others can be as high as 37%. Which leads to the question, why don’t bees forage on the most nutritious source? The answer comes from studies that show that, while excellent at distinguishing among nectar sources based on sugar content, bees don’t differentiate based on protein content. At times, bees will collect many things that aren’t even pollen, like birdseed, sawdust, and fungi.17
Figure 8 The key to a healthy colony is access to nutritious pollen. Shown here is an example of a colony collecting many different pollen species.
Although bees don’t make a distinction based on the protein content, they are attracted to an abundant source. As a result, you will sometimes observe a single pollen species on al- most every returning bee. One pollen study conducted on a large field of highbush blueberries receiving pollination services documented the allure for abundance. The results showed that bees were drawn away from the blueberries by two competing pollen sources. The data from trapped pollen showed that only 4% of the returning pollen was blueberry pollen. The bulk of the remainder was between wild dogwood and cherry pollen.18
One of the calming pleasures, bordering on meditation, is to take some time and watch as bees return with full pollen baskets, then land and walk into the entrance. If you miss the returning bee in flight, you can sometimes see the light-colored pellets flash like taillights as bees enter the hive’s darkness. It’s fun to watch, but figuring out the pollen species coming in during the season is more important.
I encourage beekeepers to first survey their area for available pollen sources by plant species. Once identified, start a log or a calendar based on bloom times and color. The most accurate way to determine the color of a pollen species in pellet form is to observe bees on the plant when actively collecting. Pollen pellets on bees actively collecting will match the color you see at the colony entrance.
An excellent place to start identifying pollen is with the first pollen of the season because there’s little blooming then. When I see yellow or light grey pollen in late February, it’s likely skunk cabbage and silver maple because, in my area, they are the only two blooming. I then look at specific specimens used yearly to verify my observations. Bloom calendars are a joy most beekeepers miss. They document the return of a dependable cycle, and they can help predict brood rearing and honey flows.
After starting your list, you can determine whether your area’s floral sources provide adequate protein. As mentioned, bees need pollen that contains ten essential amino acids for good health.19 Most pollen will have at least nine of the ten needed. If there is any scarcity, it will probably be with the single amino acid tryptophan, which frequently comes up missing in pollen. The one dependable source of tryptophan is the clover family (Trifolium), which in my area is known as white Dutch clover. Many other species in the family include peas and beans, so odds are your bees collect tryptophan in most years.
It can be challenging to figure out the amino acid content by pollen species because of sparse reference material. We need a list of regional plants with details like the amount of pollen plants generate and the amino acid content. As an alternative, I reference an Australian study called “Fat Bees Skinny Bees.”20 Although they are Australian plants, a surprising amount of similarities provide useful comparisons.
While amino acid references require some digging, there’s another way to measure the adequacy of incoming pollen — the time-proven method of simple observation. For instance, if your bees collect various pollens, indicated by different colors in cells, and the brood is pure white and plump, their protein needs are likely satisfied. Observation will also reveal the lack of adequate incoming pollen during brood rearing. Since brood food production requires lots of pollen, I look at young larvae (second instar) to ensure they are in a rich pool of brood food. Dry larvae can be seasonal and indicate a temporary pollen shortage, but if prolonged, it can suggest a more significant source-based problem with your area’s availability — your bloom calendar will help answer those questions.
Pollen’s importance in the ecosphere is undeniable, and the same is true for bees. We often equate potency with size, and in the world of fertilization, pollen’s silent exchange of genes provides the world with many riches, and its role as a life-sustaining nutrient makes pollen a biological giant.
Take care of your bees. - Bill
About the Author
Bill Hesbach is an Eastern Apicultural Society Certified Master Beekeeper and graduate of the University of Montana's Master Beekeeping Program. Bill is on the Board of Directors for the Eastern Apicultural Society, teaches bee biology and beekeeping methods at events hosted by regional organizations, and speaks about beekeeping at national seminars. He's a published author, with articles appearing in Bee Culture, Bee Craft, and The American Bee Journal. In addition, Bill has worked with native beekeepers in Kenya and Thailand.
References and End Notes
1 Angiosperm-like pollen and Afropollis from the Middle Triassic (Anisian) of the Germanic Basin (Northern Switzerland). Peter A. Hochuli, and Susanne Feist- Burkhardt 2 Plant microRNAs in larval food regulate honeybee caste development. Kegan Zhu, Minghui Liu , Zheng Fu , Zhen Zhou, Yan Kong, Hongwei Liang, Zheguang Lin, Jun Luo, Huoqing Zheng, Ping Wan, Junfeng Zhang, Ke Zen, Jiong Chen , Xi Chen. Published August 31, 2017, https://doi. org/10.1371/journal.pgen.1006946 3 Andrea D. McCue, Mauro Cresti, José A. Feijó, R. Keith Slotkin; Cytoplasmic connection of sperm cells to the pollen vegetative cell nucleus: potential roles of the male germ unit revisited, Journal of Experimental Botany, Volume 62, Issue 5, March 1 2011, Pages 1621–1631, https:// doi.org/10.1093/jxb/err032 4 Pollen nutritional content and digestibility for animals. T.H. Roulston and J.H. Cane 5 Sporopollenin is the actual name that de- scribes the biopolymer that makes up the exine layer. For further reading, see this paper: Sporopollenin, the least known yet toughest natural biopolymer. Grahame Mackenzi, Andrew N. Boa, Alberto Diego- Taboada, Stephen L. Atkin, and Thozhu- kat Sathyapalan https://www.frontiersin. org/articles/10.3389/fmats.2015.00066/full 6 Microspectrophotometric evaluation of digestibility of pollen grains. Franchi GG1, Franchi G, Corti P, Pompella A. 7 Forensic melissopalynologists were the first to discover the true national origin of different kinds of honey shipped to the U.S. under false country-of-origin labeling. 8 The role of the intine and cytoplasm in the activation and germination processes of Poaceae pollen grains.Jesus Marquez, Juan A. Seoane-Camba & Maria Suarez- Cervera.https://www.tandfonline.com/ doi/pdf/10.1080/00173139709362626 9 The pollen tube varies in length depend- ing on the plant species. One theory suggests that the amount of protein in a pollen species is proportional to the re- quirements needed to grow the tube. 10 Sperm Delivery in Flowering Plants: The Control of Pollen Tube Growth Kathleen L. Wilsen Peter K. Hepler 11 Evolution: Pollen or Pollinators — Which Came First? Simone C.Cappellari, Han- noSchaefer, Charles C.Davis. https://doi. org/10.1016/j.cub.2013.02.049 12 Honey bee hairs and pollenkitt are essen- tial for pollen capture and removal. Guill- ermo J Amador, Marguerite Matherne, D’Andre Waller, Megha Mathews, Stan- islav N Gorb, and David L Hu. Published March 23, 2017. Bioinspiration & Biomi- metics, Volume 12, Number 2.http://iop- science.iop.org/article/10.1088/1748-3190/ aa5c6e/meta 13 Displacement sensors in the honeybee pollen basket. D.M.Ford,H.R.Hepburn, F.B.Moseley,R.J.Rigby, Journal of Insect Physiology, Volume 27, Issue 5, 1981, Pag- es 339-346. https://doi.org/10.1016/0022- 1910(81)90080-9 14 Pollen nutritional content and digestibility for animals. T.H. Roulston I and J.H. Cane 15 Theproteinbalanceofthehoneybeework- er. K Crailsheim https://www.apidologie. org/articles/apido/pdf/1990/05/Apidolo- gie_0044-8435_1990_21_5_ART0004.pdf 16 Ranking pollen from bee plants according to their protein contribution to honey bees. Vasilis Liolios, Chrysoula Tananaki, Maria Dimou, Dimitrios Kanelis, Georgios Go- ras, Emmanouel Karazafiris & Andreas Thrasyvoulou 17 Shaw, D.E. (1990). The incidental col- lection of fungal spores by bees and the collection of spores in lieu of pollen. Bee World, 71, 158–176. https://www. tandfonline.com/doi/abs/10.1080/000577 2X.1990.11099059 18 Pollen preferences of honeybees sited on four cultivated crops. Larry g. Olsen, Roger Hoopingarner, and E. C. Mar- tin, Journal of Apicultural Research 18(3): 196-200 (1979). 19 The ten essential amino acids required by bees are: Threonine, Valine, Methionine, Isoleucine, Leucine, Phenylalanine, Histi- dine, Lysine, Arginine, Tryptophan. 20 “Fat Bees Skinny Bees” is a free download with lots of information on honey bee nu- trition. The plants listed in the amino acid tables are Australian, but many species are similar and can aid you in determin- ing the amino acid contents of US plants. Click “Download PDF” from the link below. https://www.agrifutures.com.au/ product/fat-bees-skinny-bees-a-manual- on-honey-bee-nutrition-for-beekeepers/