A Closer Look at Propolis
Propolis and the resin connection
When the weather turns warm, propolis can string into fragrant tendrils like fresh-from-the-oven mozzarella cheese. When cold, it can shatter like bone china, yielding knife-sharp shards. In especially hot seasons, it can run like water down the inside of hive walls before hardening into translucent droplets that glisten like gemstones.
This article first appeared in American Bee Journal, Volume 158 No 10, October 2018, pp. 1155-1158.
As fascinating as propolis is, it can test the patience of the most experienced beekeeper. It cements boxes together, glues frames to each other, sticks to fingers and tools, feeders and excluders. Worse, it makes indelible marks on clothing and bee suits. And no matter how many times you scrape it away, it will be waiting for you next time you open a hive. In the beekeeper’s world of consummate stickiness, nothing outsticks propolis.
Because propolis is so messy, and because it can easily destroy the pristine look of comb honey, early breeders spent decades trying to get rid of it. They believed that developing a strain of bees that deposited little propolis would be a boon and a blessing to honey producers and simplify the tasks of beekeeping.
First aid for plants
You often hear that honey bees collect propolis, but that’s not quite true. Honey bees don’t collect propolis fully formed any more than they collect honey fully formed. What bees collect are the raw ingredients to make propolis, namely plant resins.
Just as nectar varies according to the local plants, so do resins. Most resins are collected from leaf buds, twigs, or the bark of trees. It may appear whitish gray, tan, a variety of browns and reds, or nearly black. The resins are made from a variety of phytochemicals that are designed to protect the plant from invasion by pathogens and predators.
The resins are produced in special cells and kept in reserve. When a plant is damaged, the resin oozes out of the cells and flows over the injury, making a bandage-like barrier that keeps out harmful bacteria and fungus. Although the specific properties vary from plant to plant, resins are alike in that they are insoluble in water and they harden when exposed to air.
Many sources of resin
According to the USDA Forest Service, a wide variety of plants produce resin.1 Conifer trees are famous for it, including various species of cedar, fir, juniper, larch, pine, redwood, spruce, and yew. But other trees also produce resin, such as alder, aspen, birch, chestnut, poplar, sweetgum, and willow. In addition, a surprising variety of other flowering plants produce their own resins. Examples include balsam root, caraway, creosote bush, dill, fennel, gardenia, ginseng, mayapple, morning glory, parsley, poison ivy, poison oak, quinine, rabbitbrush, sarsaparilla, sunflower, and tarweed.
Before the days of commercial chewing gum, spruce sap was a popular chew. The mechanical act of chewing along with the addition of saliva made it soft and pliable, just as the honey bee discovered. In humans, spruce resin turns the mouth red in the same way it stains woodenware and bee suits. Commercial chewing gum couldn’t come soon enough.
Collecting the goods
A small percentage of the work force in each colony has the non-enviable task of collecting resin. Because its consistency varies with temperature, resins needs to be collected on warm days when they are soft enough to be worked. On the right kind of day, the honey bees use dance language to describe the location of a rich source. Then the select few are off to meet their challenge.
First, the worker bites off a chunk of resin. She works this with her mandibles, adding saliva for softening. Once it is pliable, the worker stores it for transport. According to entomologist Rosanna Mattingly, resins are carried in the pollen baskets. But loading resin into pollen baskets is not the same as loading pollen.2
Pollen changes sides as it is passed from the forelegs, to the mid legs, the hind legs, and finally to the baskets. Side-to-side transfer of pollen occurs as the honey bee brushes pollen from one collection spot to another, and then into the pollen press. But a load of resin stays on one side of the body and goes straight back, perhaps to limit the number of sticky transfers. From the mandibles, it is passed to the forelegs and then to the inner side of the basitarsus of the rear legs, and then into the corbiculae.
The transformation to propolis
Once in the hive, the worker bee needs help to offload her gooey cargo. Instead of kicking her load into an awaiting cell as she would a pollen pellet, she requires the assistance of other bees who bite at it with their mouthparts. Once the resins are removed from the corbiculae, other bees begin the job of transforming the raw resin into propolis.
Sometimes called “bee glue,” the finished product is a mixture of resins, beeswax, salivary secretions, and a pinch of pollen. The bees continue to work the dough until they achieve the consistency they need. The amount of beeswax and saliva added to a batch depends on the source of the resin but on average, finished propolis contains roughly 50% balsams, 30% waxes, 10% essential oils, and 5% pollen. The rest is a vast composite of amino acids, vitamins, and minerals.3
When the transformation to propolis is complete and the material is soft and malleable, the workers smear it on rough surfaces, cracks, holes, or any place where they want a smooth, waterproof, or antimicrobial surface. Propolis is produced on an as-needed basis, and is not stored in the hive like pollen or nectar.
Putting propolis to work
A colony of honey bees takes full advantage of the many properties of plant resins, both chemical and physical. Depending on the source, resins have been shown to have antibacterial, antifungal, antiviral, and anti-inflammatory properties. And in addition to waterproofing, it can lend structural support to combs and joints. Using plenty of propolis, the bees can build a protective envelope that helps shield the colony from disease, sharp edges, damaging moisture, and melty combs.
In feral honey bee colonies, the deposition of propolis begins on the outside of the nest cavity. You can often see a ring of propolis completely surrounding the entrance. This ring can be many inches wide and several layers deep. In theory, the propolis ring suppresses microbes from freely entering the nest.
In managed hives, honey bees distribute the propolis in different ways. Since there is no tree to coat, the bees spread it on rough surfaces, they use it to caulk cracks, and they (annoyingly) glue frames to the frame rests. In addition, they may use it to strengthen wax combs, sequester unwanted substances, or adjust the size of their entrance.
Honey bees are also famous for coating dead animals in propolis. The carcasses of too-big-to-move creatures such as mice, voles, or snakes, are frequently found coated in a thick layer of propolis which keeps the harmful bacteria separated from the colony. In addition, pollen has been found entombed beneath layers of propolis, possibly because it was contaminated by pesticide or some other dangerous substance.
The cooperative use of external agents to protect a colony from disease and predation is a form of social immunity. Compared to traditional immunity, social immunity is a form of external defense similar to a family locking its doors or cooking its food. It is social practice that helps protect all the individuals in a living unit.
Propolis collection is a vital part of a honey bee colony’s immune defense. Similar to the hygienic removal of sick larvae or altruistic suicide, it most likely evolved to combat the increased risk of disease transmission that results from group living. By building an antibiotic envelope around the brood nest, eliminating sharp edges, and sequestering sources of contamination, each individual bee benefits as does the colony as a whole.
Honey bees are not unique in their quest for plant resins. In fact, resin collecting is a fairly popular activity in the bee world, having evolved in several different lineages. Most resin bees are found along with other mason bees in the family Megachilidae.
The so-called resin bees, of which there are many, use resins in basically the same way as honey bees. Even though most resin bees are solitary, they collect resins from plants and use them to seal their nests against diseases, predators, and moisture. Most species use the resins both within the brood area and at the entrance. Unlike honey bees, resin bees carry balls of resin back to the nest firmly clasped in their large mandibles.
For example, the genus Dianthidium is a type of mason bee that uses resin to glue tiny pebbles together to seal nest entrances. Some species build exposed nests of resin and pebbles attached to a twig, or they dig in the ground and build their structure attached to a root. The result is similar to a mortared rock wall.
Another genus in the same family, Heriades, builds nests in hollow stems or cavities. But instead of using leaves or mud to construct partitions between egg chambers, these bees use resin. When the nests are complete, they seal the entire cavity with another plug of resin. These waterproof and disease-resistant barriers help the immature life stages overwinter in less-than-perfect conditions. Heriades are economical bees and will even reuse resin from an old nesting cavity.
Honey bees recycle old propolis
When it comes to recycling resins, the industrious Heriades are not alone. Honey bees will reuse propolis by scraping it from one place, reworking it, and moving it elsewhere.
In an amazing series of photographs and a must-see video, UK beekeeper Christopher Wren captured honey bees in the act of recycling. He writes, “The bee first chews off a bit of propolis with her mandibles, then she transfers it to her front feet. The next move is very rapid as she moves it back to the inside of one of her middle legs. Then it is moved to the corbicula (pollen basket) on the same side and patted into position.”4
Wren discovered this practice while he was preparing an empty nucleus hive. By chance, he noticed a honey bee making repeated trips to the crown board to collect basket-loads of old propolis. Since the weather was warm and plant resins were available, he speculated that recycled propolis may be easier to collect than new resin.
Resin bees in the news
A pair of east coast resin bees made the news in recent years. The giant resin bee, Megachile sculpturalis, is a recent import to North America, having first arrived in North Carolina in the 1990s. These large cavity-nesting bees use resin to line their nests, just like other resin bees. But in addition, they have been seen smearing resin on native carpenter bees and rendering them immobile. Since the giant resin bees cannot drill into wood themselves, they steal the nests of carpenter bees by attacking or killing the builder. Both their size and aggressive behavior are worrisome to beekeepers who fear attacks on their colonies, but honey bees don’t have what the giant resin bee wants.
In 2013 the bellflower resin bee, Megachile campanulae, made the news for a different reason. Entomologists discovered that urban populations of this bee, which is native to the northeastern US and parts of Canada, were using man-made materials—such as builder’s caulk—in place of resin. They speculated that this was a form of adaptive behavior compelled by urbanization: when the normal plant materials became scarce, the bees found alternatives. Unfortunately, the chemical polymers in such products are not good for bees.
Although I have not heard of it, I wouldn’t be surprised to find honey bees collecting similar materials. Since we know honey bees will occasionally collect sawdust and coffee grounds when pollen is scarce, it doesn’t seem far-fetched that they might look for resin substitutes as well. Honey bees, however, have the advantage of being able to travel great distances, something that most wild bee species cannot do.
Setting the record straight
We’ve come a long way in our understanding of honey bee health, and now the idea of breeding away propolis deposition sounds ludicrous. In fact, a 2017 American Bee Journal article by Thomas Seeley suggests that beekeepers build bee boxes of rough-sawn lumber because the irregular interior surface will encourage propolis deposition and the formation of an antimicrobial envelope. Some of the forward-thinking manufacturers, such as the makers of the Valkyrie long hives, are even roughing up hive interiors during construction.
Fused through time
As you can see, resin is a common feature in the world of bees. To me, the existence of resin collection in multiple species is a strong indicator of its evolutionary value. We should pay attention. Far from being an annoyance, resin may be the beekeeper’s best friend.
Honey Bee Suite
Notes and references
Mattingly RL. 2012. Honey-Maker: How the Honey Bee Worker Does What She Does. Portland, Oregon. Beargrass Press. Huang S, Zhang CP, Wang K, Li GQ, Hu FL. 2014. Recent advances in the chemical composition of propolis. Molecules 19: 19610-19632. An excellent video of a honey bee collecting used propolis, along with many more recycling photos, can be seen on Christopher Wren’s website. More resin bee photos by Mark Berkery can be seen at his website Being Mark.
How to determine the quality of propolis
According to the Polish Standard PN-R-78891, the classification of raw propolis to quality class I or II is based on the content of ethanol insoluble matter. In other words, it depends on the amount of wax and impurities.
The desired components of propolis, which means mainly resins, dissolve in ethanol (spirit), while wax and various solid impurities – such as wood splinters, pollen or chitinous fragments of bees – do not dissolve.
According to the Polish Standard, the 1st class includes propolis which contains not more than 30% of ethanol insoluble matter. It’s very little. It must be remembered that the bee, when depositing propolis in the hive, mixes the resins brought from the environment with the wax it produces. Thus, even a clod of seemingly pure propolis contains at least 25% of wax. For this reason, 1st class propolis, according to the Polish Standard, is extremely rare in wholesale trade.
The 2nd class includes propolis with the content of insoluble substances between 30% and 50%. Such goods are usually delivered by beekeepers to purchase centers.
How to determine the quality of your propolis? Unfortunately, judging by appearance can be deceptive. The only effective method is to dissolve a sample of propolis in spirit and weigh the undissolved residue. Such a method is described in the Polish Standard. It is quite simple and can be successfully made with minor modifications at home.
We will need:
– balance with an accuracy of at least 0.01 g. It can be, for example, a pocket jewelry scale available in many variants on the market at a price of about 25$
– white tissue paper (from the stationery store)
– approx. 40 ml of 95% alcohol (ethanol)
– two vessels with a capacity of 50-100 ml (one must have a tight closure)
1) Pour 20 ml of alcohol into the first vessel (the amount doesn’t have to be very precise).
2) Take 2.00 g from the propolis you want to evaluate (accuracy is important here) and place it in the first pot with alcohol. The sample should be crushed.
3) Close it tightly and leave it at room temperature in order for the propolis to dissolve. Mix it up from time to time. The amount of time depends on the intensity of mixing. According to the Polish Standard it will be 24 hours, but on an automatic agitator that mixes continuously. In home conditions, the time of 48 hours will work well, with repeated mixing by hand.
4) After that time, put it in the fridge for an hour.
5) Cut from the tissue paper a square of funnel dimensions, but not larger than 15×15 cm, weigh it and write down its weight. You can just write it on the tissue paper, but only with a pencil.
6) Fold the tissue paper in four and put it in the funnel, which in turn should be put it in the second vessel. Filter the contents of the first vessel. It will take a while.
7) When all the liquid is in the second vessel and only the insoluble residue is left on the tissue paper, remove the tissue paper with its contents (we can wait until it dries a little).
8) Twist the tissue paper so that the contents do not fall out and rinse in a small amount of fresh spirit (the first vessel can be used for this).
9) Leave the tissue paper to dry, and when it is completely dry, weigh it. Accuracy is important here.
10) Use the following formula to see the result:
y = (x-z) / w
y: impurity content
x: weight of tissue paper with undissolved residue
z: weight of tissue paper itself
w: starting mass of the propolis sample (here 2g)
This way you will get a fractional result. Multiply this result times 100 to get the percentage.
A Closer Look at Propolis
Author: Dr. Lane Kreitlow Propolis is the sticky resin honey bees collect from trees or other botanical sources and bring back to the hive for multiple household uses.
Propolis is an antimicrobial, multi-purpose product used inside the hive to seal cracks, strengthen structures, repair comb, seal up any alternative entrances to the hive, and serves as a preserving or repellent agent. The wide use of propolis inside the hive is, in fact, one of the reasons why bees can stay healthy, in spite of living in a crowded space where germs can spread rapidly.
Bees collect propolis by scraping it off trees with their mandibles and packing it into their pollen baskets (corbiculae), where it is carried back to the hive. Propolis is not stored inside cells in the hive but is brought in and used as needed. It is widely used on hard surfaces inside the hive. Because propolis has antimicrobial properties, a thin layer painted over the inner surfaces serves to keep microbes in check.
Propolis is collected from a wide variety of sources, but some of the best sources include poplar, willow, alder, conifer, elm, horse chestnut, beech, and birch trees. Propolis ranges in color from dark brown or black, to reddish, and even green, depending on the source from which it was collected.
At or above room temperature, propolis is sticky, soft, and pliable, and can be easily manipulated by bees to fill gaps or varnish surfaces. Propolis becomes hard and brittle as it ages, or when the temperature drops below about 75°F.
My first experience with propolis was early in graduate school many years ago. One of the duties imposed on us “lowly” students by our lab’s technician (who shall remain anonymous) was to scrape off the propolis from the inside of the inner covers and tops of frames of all of the hives in our research apiary, using a hive tool. Here is where I learned two important lessons about propolis: 1) propolis is, indeed, very sticky and almost impossible to remove while in this state but is much easier to remove when it is old and brittle and, 2) bees do NOT like it when you scrape off propolis from their home! Not surprisingly, we frequently got stung during those exercises, and thus approached it with dread. (I always got the feeling that our “propolis duty” was more about testing our bee fortitude than anything else.) But I digress.
Bees have many uses for propolis, including the following:
Inside the hive, bees use propolis to seal up cracks and crevices that are smaller than “bee space”. Bee space is the amount of space needed by a bee to move around easily inside the hive. Roughly 3/8”, bees seal with propolis anything smaller than bee space. Bees typically build comb in spaces larger than bee space. You will notice this phenomenon if you ever leave a spacer or partially filled hive body on the hive too long.
Like all insects, honey bees are cold-blooded, though they have mechanisms that allow them to regulate the hive temperature. One of these behaviors is the tendency to reduce potentially deadly drafts by sealing cracks with propolis. Beekeepers should remember this during cooler weather and refrain from unnecessary and too-frequent hive inspections, as every time you remove the inner cover, you are breaking the propolis seal.
In addition, left unsealed, cracks, and crevices can create rough edges, which irritate or tear the delicate wings or exoskeleton as a bee rubs up against it. A layer of propolis will smooth out surfaces to prevent this from happening.
Propolis is commonly referred to as “bee glue”, a name of obvious appropriateness to an experienced beekeeper. Bees seal with propolis gaps formed between the inner and outer covers, hive boxes, frames and anywhere else that might make the hive drafty or let in water. The familiar “cracking” sound created when you pop off the covers of a hive occurs when you break the aforementioned propolis seal. It is nearly impossible to open a hive or remove frames without a hive tool, due to the widespread use of propolis within the hive.
Propolis is well known to have antibacterial and antifungal properties. Bees commonly spread a thin layer of propolis on hard surfaces of the hive to keep down the level of microbes inside the hive. Annoying as it may be, it is best for the bees for the beekeeper to leave the propolis inside the hive, even if it makes hive inspections more difficult.
One of the many remarkable qualities of honey bees is their keen sense of sanitation. As individual bees die, they are carried out of the hive so that their decomposing remains- and the ensuing reservoir of germs- do not remain inside the hive. In fact, one of the telltale signs of various brood diseases is the presence of dead, diseased larvae or pupae outside the front of the hive. Bees with a high level of hygienic behavior are more disease resistant because of this propensity to remove decaying and potentially infectious matter from the hive.
Sometimes, though, small animals that are too heavy for the bees to remove die inside of the hive. The highly antimicrobial properties of propolis make it an ideal substance with which to embalm dead creatures that are too large to be carried out of the hive. It is not unusual to find a mouse that has been entombed in propolis inside a hive.
Medicinal Uses by Humans
The medicinal properties of propolis have been known for centuries. The ancient Greeks, Romans, and Egyptians knew about it. Pliny the Elder knew about. Aristotle knew about it. Even Hippocrates, the father of modern medicine, is said to have used propolis to cure internal and external wounds. The first recorded human use of propolis dates back to 300 BC and continues to this day. Its use is particularly popular in herbal and natural medicine.
Advocates believe that propolis has antiseptic, antibacterial, astringent, anti-inflammatory, antioxidant, anesthetic, and possibly even anticancer properties in humans. Though scientific evidence is sparse with some of these claims, it is well established that propolis has antimicrobial properties. Propolis is included in ointments, creams, and tinctures in a variety of products used for skin problems, burns, and wound healing.
Propolis is usually harvested with a propolis trap, a thin, plastic sheet that is perforated with narrow slits just slightly less than-you guessed it- bee space. They look similar to queen excluders, but with smaller gaps.
The idea is that bees will propolize the gaps in the trap when it used in place of the inner cover. The outer cover can be propped up just a bit to allow light into the hive. Bees will readily seal the trap with propolis in order to block out the light, as well as drafts created by the holes in the trap. When the trap is full of propolis it is removed from the hive for harvesting.
For propolis that cannot be easily scraped off, the trap can be placed in a bag and frozen, and the residual propolis removed from the slits by banging it onto a hard surface. Autumn is a good time to trap and collect propolis since this is the time of year when bees seal drafts in preparation for winter.
Propolis may also be harvested without the use of a trap by simply scraping it from areas where it has been placed in the hive.
Propolis is particularly popular in the natural products industry, though the medicinal use of propolis tends to be more popular in Europe than in the United States.
To some beekeepers, propolis may be an aggravation, but it is a crucial hive product that helps bees stay healthy. Think about this the next time you have difficulty removing your frames, and it might curb your frustration. As for my fading memories of scraping propolis all of those years ago, it did indeed instill a sense of bee fortitude but even more, an unwavering appreciation for all things honey bee.