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.
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.