Hive Mending, Antimicrobial Activity & Disease Prevention
Propolis is a combination of plant resins and wax used by worker bees inside the hive to mend cracks and defects, preserve the dead, and sticking frames together to annoy beekeepers. Foragers collect resin from wounded trees or new leaf buds and carry it in their corbiculae (the hind leg baskets) back to the colony for further processing.
Inside the hive, middle aged bees scrape the sticky resin off the returning forager, mixing it with wax and salivary enzymes to make it softer and more workable to produce the end product: propolis. This can now be used around the hive for repairs, narrowing the hive entrance (defensive measures) or mummify objects too big to remove (e.g. dead mice).
This article explores propolis formation & chemistry, as well as its role in the colony’s “social immunity”. I didn’t want to this to become a one-sentence paragraph1, so I’m including a fun trivia on the meaning of the word “propolis”: the Greek prefix “pro” means “before” or “in front of” and also the Greek suffix “-polis” meaning city, thus giving us “propolis”, meaning “before the city”.
There’s a shirt that goes with this article, designed & printed by yours truly:
How Resin is Collected & Used Inside the hive
Resin is collected by forager bees from plant barks, new buds, stems and so on2. It appears as droplets on the plant’s surface that can be scraped to a forager’s hind legs, and when extracted from budding leaves, the forager uses her mandibles to fragment the growth and extract its resinous sap (reviewed in Simone-Finstrom & Spivak, 2010). The general process is as follows: A resin particle is broken off with the mandibles and then passed on from the forelegs back to the hindlegs (via the oft-ignored middle legs!). In the hindlegs, they are packed onto the corbicula from each side (those pollen baskets have now become resin baskets), rinse and repeat until the baskets are full.
Back at the hive, the resin is unloaded to a receiver bee, and the duration of this unloading is dependent on colony needs (the more need, the faster the unloading). This is not like pollen, water or nectar gathering whereby the forager waits at the hive entrance; resin foragers unload in areas where it is required (be it a hive wall, brood area or a mouse corpse; Nakamura & Seeley, 2006).
As the “cementing bee” removes resin from the forager, she starts manipulating it with her mandibles, adding salivary enzymes and wax to turn it into propolis. Once the forager has completely unloaded, she takes part in these repairing activities. Hive walls are lined with propolis prior to comb building3 (NOT “brood cells are lined with propolis”, as AI will tell you- see Simone-Finstrom & Spivak, 2010), nest entrances are reinforced/shrunk, and any rough patches are smoothened (after all, the inside of a tree cavity is all sharp and pointy). If they’re housed in Langstroth boxes, they’ll also glue frames together to show their love and appreciation for you, the beekeeper.
Resin Use Outside of A. mellifera
Other Apis species use resins and propolis differently. A. mellifera’s eastern cousins A. cerana don’t use it at all, whereas their giant cousins A.dorsata (who are also eastern) use it occasionally for strengthening comb suspended from a tree branch, or coating foreign objects. Their dwarf cousins A. florea use propolis as a predator-deterring coating on the branch from which they suspend their comb. The references here are early works (& books) by Seely, as well as Eva Crane’s 1990 text “Bees and Beekeeping”. Formal research on other Apis species is fewer than A. mellifera, and scarcer still when looking at how these species use propolis.
Propolis is a Part of the Colony’s Immune System
Propolis is proposed as a form of “social immunity” to the bee colonies (i.e. the collective defences of a group to protect themselves from pathogens; reviewed in Simone-Finstrom & Spivak, 2010). In one study, the presence of propolis on hive walls lowered adult bee immune gene expression (Simone et al., 2009), which reduced the energy spent by bees on immune system activation. Maintaining an immune system is metabolically costly, so when less energy is required for normal adult bee function, that remaining energy is diverted elsewhere: nectar processing, brood care, wax comb building, queen bee feeding, water collection… the list goes on.
Propolis Reduces Pathogen and Pest Burdens
Hives with propolis envelopes had reduced microbial growth for pathogens that cause American Foulbrood & Chalkbrood due to antimicrobial activity from compounds like flavonoids, esters and large fatty acids. Al-Ani and colleagues showed that propolis works on both Gram-positive and Gram-negative (meaning those bacteria with and without a certain type of cell wall), as well as aerobic and anaerobic (oxygen-using versus non-oxygen using; Al-Ani et al., 2018). The effect is more pronounced in gram-positive bacteria, which makes me think that one antibacterial mechanism involves cell wall disruptions (amongst many other mechanisms- propolis is a complex compound after all).
When Varroa mites are exposed to a 10% propolis extract, all mites were killed (reviewed in Simone-Finstrom & Spivak, 2010). Further experiments found that increasing propolis concentrations within a managed beehive reduced Varroa loads, as well subsequent viral loads (e.g. Sacbrood & DWV; Drescher et al., 2017). Drescher & colleagues reported that these colonies had a stronger immune system, and other studies showed increased brood survival & increased worker life expectancy in high-propolis producing bees (Nicodemo et al., 2013).
Propolis-as-a-treatment is promising since the bees make it themselves, reducing the need for chemical applications. The hundreds of compounds in propolis means that a mite or bacterium will have a hard time developing resistance, compared to the single regimen of flumethrin or formic acid. If you’re still set on treating your bees, consider spraying them with propolis extracts (Ayad et al., 2024). In this study, Ayad and colleagues sprayed adult A.m. intermissa (the north African honey bee subspecies) with a 10% propolis solution and found >85% mite mortality with no impact on the bees. The authors suggest that 10% is a viable concentration, citing other studies from Argentina & Germany which reported propolis sprays as a potential mite treatment method.
So, whether it is poplar-based propolis (which is more studied because… most research comes out of North America) or propolis from mango tree resins, the compound is effective and promising as another layer of immunity to the colony. I wonder when we will start being sold propolis sprays as a treatment for Varroa, or a tincture for general colony health… More than that, what’s stopping you from making a spray of your own?4
Plant Biology: Resins & Exudates
Resin is released by the plants as a protective measure. It protects young leaves from pathogens, pests and herbivores, the latter by giving it an unpleasant taste/digestive unpalatability (“tummyache”). Resin’s chemical profile is based on the plant source, with the base contents being flavonoids, aliphatic acids and esters, aromatic acids and esters, chalcones, terpenes, lignans, shmignans, JoeMamanoids, amongst others. Multiple reviews on the benefits of these components for human health exist, and the interested reader is directed to the literature for more information (e.g. Adetunji et al., 2021).
A generalised plant resin production process is as follows:
- Resin compounds (Terpenoid, Phenolics & other complex acids/aldehydes/lipids) are synthesised within their respective specialised cells
- They are stored in specialised secretory structures (with names like “idioblasts” and “Glandular trichomes”, or more the straightforward “resin ducts”, depending on the plant species)
- The resins are released when:
- A: The plant is damaged from herbivores, pruning, fire or a cheeky forager bee’s mandibles (many Australian native trees fall in this category ([gum trees, wattles, pines etc..])
- B: as a continuous flow to protect new leaf growth (North American Poplar tree species are one such example)
- Once released, the resins harden and cover the surface. If it becomes too hard, the forager won’t gather it and the plant carries on. When it is still soft, they begin filling their corbiculae.
Resins are not required for plant survival (“secondary metabolites”) and are there for plant defence and wound healing. Good for tree, good for bee.
Resin and Subsequent Propolis Varieties
The literature identifies eight categories of propolis on the basis of botany and chemistry: poplar-type propolis from temperate regions (North America), “aspen” propolis from northern Europe, Brazilian green propolis from southeastern Brazil, South American red propolis from Cuba and Brazil, Mediterranean propolis from southern Europe, Pacific propolis from Pacific islands, Mangifera indica (i.e. mango tree) propolis from Indonesia, and hybrid types such as aspen-poplar and Pacific-Mangifera indica propolis (taken from Bankova et al., 2016).
As mentioned above, the chemistry of the propolis mixture (“volatile fraction”) is dependent on the tree. Alqarni and colleagues reported that bits of road (asphalt) were found in propolis from urban areas of Saudi Arabia (Alqarni et al., 2015). Clever girls… This suggests that propolis is not just resin and wax alone, rather a blend of chemicals that the bees deem acceptable for their antimicrobial and preserving effects.
Stingless Bees: Geopropolis & Cerumen
Propolis is what A. mellifera uses. Geopropolis is propolis with extra steps (added soil) and is used by stingless bees (Meliponines), whereas “cerumen” is standard propolis (resin blended with wax) that’s made by non-honey bee species (reviewed in Chuttong et al., 2023). Geopropolis incorporates soil, making it less malleable than standard propolis yet still provides similar functions inside the hive.
Chuttong & colleagues mention that despite showing similar antimicrobial/immune function benefits in both bees and humans, these non-A. mellifera propolis counterparts are not well studied… I wager that there’s too many species and too much diversity in each stingless bee tribe, coupled with their relative rarity and niche ecological ranges… it’s makes A. mellifera much easier (and appealing) to study. Nonetheless, geopropolis may end up being more promising than regular propolis due to the presence of soil microbes- we’re just a few decades early.
What to say of cerumen? The Meliponini tribe of stingless bees combine plant resins with their brand of beeswax to produce the propolis we know and love. Stingless bees produce both propolis and cerumen, and while the literature uses them interchangeably (as they do with resin and propolis!), they are used differently: cerumen is for building nest materials (combs, pillars, storage pots etc…), and also for mummifying dead intruders, whereas their propolis is more for mummifying and repairing cracks/hive deficiencies (reviewed in Chuttong et al., 2023).
Want More Propolis from Your Hive? Damage the Box!
…or buy commercially-roughened hive box. So, you’ve read the above and now want to get practical by stimulating propolis production in your hive walls, how do you go about this? I propose you follow the methods of Shanahan & colleagues, who housed colonies in rough wooden boxes & also “propolis-trap” boxes to maximise propolis deposits (Shanahan et al., 2024).
Commercial-grade rough boxes (in this case from the Propolis Hive Company, USA… I can’t find them on Google?) contain deep vertical grooves measuring 0.3175 cm wide by 0.3175 cm deep. Grooves were spaced every 0.635 cm on the interior of the four walls. The inner surface of was not planed or sanded so it was rough, even slightly splintery (paraphrasing Shanahan et al., 2024). Colonies were placed in their respective boxes and the bees were monitored. After a year, they found that these “rough” wooden boxes had a positive outcome on colony health, presumably due to increased propolis deposition. They performed better than commercial propolis traps, and the authors note that future models “should strive to imitate the combination of rough wood textures, cracks, and crevices found in the hollow tree cavities”.
Likewise, the authors note a reduction in Varroa loads within propolis-enveloped colonies, suggesting that a honey-bee made (“natural”) propolis envelope does better than coating hive walls with propolis, or by stimulating propolis deposits with commercial propolis traps. Lower EFB-causing bacteria (M. plutonius) genes were detected in the propolis-rich versus propolis poor colonies, lower viral loads and so on… I can go and on and, but you get the picture – more propolis deposits, better colony health outcomes.
So, if you want to achieve the same, I reckon buy a commercial trap, or look for companies that sell these roughened hive boxes OR, make it yourself if you have the tools (& patience) – the dimensions are above, should be simple, right? Add in a few cracks and crevices as in a natural beehive and watch your colonies perform.
One thing I’d like to know from their was how hive inspections were conducted– did the propolis-rich hives make it more difficult to move frames and inspect them? I’d say yes, based on how the propolis sticks to the ends of each frame in my hive… All up, it is a small price to pay for increased colony survival. It is also possible that keeping small & healthy (as opposed to “small & strong”) rough box colonies will produce less honey in their first year as they focus on resin collection to seal the cracks, then produce larger yields next year… many future possibilities, indeed.
Human Consumption of Propolis
I’ve had a dropper bottle of propolis for a few years, purchased from a member of my beekeeping club who has a company in Uganda(!!), sourced from (I assume) local A.m. scutellata populations. This propolis is collected using propolis traps inside the hive, extracted, and then processed into a human-safe dosage by first freezing the blend and then crushing it to remove hive debris, followed by extraction of the bioactive compounds (whether alcohol or non-alcohol extraction methods are used, I don’t know).
The resulting liquid is bottled into a dropper bottle and can be consumed directly, or mixed in water as I do. It has a sharp, strong taste and alcohol-like odour (oh look, I’ve figured out it’s alcohol extracted), and tends to stick to everything- teeth, mug, benchtop… be sure to clean up quick!!
The benefits of propolis are many: antiviral, antibacterial, wound healing, oral health, and so on. I personally use it when I have a cold; 3-4 drops twice a day in water. Whether it works or not, that can be debated… maybe it does, as a placebo effect. I do enjoy its taste and the fact that I’m consuming something made by bee!
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Footnotes
- After completing the article I realised that I do have more to add to the formerly-one-sentence-paragraph and since I didn’t want to delete the fun trivia, here’s a new paragraph: The literature appears to reference to “resin” and “propolis” interchangeably (looking at you, articles written pre-2015) I do not agree with this, as “resin” is the plant exudate in its raw form, whereas “propolis” is resin with wax and saliva added by the bees (or soil, as you’ll find out soon enough). ↩︎
- Keeping it simple, the resin sources vary between regions, with “temperate regions” coming from plants like Poplar spp. (e.g. cottonwoods, aspens) and other varieties like pines, elms, beeches & birches; the environment determines the propolis profile. ↩︎
- This is the “propolis envelope” in natural beehives- The idea being that by placing propolis on walls prior to comb attachment, a clean & smooth surface is created for the wax to adhere to. The stickiness of propolis also adds additional support to the comb. Why is there no such envelope in in conventional beekeeping boxes, you ask? It’s because the smooth inner walls of the box do not trigger the “I need to fix this” propolising response, and so the bees see the box “okay as is”. You’ll only find propolis deposits in cracks, crevices and between frames. ↩︎
- If any chemists or beekeepers-who-happen-to-know-chemists read this, please get in touch! ↩︎