Propolis is a mysterious material, not so much a thing bees produce but literally a collection of ‘things’ they use. Beekeepers view it as a bit of a nuisance and frequently selectively breed honeybees that use as little as possible. In some respects, that’s not a good idea.
Apis, euglossine, meliponine, and megachilid bees, and, occasionally, other social insects, all use a kind of propolis to a greater or lesser extent, which in its simplest description consists of plant resins mixed with wax (propolis and cerumen) or mixed with clay or sand soils (geopropolis or batumen). There are plants that use the desirability of these useful resins to attract pollinators but in many, many plants they are just ‘there’ as a part of normal metabolic processes. It is an essential nesting material, but also has some interesting biological activity as well as its physical function.
It’s the later that has been the reason for most of the research. It seems Man has always regarded propolis as a therapeutic product, not without reason, and there is plenty of study about its constituents that, so far, seems to have led nowhere. Propolis has been observed to be anti-bacterial, anti-viral, anti-fungal, cytotoxic, anti-inflammatory, anaesthetic, immunomodulatory, and anti-oxidant. Oh, and allergenic, particularly after ingestion. Propolis is quite complex, containing so far at least 300 common phytochemicals, and is never quite the same, varying qualitatively and quantitatively according to exactly where and when it was collected, and by whom, all across the world. Many of these chemicals can be shown to be pharmacologically active, in the right dose even toxic. It’s also true that some are known to have a therapeutic effect for common bee diseases and pests. Unfortunately, it’s also possible to detect less desirable contaminates in propolis, including various classes of residues including pesticides, antibiotics, and heavy metals. At the moment we are never in a position to anticipate its properties or effects and it’s a fair way from being systematically useful because it’s so unpredictable.
European honeybees collect the ingredients for propolis from several plant structures where they are actively secreted or exuded from wounds. They can be waxes on leaves or buds, gums and resins from the bark, from fruit, or around trichomes and the ducts of new leaves. In a pinch, even paint, asphalt, and oil. For temperate bees like ours by far the most important sources are the bud exudates from poplars, birch, horse-chestnuts, willow, pine, elm, and alder. I have seen mine working the seed capsules of pittosporums. They collect at any time of year, but mostly while temperatures are warm enough to render the materials malleable. We don’t know how they find sources but the assumption made is that they detect the odour. Resin foraging bees seem to be unoccupied intermediaries between nest construction and foraging, with atrophied wax glands.
The geographical dependence of propolis ingredients suggests that New Zealand’s ‘blend’ might be uniquely different and in 1995 Comvita supplied various samples (as tinctures – ethanol extracts) from the North Island (Bay of Plenty, Coromandel, Waikato, Auckland, Taranaki and Northland) to the University of Waikato so they could have a look. They found very little variation between the samples, and actually that they look much like samples from Europe and North America, suggesting that the majority of plant sources were introduced. The only feature of note was that they were all relatively high in some dihydroflaviniods - some of the characteristic phenolic compounds found in nearly all propolis. The problem is that’s a difficult comparison to justify given propolis samples vary so much but it’s interesting if you’re thinking about selling it under a ‘same but better’ banner. It seems to me highly unlikely there is a consistent and reliable difference from anyone else’s honeybee propolis but you never know.
There are quite detailed descriptions of collection and use. The majority of bees collecting resin seem to be both collectors and users, swapping roles from time-to-time. A bee will break or scrape a particle or resin off the substrate, pass it along their legs to their corbicula, and then get another. She may hover about the substrate seemingly testing the weight before collecting more. The process may take from seven minutes to an hour before flying back to the hive. There, she will take it to where it is needed and wait to be unloaded by other workers acting as ‘cementing bees’. These bees will take it and apply it, or stash it in a storage area for use later. It can be used to line the nest cavity, reinforce comb, and entomb old pollen or the bodies of invading pests. It doesn’t appear that honeybees modify the material with their own secretions, but other bees do. The cementing bees probably just create an amalgam of the materials available to them. They patrol the nest using their antennae to probe the surfaces testing for gaps that need to be ‘glued’ but how they decide when there is a ‘shortage’ of material, and translate that into a ‘need’ for foragers, is unknown. Resin collectors can be seen dancing at the unloading site (away from the nest entrance and the nectar-forage dancing). The most complete modern description we have of the organisation of propolis use is from Jun Nakamura with Tom Seeley published in 2006 (unfortunately pay-walled).
While all honeybees use propolis some strains and races use more or less than others. Apis mellifera caucasia , the Caucasian honeybees from Georgia and Turkey, are notorious for their extensive use of propolis. I had some ligustica from Hawai’i that blocked up most of their entrance each winter. It’s not unusual to find one colony in an apiary that deposits more propolis than its neighbours. Propolis covers small holes and cracks, seals and reinforces the surfaces to which comb is attached, and in a natural nest covers the internal wall in a ‘varnish’ that controls fungal growth and controls water penetration - a propolis ‘envelope’. The most effective demonstration of this that I have seen was and old straw skep hive so completely sealed that it could be filled with water and used as a bucket. Bees in natural nests control the entrance size by restricting it with propolis walls (the word means ‘before the city’ – ‘pro’ -‘polis’ in Greek). Old brood comb incorporates a fair amount of propolis, and in new comb a red/orange tint around the edge of cells can be quite attractive! It’s what makes wax candles slightly yellow and imparts a distinctive scent. When propolis can’t be found bees attempt to collect similar compounds as substitutes.
In the comb propolis has been shown to depress the metabolic rates of wax moth adults and larvae, and cause higher larval mortality, and it seems quite clear that it is a significant part of the colony’s general social immunity. The expression of genes related to an individual bee’s immune response is reduced in a propolis-rich environment, probably because the overall bacterial load is reduced, and leaves the bees better able to respond to any additional challenge. While propolis has been shown to have a direct effect suppressing Paenibacillus larvae in a laboratory, it doesn’t appear it would be applicable in a live colony. However, it’s likely in general that the contents stored in combs like honey, brood food, pollen, and larvae, benefit from the addition of propolis and its antimicrobial properties. Although there are some portrayals of honeybees self-medicating with propolis I think these are apochryphal. Honeybees have not been observed either ingesting propolis, or increasing their rate of collection and use in response to disease, although we can increase it by providing a suitably ‘ragged’ surface.
Infuriating as it might be, it’s quite clear that propolis use – ‘bee-glue’, is an essential component of naturally healthy honeybee colonies.
MC Marcucci (1995), Propolis: chemical composition, biological properties and therapeutic activity. Apidologie 26, 83-99
Vassya S. Bankova, Solange L. De Castro, Maria C. Marcucci (2000), Propolis: recent advances in chemistry and plant origin. Apidologie 31, 3–15
Markham, K.R., et al (1996), HPLC and GC-MS indentification of the major organic constituents in New Zealand propolis. Phytochemistry, Vol. 42, No. 1, pp. 205-211
Jun Nakamura and Thomas D. Seeley (2006), The functional organization of resin work in honeybee colonies. Behav Ecol Sociobiol. 60: 339–349 DOI 10.1007/s00265-006-0170-8
Michael Simone-Finstrom, Marla Spivak (2010), Propolis and bee health: the natural history and significance of resin use by honey bees. Apidologie 41, 295–311 DOI: 10.1051/apido/2010016
Soumaya Touzani, et al, In Vitro Evaluation of the Potential Use of Propolis as a Multitarget Therapeutic Product: Physicochemical Properties, Chemical Composition, and Immunomodulatory, Antibacterial, and Anticancer Properties. Hindawi BioMed Research International, Volume 2019, Article ID 4836378, https://doi.org/10.1155/2019/4836378