By Janine Kevits

Propolis: origin and nature.

The term propolis [1] refers to the product made by the bee, consisting of resins collected from plants and mixed with wax. Propolis therefore denotes the substance found in the hive; what the bee brings back in its pollen baskets is resin.

In temperate climates, poplar is considered in the literature to be the primary resin source, but other trees also provide it: birches and aspens, willows and pines; and in more southern European climates, in addition to the species already mentioned, cypresses and rockroses, eucalyptus and chestnut trees [3]. Resin is collected from the bark of trunks and branches, from the surface of certain fruits, and especially from buds and young leaves; it is secreted for protective purposes, either on injured parts of a tree to protect it from the intrusion of parasites or pathogenic agents, or on young buds and leaves, which it protects against bacterial and fungal diseases.

In tropical or equatorial climates, resin is also produced by flowers and might be a way for certain plants to attract bees that ensure their pollination. While resin collection is marginal in our climates, it is indeed practiced at a much more sustained rate by tropical bees, as will be seen below. It is therefore not surprising that some flowering plants have acquired, through co-evolution with bees, mechanisms for resin production aimed at attracting their pollinators, just as other, more numerous, plants have done with nectar.

 

Dalechampia scandens, a resin-producing flower, is commonly pollinated by bees that collect this resource.

 

Generally, bees have no trouble finding resin sources. However, when deprived of them, they may collect rather undesirable substitutes such as asphalt or paint, which they mix with “true” propolis, thereby obviously altering its quality [1][3]. It is therefore preferable to ensure that bees have suitable vegetation available near the apiary, a precaution that is especially relevant for bees kept in urban areas.

A laborious harvest

Only a small proportion of foragers collect propolis (1% or less) [13]. Indeed, by weight, it is the most marginal collection activity in our colonies: 50 to 150 g/year on average; but the most “propolis-producing” of the honey bees, the Caucasian, could gather up to 1 kg [3]. Nothing is known about the criteria they use to choose their resin sources.

The harvest is not easy, as noted. The bee must first detach the resin from the plant, extract it if necessary from the hairs covering the bud or leaf, and sometimes fragment it using the mandibles. Once detached, the resin particles are kneaded with the mandibles, then taken up by the front legs, passed to one of the middle legs, and from there to the pollen basket of one of the hind legs [15]. The bee then takes off and sometimes circles around the source to land again and complete the load; this “pre-flight” could be a way for it to assess the magnitude of the load already collected. The whole process takes from a few minutes to an hour. Some of the propolis may also be carried between the mandibles [10].

Back at the nest, the bee goes to the place where the propolis is to be deposited; it is unloaded there by other bees. The forager is in fact unable to detach the resin from its pollen baskets by itself. Removing a load of propolis is a lengthy process: it can take from one to seven hours, and it sometimes happens that several workers collaborate simultaneously to unload a single forager. If it is not unloaded quickly enough, the forager performs shaking dances, exactly like nectar foragers; these dances recruit new propolis receivers. Most propolis foragers do not merely collect it, but also proceed to use the product. Becoming “cement workers,” they cut and knead, using the mandibles, pieces of resin that are mixed with wax to make propolis proper; they then use this material to carry out the cementing and caulking operations necessary for the comfort of the hive. Sometimes propolis is also deposited temporarily in a storage location from which it will be retrieved as needed. Propolis foragers therefore do not constitute a category distinct from the cement workers, a class of workers responsible for nest maintenance. However, workers that handle propolis are on average somewhat younger than foragers; they also include middle-aged bees, such as the store bees that process nectar. Some of these young cement workers, having become foragers, will not collect resin at all, but will turn to other resources, pollen or nectar [10].

 

The forager is unable to rid itself of its load of propolis; other workers carry out the unloading.

 

Propolis work thus differs from that of pollen or nectar in that those who use the product largely overlap with those who collect it. It should also be noted that unloading, while performed by other workers as with nectar, takes place in the center of the nest (and not at the entrance as is the case for nectar), which can be understood from two perspectives. First, this system rationalizes movements inside the hive, since a single load is divided among several workers—the bulk of the journey is therefore made by a single bee, the forager, whereas it would be made by several cement workers if they had to go to the hive entrance to obtain supplies. Second, this system allows the forager to learn about the colony’s needs for propolis.

For resin collection, like that of pollen, nectar, or water, is regulated. The trigger for foraging is likely provided by the state of the nest: rough surfaces requiring smoothing, cracks to be sealed, drafts to be countered, foul-smelling objects to be covered (the thymol strips!). Some of the propolis foragers observed by Nakamura and Seeley (2006) were seen inspecting crevices by inserting their antennae, which, it should be recalled, are the bee’s main tactile organ—in the laboratory, these specialized foragers discriminate tactile stimuli better than others [12]. Regulation of foraging would therefore be carried out by the foragers themselves, through direct assessment of demand, a system obviously facilitated by the fact that these foragers are also most often the ones that will apply the material. But this does not preclude regulation by dancing, as for nectar or pollen. Indeed, waggle dances are performed by propolis foragers, but, unlike dances for nectar or pollen foraging, they take place not on the dance floors near the hive entrance, but near the sites of use [10]. This suggests that recruitment targets not idle foragers frequenting dance floors, but rather cement workers engaged in caulking. One reason for this particularity could be that experienced cement workers would also be better foragers, since handling resin requires special skill. It is therefore not surprising that resin foragers are ordinarily very faithful to this task: of 80 bees observed over several consecutive days by Nakamura and Seeley, 63 collected resin on several successive days, with only 17 switching to another resource after just one day of resin collection. The most persistent collected only resin for 18 consecutive days! An expert, certainly…

 

The trigger for propolis foraging is likely provided by the state of the nest: rough surfaces requiring smoothing, cracks to be sealed, etc.

 

If the two hypotheses regarding the triggering of foraging (needs assessment and dancing) were jointly confirmed, this would mean that bees are able to integrate information from several distinct sources to adjust their behavior to the needs of the colony: another point in favor of the mini-brain of our little friends! However, the demonstration is not complete to date. The effect of dances on foraging has not in fact been demonstrated; it may be nothing more than a remnant of more general foraging behaviors [12].

Composition of propolis

Propolis is a highly complex mixture; more than 300 different molecules have been identified in it. Broadly speaking, it contains 30% wax, 50% resin, 10% essential and aromatic oils, 5% pollen, and 5% various organic and mineral substances. It is of course the resins and aromatic oils that confer its medicinal properties. Their main constituents are terpenes, which give propolis its particular odor, and flavonoids (see box). It also contains phenolic acids, including caffeic acid and its derivatives, which are responsible for a large part of its pharmacological properties (for detailed composition see [9]). Propolis also contains vitamins (B1, B2, B6, C, E, nicotinic and folic acids), minerals (calcium, magnesium, iron, copper, zinc, manganese, nickel, cobalt, vanadium, and strontium), and enzymes [8]. About 10% of its compounds (mainly terpenes) are volatile.

Propolis varies substantially in composition depending on the region of origin and the plant species on which it was collected; this indeed poses problems for standardizing the product for pharmaceutical purposes. It comes in various colors (brown, red, yellow, or greenish) and different textures—in our climates, red propolis is thus harder and more brittle than yellow-green propolis, which is softer and stickier.

 

The chemical composition of propolis varies according to the plant species collected. These variations are reflected in its color and texture.

 

However, at low temperatures, all propolis types become hard and brittle (which is why beekeeping equipment is cleaned cold; hot water, if needed, is used only in a second step).

Surprisingly, despite these physico-chemical differences, all propolis samples studied have broadly similar medicinal properties [2].

Long-known properties

These properties, notably antibacterial ones, have been known since Antiquity.

Since when, and by whom, is not easy to determine. One often reads, for example, that the ancient Egyptians used it for embalming their mummies. In a search—limited, it is true—I nevertheless found no credible confirmation of this use; while it is certain that the mummy, once wrapped in its bandages, was treated with an antibacterial resin, the fact that this resin was, or contained, propolis does not appear to be attested.

But ancient authors do mention it, and pertinently. Aristotle, for example (384–322 BCE), says this: Thus, when they are given a hive that is completely empty, they build the wax cells, bringing to it the tears of all flowers and those of trees, such as willow, elm, and the trees that produce the most viscous matter. They carefully coat the floor of the hive with this matter, in order to defend themselves against other animals. This is what breeders call collisis (in Greek κόλληση means “gluing,” Ed.); the bees also use it to build the entrances of the hive when they are too wide[2]. And further on, he evokes what he considers another substance but is nonetheless also propolis: At the mouth of the hive, the rim of the entrance is coated with mythis. This matter, which is fairly dark black, is like a purification of wax for bees, and the smell of it is very strong. It is a remedy against bruises and suppurating wounds.

Four centuries later, Pliny the Elder (23–79 CE) says no different. He too speaks of the tears of trees that produce a glue, with the sap, gum, resin of willow, elm, and reed. With these substances and other more bitter saps, they (the bees, Ed.) first make a coating with which they line the entire interior of the hive, as a defense against the greed of other small creatures; for they know well that they are going to produce something that can be coveted. Then with the same matter they narrow doors that are too wide. He also speaks of propolis as being one of the layers of wax, which is false, but cites as its origin the gum of pines and poplars, which is true, and says that it is much used in medicinal preparations.[3]

The Greeks and Romans of Antiquity therefore knew not only the substance, but also its origin, the use made of it by the bee, and its medicinal properties. The latter are still current today, more than ever in fact; propolis is sold in various forms—raw, as an alcoholic solution, or in capsules. It is somewhat of a fashion, but not without foundation: its medicinal properties have been the subject of numerous scientific studies, carried out mainly on cell cultures and in animals. They come primarily from compounds extractable by alcohol, which explains the success of alcoholic solutions. Antibacterial, antiviral, antifungal, it also has antioxidant, anti-inflammatory, and antitumor properties. It protects organs such as the heart and liver, improves the regeneration of bone cartilage, and is healing, notably for injuries of the mouth or the cornea. It is, however, also allergenic, and can prove dangerous for people who are hypersensitive to it [6].

We will not dwell further on this point; readers interested in the pharmacological properties of propolis may refer to the review by S. Bogdanov (2015), which lists them and also lists the scientific studies that support them. Let us instead turn to the small world of insects, which also actively make use of these properties.

Insects and resins: a widespread association

The evolution of resin-collecting behavior is poorly known. In particular, it is not known whether this type of collection predates or postdates the separation between ants and bees within the Hymenoptera [12]. For indeed, bees are not the only ones that use plant resins; ants do so as well.

The wood ant Formica paralugubris, for example, incorporates pieces of solidified pine resin into its nest, which inhibits the development of parasitic microorganisms. A large nest may contain up to 20 kg [7]. The presence of this resin reduces the immune effort provided by the individual. The resin also protects the brood; worker ants increase collection when brood is present in the nest, and place it preferentially near it [4]. The use appears prophylactic rather than curative: resin collection does not increase in response to contamination by two micro-fungi pathogenic for the ant (they are not for the bee): Beauveria bassania (Ibid.) and Metarhizium anisopliae [5].

In the genus Apis, propolis use varies among species: A. cerana does not use it at all; A. dorsata uses it occasionally to strengthen the anchoring of its comb to the branch that supports it. In A. florea, by contrast, use is systematic: its single comb is also suspended in a tree, and it coats with a sleeve of propolis any branch that could give access to its nest, thereby blocking access to ants [12].

Apis mellifera also uses propolis systematically in its nest, for a variety of purposes.  In managed colonies, the most frequent use is sealing cracks in the hive; and propolis pillars are sometimes built to narrow the entrance. Propolis is also used to embalm the corpse of an animal killed inside the hive and too large to be removed by the bees, such as a mouse. Finally, the edge of the cells is generally coated with a thin layer of propolis, which thickens and strengthens it, forming the hexagonal mesh network on which the bee moves; propolis helps give this network the strength characteristics required for the transmission, through the comb, of vibrational waves that are one of the means of communication among bees, used notably during the dance [14].

 

The bees add a little propolis to the rim forming the edge of the cells in order to strengthen it, which serves as a support for the bee’s movement.

 

Feral or wild colonies coat the inner wall of the cavity that houses the nest with a layer of propolis over the full height of the combs, thus creating a thin “envelope” less than one millimeter thick. This envelope helps strengthen the anchoring of the combs and probably also protects the nest against external moisture, notably that which could come from sap when the nest occupies the hollow of a tree [12].

The domestic honey bee (in the sense of “kept by humans”) collects less propolis than its congeners living in nature. One possible reason is the selection through which beekeepers have sought, among other objectives, to reduce the production of this sticky substance that complicates hive opening and frame handling [12].

Meliponine bees[4] also collect resin, and for some species it is such an important substance that sources are exploited daily and are guarded. Use depends on the species and is highly varied, sometimes even extravagant. While some use it to protect the nest from ants, shape the entrance, or embalm a small predator just like our bees, others use it for more warlike purposes. Bees of the genus Melipona roll it into pellets that they keep near the entrance: in case of attack by a predator, they stack them to form a defensive wall. Others apply sticky resin to the hairs of the predator they attack (they do not sting, but some species bite or emit caustic secretions). Others still use it to form an interior wall behind the entrance. The resin is mixed with wax, but sometimes also with mud or small stones [11].

If in the world of insects propolis is used for a bit of everything (glue, mastic, and even a weapon of war), the most important effects of its presence in the nest are, as will be understood, sanitary in nature. For in the bee as well, the pharmacological effects of propolis are demonstrated. What are they? That, dear readers, will be for a future instalment of LSA. For to understand them properly, one must first explore what the bee’s immune system is—an exercise we promise you soon. Until then, stay well… with propolis within reach, colds should not poison your winter!

 

Terpenes, flavonoids, and other components of propolis   Raw propolis   Terpenes are hydrocarbon compounds with a linear or cyclic chain, generally fragrant (one of them is responsible for the characteristic smell of oranges; others for the aromas of basil, camphor, thyme, eucalyptus…). They are abundant in resins, notably pine resin, but also in the essential oils of flowers. Vitamin A is a terpene; the aroma and flavor of hops, sought after in beer, are due to terpenes. They are widely used in the food, pharmaceutical, and cosmetic industries, and in biotechnology. The medicinal properties attributed to them are numerous: cancer-preventive, antifungal, antiviral, antibacterial, anti-hyperglycemic, and anti-inflammatory. Some of them modify brain chemistry: terpenes produce aromas, but also the effects of cannabis. Flavonoids are polyphenolic compounds (organic compounds containing several 6-carbon rings) that are very common in plants. Among them are pigments that give flowers and fruits their yellow, orange, and red colors (tomatoes, raspberries…), and others responsible for blue or purple colors (blueberries, blackberries…). A single pigment can produce a range of different colors depending on the acidity (pH) of the medium. They have high nutritional value, although they are poorly absorbed by the human body; in particular, they have antioxidant and anti-inflammatory properties, and their consumption is associated with a reduced risk of developing certain types of cancer. This is why one must eat fruits and vegetables; five drops of propolis per day do not compensate for a dietary deficiency in plant products! Phenolic acids have roughly the same properties as flavonoids. Caffeic acid—so named because it is very present in coffee, but distinct from caffeine—is one of them. It is the main factor responsible for certain curative properties of propolis (antioxidant, antimutagenic).

 

[1] The term comes from a Greek word meaning “in front of the city,” because bees sometimes use this material to partially block the flight entrance.

[2] History of Animals, Book IX, Chapter XXVII, §6.  Source: the website of Philippe Remacle et associés: http://remacle.org/bloodwolf/philosophes/Aristote/animaux9ab.htm#XXVII. This site provides French translations of a very large number of authors, notably Latin and Greek—but many others as well. It is worth visiting.

[3] Natural History, Book IX, § V and VI. Ibid.: http://remacle.org/bloodwolf/erudits/plineancien/.

[4] Meliponines, or stingless bees (the “stingless bees” of English speakers), are indigenous to the African and American continents. There are some 800 different species.

 

See also:

► Wax production by bees

 

 

 

Bibliography

[1] Alqarni AS, Rushdi AI, Owayss AA, Raweh HS, El-Mubarak AH et Simoneit BRT, 2015 : Organic Tracers from Asphalt in Propolis Produced by Urban Honey Bees, Apis mellifera Linn. PLoS ONE 10(6): e0128311. doi:10.1371/journal.pone.0128311

[2] Bogdanov S, 2015 : Propolis : Composition, Health, Medicine, a review. Cet article est disponible (en anglais) sur le site Bee-Hexagon de Stefan Bogdanov, chercheur du centre suisse de Liebefeld  (http://www.bee-hexagon.net/)

[3] Bogdanov S et Bankova V, 2015 : The Propolis Book Chapter 1 : Propolis: Origin, Production, Composition. Cet ouvrage de premier intérêt est également disponible sur le site Bee-Hexagon (http://www.bee-hexagon.net/)

[4] Brütsch T et Chapuisat M, 2014: Wood ants protect their brood with tree resin, Animal Behaviour 93: 157-161

[5]  Castella G, Chapuisat M et Christe P, 2007: Prophylaxis with resin in wood ants, Animal Behaviour 75: 1591-1596

[6]  Castro SL, 2001 : Propolis: biological and pharmacological activities. Therapeutic uses of this bee-product, Annual Rev Biom Sci 3: 49–83

[7]  Christe P, Oppliger A, Bancala F, Castella G et Chapuisat M, 2003: Evidence for collective medication in ants, Ecology Letters 6: 19–22

[8]  Farooqui T et Farooqui AA, 2012: Beneficial effects of propolis on human health and neurological diseases, Frontiers in Bioscience E4, 779-793

[9]  Huang S, Zhang C-P, Wang K, Li GQ et Hu F-L, 2014: Recent Advances in the Chemical Composition of Propolis, Molecules 19: 19610-19632; doi:10.3390/molecules191219610

[10]  Nakamura J et Seeley TD, 2006: The functional organization of resin work in honeybee colonies, Behav Ecol Sociobiol 60: 339–349

[11]  Roubik DW, 2006: Stingless bee nesting biology, Apidologie 37(2): 124-143

[12]  Simone-Finstrom M et Spivak M, 2010: Propolis and bee health: the natural history and significance of resin use by honey bees, Apidologie 41: 295–311

[13]  Simone-Finstrom MD et Spivak M, 2012: Increased Resin Collection after Parasite Challenge: A Case of Self-Medication in Honey Bees? PLoS ONE 7(3): e34601. doi:10.1371/journal.pone.0034601

[14]  Tautz J, 2009: L’étonnante abeille, De Boeck éd.

[15]  Weinstein Teixeira E, Negri G, Meira RMSA, Message D et Salatino A, 2005: Plant origin of green propolis: bee behavior, plant anatomy and chemistry, Evid Based Complement Alternat Med. 2: 85–92.