The Role of Propolis in the Beehive
In between acting as teacher to Alexander the Great and writing his great works on ethics and philosophy, Aristotle (384–322 bc) found time to write the ﬁrst detailed natural scientiﬁc study of animals. This work included substantial and detailed references to the working of the honeybee. Aristotle’s great goal was to under stand the intimate workings of the hive and in particular the role of the ‘mother bee’ or, as we know it, the queen bee.
Aristotle recognised two types of propolis and had clearly arrived at a detailed understanding of their role and purpose. Conosis propolis, he tells us, is used by the bees to ﬁll cracks and holes in the hive, keeping out the light, weather and potential intruders such as spiders, ants and other insects which might wish to invade the warmth, shelter and food supply of the hive. Mitys propolis, on the other hand, was a richer, darker, more aromatic substance. This was used to narrow the entrance to the hive and was also used in the cell walls.
Ad Amiri, an Arab writing some years later, provides us with a revealing insight both into the way Aristotle worked as a scientist and into the use made of propolis. In his attempt to observe the bees at work, Aristotle built an observation hive made of glass. But the bees did not approve and refused to work until they had smeared the inside of the glass with a dark, sticky substance, a clear reference to the use of propolis to block out the light in the hive.
The Bees’ External Immune System?
The role of propolis in the hive is linked to a central conundrum. How does a colony of bees, usually of 50,000 or more, crowded into a virtually hermetically-sealed space, at a constant temperature of 35°C (95°F) and with moisture levels of 90 per cent, manage to survive? Surely these conditions are perfect for the growing of bacteria and the spreading of disease, and yet bees have not only survived but prospered in such conditions for millions of years. In many ways, conditions in the hive mirror conditions in the human body (the brood-rearing temperature is actually the same as our body temperature) and present similar problems—constant high temperature and levels of moisture. The answer for us humans is that we have our complex immune defence mechanism, while the bees use propolis.
Dramatic as this may sound it is hard to avoid the conclusion that propolis effectively provides a form of externalised immune defence mechanism for the beehive, fulﬁlling its role in a multitude of ways. It does this at times by using very simple, practical, structural mechanisms. At other times it involves highly complex, pharmacological processes, aspects of which are as yet not entirely understood.
Defending the City
The labyrinthine passageway into the hive, the entrance to the city, incorporates both mechanisms. The winding passageway is built entirely of propolis. It should provide the only entrance into the hive. Through it must pass all the bee trafﬁc between the hive and the outside world. Practically and structurally, by narrowing the entrance down with propolis the entrance can be defended by the— guard bees against intruders—mainly other bees, ants and other insects. But it defends the hive in another more subtle way. Being made out of resin, it also acts as a kind of sterilizing chamber through which all the bees must pass—a long, narrow and circuitous chamber which slowly cleanses the bees of bacteria as they move towards the heart of the hive and an environment which is recognised as the most sterile known in nature.
As we have seen from Aristotle’s observations the honeybee does not like to be exposed to the light. In the wild, bees will build their hives in protected spaces like hollow tree trunks, caves or crevices. In urban environments they may seek the safety of roof spaces, as they have done in my own home in North Yorkshire, or redundant chimneys as one colony did many years ago at the ofﬁces of the International Bee Research Association in Cardiff ! Propolis is used to seal up the unprotected sides of the colony, keeping out the light as well as providing protection from the weather and intruders.
In commercial hives bees seek to protect themselves in a similar way, sealing up any external holes in the hive and closing any internal gaps which may cause unhealthy draughts, for example between the frames and the hive walls. However, they also use propolis to create the right amount of air circulation in the hive by restricting or enlarging, according to need, ‘ventilation’ holes through which air can enter from outside.
Beekeepers groan when you mention propolis. For most of them it is not a valuable natural medicine but rather a huge nuisance which increases their work in managing the bees and collecting the honey, by ﬁrmly gluing all the moveable parts of the hive to each other and to the hive walls. We have already heard how some beekeepers have tried to breed this tendency out of the bees. With current demand for good-quality propolis running high perhaps they should be doing the reverse.
Inevitably, however, not all would-be intruders into the hive are deterred by the natural fortiﬁcation provided by propolis. Felix Murat3 tells us of one variety of bees, Apis ﬂorea, which has a particular problem with ants. These ants have worked out that they stand no chance singly against the bees guarding the hive entrance so they have evolved a counter-strategy. They gather en masse near to the hive and then swoop on the hive entrance. Some ants are stung to death by the guards but inevitably the sheer numbers of ants are able to overwhelm them. When this happens the ants enter the hive killing the queen and removing everything edible, including larvae, honey and other bees. Apis ﬂorea however, not to be outdone, has itself devised a countermeasure. The bees smear propolis all over the area surrounding the entrance to the hive, creating something resembling a giant ﬂy trap, or in this case ant trap. The invading ants stick fast in the propolis and can be more easily stung and killed. The dead ants are then coated with propolis and can remain effectively mummiﬁed, sometimes in heaps around the entrance to the hive, until they are consumed by the voracious larvae of the wax moth which lays its eggs in beehives.
Dealing with Invaders
Unfortunately, not all intruders are kept out of the hive. Bees and other insects as well as snails and small animals—frogs, mice and lizards—sometimes ﬁnd their way into the hive in search of warmth and food. In the case of smaller insects this is not too much of a problem. They can be stung to death and even physically removed. It is the demise of the larger intruders which provides the most graphic and symbolic illustration of how propolis protects the body of the hive from infection. In the case of a mouse, for example, the bees may be able to sting it to death but cannot hope to remove it. Left to rot, the dead mouse would present a serious bacterial problem within the hive. The colony responds to the threat with a rapid transfer of all available propolis to the site of the corpse where they proceed to cover every exposed surface of the dead mouse, ﬁnally giving it a top coating of wax. The mouse, like the ants caught outside the hive, is effectively mummiﬁed, sealed up within the hive. The process of decomposition is stopped, the corpse is no longer a threat to the colony.
The role of propolis at this macro level is mirrored on a micro level within the hive. When we come to look at the pharmaco logical properties of propolis we will see how bacteria in the hive are not simply destroyed or annihilated as they might be by synthetic antibiotics; rather, they are surrounded, neutralised and disarmed in the same way as the mouse is—a powerful metaphor of the relationship between the natural and the man-made.
A High-Tech Building Material
Perhaps the least-known but most intriguing aspect of the role of propolis in the hive is its use as a building material. In 1964 a Russian scientist, J. K. Leipus,4 worked out that 1 lb of beeswax honeycomb was capable of holding 25 lbs of honey which, in engineering terms, makes honeycomb a remarkable substance. The interlocking hexagonal cells no doubt contribute something to the honeycomb’s strength but he suggests it is propolis which enables the comb to carry the load it does. 90–95 per cent of the comb is made of beeswax while the remainder is composed of propolis. The propolis acts like a natural ‘carbon ﬁbre’ reinforcement both within the cell itself and in connecting it with surrounding cells. Leipus also demonstrated that all the brood cells and honey cells were coated with a very thin but complete layer of propolis resin. Apart from the sterilizing function of this lining the naturally impervious and adhesive tension of the resinous envelope probably also adds to the internal strengthening of each cell. It is interesting that resins are being incorporated in a new generation of building materials for the construction industry and that they are noted for their high-tensile and load-bearing properties. It seems once again that nature got there ﬁrst.
Sterilizing the Hive
Inside the hive, propolis is used in its most complex pharmaceutical form. A very ﬁne resinous material is used to ‘polish’ and effectively sterilize every surface in the hive. It is particularly important that the brood cells into which the queen will lay her eggs are bacteria-free. The queen bee can lay upwards of 3,000 eggs per day—over twice her own body weight. The worker bees ensure that every cell into which an egg is placed is lined with propolis. Ghisalberti,5 in his 1979 review of propolis, has suggested that an additional reason for coating the cells with propolis is to provide an impervious lining, limiting the escape of water vapour needed for the developing brood. Once the eggs have been laid, a wax and propolis mixture is used to cap each cell, providing a ﬁnal antibiotic and anti-fungal seal. Even the honeybee’s most determined bacterial enemy, Bacillus larvae—the cause of American Foul Brood,6,7 has been shown to succumb to propolis. The cells into which honey is stored receive a similar treatment— lined ﬁrst with propolis and then capped with wax and propolis. A traditional and highly effective remedy for bronchitis was for the sufferer to eat the comb cappings after the honey had been extracted.
One cannot help but marvel at the ingenuity and wisdom of the honeybee in the way it utilises propolis in so comprehensive and effective way within the hive.