The Remarkable Honey Bee - May 2013

Propolis

by Larry Connor

(excerpt)

Propolis is a plant product collected by bees from plants within their normal foraging territory. The substance, understood to be largely unaltered by the bees, is comprised of resin from various species of plants, the exact sources of which are difficult to determine due to logistical reasons. However many of the North American propolis samples are collected from the tree genus Populus, commonly known as poplars. Propolis sources from other parts of the world appear to possess properties directly related to the available plant forage visited to obtain the material. Not all plant resins are identical.

Plant bud resins and their functions
Plant resins are believed to protect a plant’s buds and leaves from water loss and have a component of insect repellency. The resins are found in trees like the poplars and cottonwoods, trees that form new buds in the summer and early fall. The primordial tissue inside the bud does not produce propolis, but the stipules that form the bud scales secrete a resin that fills the bud. As the leaves grow and unroll in the spring, they produce glands that secrete resin. There may be extra floral nectaries located near the resin glands that are responsible for the guttation of sap (seen as drops of water at leaf edges, often exuded when there is high soil moisture) and nectar secretion1.
Humans use poplar buds for medicine, and continue to make poplar bud oil from cottonwood poplar buds, especially from Populus trichocarpa. They collect winter buds, macerate them and cover them with water and oil. This may be heated at a low level for several days and then stored for a month or more. Then the oil is carefully removed from the mixture and bottled. Any medicinal properties are attributed to the plant bud resins and salicylates in the bud tissue. Some people are allergic to this solution and may develop anaphylaxis2.
As a group, plant resins are known to be powerful chemicals. They are the source of the greasy build-up that forms when burning buds of cannabis, or while smoking leaf tobacco. As insect repellents, plant buds must contain powerful molecules to successfully minimize leaf feeders, either by a repellency (by an odor or tactile reaction) or toxic effects when ingested.

How do bees collect and use propolis?
Bees collect propolis in the same manner as pollen, packing it into their corbicula (pollen basket) and flying back to the hive. Upon arrival, the forager bee is assisted in the removal of the sticky substance that is then applied to the sides of the nest chamber, on the top of the wax brood comb (apparently to strengthen the hexagon-shaped wax to withstand bee foot traffic) and wherever a hole or crack needs to be filled. Beekeepers are all too familiar with propolis, especially when the bees have fastened combs together with the material (to keep them from moving in the wind?), securing the nest. Bees of certain races use large amounts propolis to reduce the size of the entrance of the hive, blocking potential intruders and cold winds. Hive invaders that die within the hive, and are too large for the bees to remove, are encased in propolis to arrest their decay and prevent the byproducts of decomposition from affecting the population of the hive. Beekeepers have found mice and large insects preserved, mummified in effect, in propolis.

Distribution, where and possible functions
The most apparent use of propolis is the patching of small holes, cracks and spaces less than 3/8 inch in the hive. Bees tend to use beeswax to fill spaces larger than 3/8 inch. While reducing the hive entrance is somewhat useful in deterring pests and predators, hives require ventilation to thrive, which brings our focus to another consideration: propolis and colony health. Propolis serves as an anti-fungal and anti-microbial agent, effectively working as a prophylactic tool used against a number of non-specialized threats to the bees’ health.
In feral bee colonies, in tree cavities or rock outcroppings, bees coat the top and sides of the structure with a thin layer of propolis. In addition to the anti-microbial action of the resin, the layer serves as a water and vapor barrier. This ensures that the dankness of a rotting tree or a rock cavity does not promote unhealthy conditions in the colony. The resins thwart the growth of fungi, bacteria and viruses. The propolis is like an envelope or biological shield that provides colonies with a more stable environment in which to grow.
Bees also scrape the entrance of the nest to remove loose bark or dirt and coat the hive entrance with propolis. This may provide the bees with a smoother surface for takeoff and landing activities as they forage.
Propolis is used in the comb area of the hive, as evidenced by thin layers of resin on the brood comb and between comb and their attachment to the tree or rock homesite. In managed bee colonies, the bees’ use of propolis is considered an essential part of colony health. Within the first season of use with new combs, most colonies add abundant propolis along the frame ends, where the wood parts of the frame contact the frame rest. With additional beekeeper manipulations, there will be additional layers of propolis added to the initial layers, used to re-secure the frames.