Upon opening your first hive, you may, like me, be surprised to find how difficult it is to separate the layers of boxes. You will quickly discover that the oddly-shaped hive tool is crucial to the operation -- mere fingers are seldom successful in prying apart the boxes and frames. At first, it is easy to assume that dried honey is the culprit. However, upon closer inspection, you find that the darker substance acts like an adhesive in the hive and is much more brittle (when cold) and tackier (when warm) than honey. This substance is called propolis, or “bee glue,” and it is essential to the wellbeing of hives.
Where does propolis come from? Like honey, it is a byproduct of bees foraging activity on plants. While worker bees are out collecting pollen to make honey, they may also collect resin -- a substance produced by specialized cells in plants. Resin is often excreted by injured trees, flowers, or buds as a way to protect the injured area from mechanical and biological stressors. Bees first gather the resin, and then pack it into their pollen baskets (a hollowed area) on their back legs. Then, upon returning to the hive, they can manipulate the resin with their mouthparts, and mix it with their saliva and beeswax (that is produced through wax-secreting glands on their abdomen), creating propolis.
Once the resin has been processed into propolis that is soft and workable, the bees smear it on surfaces inside the hive. It may be used to smooth rough patches in the wood, fill in holes, or patches inside of the comb. While beeswax is used to seal large, unwanted gaps in the hive, propolis is used for sealing small gaps, usually less than ¼ inch. This is the final weather and pest-proofing layer that allows the bees to keep a clean and warm home.
But what advantages does propolis possess over simply sealing with beeswax? Well, bees are immaculate housekeepers. The inside of a hive is constantly in a cycle of cleaning, spearheaded by the worker and nurse bees. For instance, after a bee is born out of a cell, the nurse bees soon visit the cell to “polish” the inside, making sure it is free of waste products and pests, and coating it with propolis. This process prepares it for either the queen to lay the next egg inside of it, or for pollen or honey to be stored inside of it. Because propolis has incredible antifungal and antibacterial properties, this substance is ideal for making sure their brood rearing and food stores are protected against disease.
(Bottom left of picture -- typical dark, sticky propolis in our hives. Photo by Deidra Ressler).
Propolis may also be used for some more bizarre purposes. If a large organism dies inside of a beehive (ex. a mouse), and the bees are unable to drag it out of the hive, they may completely coat the dead body in propolis. This “mummification” makes sure that the smell and decomposition does not create problems inside of the hive, as it is completely sealed from the air. Additionally, it may help in temperature regulation and insulation of the hive and reduce vibrations of honeycomb that may lead to fracturing of the beeswax.
Propolis production between hives varies greatly. Even within our own little bee yard, one hive is always more challenging to open than any of the neighboring colonies because they lock it shut with so many layers of propolis. However, though it necessitates a bit more muscle for the beekeeper to maneuver, propolis abundance is a sign of a thriving and vigorous hive.
Sources:
Marcucci, Mc. 1995. “Propolis : Chemical Composition , Biological Properties and Therapeutic Activity.” Apidologie 26: 83–99.
Michael, S, and S Marla. 2010. “Propolis and Bee Health : The Natural History and Significance of Resin Use by Honey Bees.” Apidologie 41: 295–311.