Honey Bee Biology
Honey Bee Biology - June 2013
Propolis Protection: A New Perspective on Bee Glue
The summer heat and humidity finally rolled in with a vengeance. I had retreated under the shed to do my sweating in the shade while scraping propolis from stacks of empty hive bodies (see Figure 1). A couple of days of this monotonous mind-numbing work made my hands bone sore. A blister even formed in the palm of my right hand from clutching the hive tool as a scraper.
The nectar flow had stopped, freeing up thousands of forager bees. As I scraped another hive body, bored to oblivion, I imagined these idle bees would find another job – collecting tree resins, which bees use for propolis once the material is in the hive. And good fortune would favor these bees. Instead of wandering off into the woods on a resin quest, an uncertain and time-consuming task (I have seen them searching way up in tall trees because I climb them), an “instant” prize was close by – my pile of propolis scrapings. And true to their perceptive nature, dozens of bees found it. As expected, they showed a relentless single-minded zeal, chewing and pulling tiny bits of propolis from the pile, packing them on their pollen baskets, and flying the loads back to the hives. “Best of all,” they could come right back for more (see Figure 2).
So there we were, all the comedic players in some sort strange inadvertent competition: me scraping out that annoying junk, blister stung with every swipe of the hive tool, versus squadrons of bees, working like an aerial conveyer belt, hauling their treasure trove of tiny goo balls back to the hives – a propolis version of the circle of life.
While I try not to work against my bees, I actually do not mind quite a bit of propolis in the hives because I know they need it (and the new scientific research reported below shows this view in finer detail). Furthermore, for years, my policy has been if I need to scrape propolis from active hives (during colony inspections), I leave it in the apiaries for the bees to reclaim, not a typical beekeeping practice (although often the bees will ignore it). I leave the small pile of scrapings in partial sunlight and sheltered from rain, helping to warm it, which makes the material easier to manipulate, and to keep it dry. I leave the pile with a lot of edges as it was scraped (not compressed in a tight ball). From watching the bees, it helps them chew pieces of the material from edges. Sometimes bees will even chew and dismantle discarded combs and bring back bits of comb packed on their pollen baskets in a manner similar to packing propolis from the scraping pile (see Figure 3). These combs had a high propolis content, which probably stimulated the collection behavior (rather than recycling the wax the way Apis florea does since Apis mellifera does not display that behavior). Plus combs are easy for bees to dismantle with lots of edges to chew from. This intense collection suggests an interesting question with obvious and subtle answers: where are the bees putting the propolis?
It is well known that bees seal the interior of the hive cavity in a thin layer of propolis. Comb attachments to the substrate (the wood) have a high content of propolis too. In the origin of the name, propolis, from the Greek, has “pro” meaning “before” and “polis” meaning “city” to suggest an additional use of propolis. The bees sometimes restrict the size of the hive entrance (“before the city”) with walls and columns of propolis. Some subspecies of honey bees use propolis extensively to reduce the entrance size (see Figures 4 and 5). Furthermore, in lands where the Langstroth frame hive dominates the experience of most beekeepers, it may not be apparent that bees spread a layer of propolis several inches from the hive’s entrance. This behavior is much more apparent with hives made of wicker, as with skeps, or hives made of other woven material like cane (Mangum, 2012). Closer to home, a careful examination of the area around the entrance hole of a bee tree shows a similar layer of propolis. While these applications of propolis are commonly found in a hive, a more rare usage occurs when something dies inside of the hive, and is relatively large so that the bees cannot remove it. A dead mouse is the classic example, and having it decay in the hive would be a considerable problem. So the bees seal up the unfortunate creature in layers of propolis. Interestingly though, features of the mouse may remain visible: teeth, tiny ribs, and rarely even whiskers (see Figure 6).
Without the convenience of my propolis pile, bees collect tree resins for propolis. In the hive, the bees chew the resin, and in the process mix it with salivary secretions, incorporate it with beeswax and other substances (Langenheim, 2003). (That is why we call it propolis once the resin is in the hive.) For the particular trees, the literature reports poplar trees as a typical source, but others are pine, birch, elm, alder, beech, and horse-chestnut (Langenheim, 2003; Simone-Finstrom and Spivak, 2010). Near my apiaries in Piedmont, Virginia (where poplar trees abound), bees collect resin from injury sites on sweet gum trees (Liquidambar styraciflua), as Figure 7 shows. In the same foraging area, other resin sources are small branches of sumac. (It was not Staghorn sumac, Rhus typhina, so perhaps Rhus Vervix, Poison sumac.) (See Figure 8.) In both cases, one way to find these resin collectors is to listen for the hum of only one to six bees under the foliage when out in the woods. (Anything that hums like a few bees gets my camera-ready attention.) The timing and the tree size (for the sumac) were in the summer, far past swarm season. So I knew the hum did not emanate from nest-site hunters of a reproductive swarm. And the sound was too faint for a summer usurpation swarm. (A new occurrence in Virginia beekeeping. Nevertheless, one that now needs consideration in this kind of reasoning.)