What is entombed pollen?

Entombed pollen is pollen that is stored in a honey bee hive and encapsulated under a layer of propolis.

The phenomenon was first described in a paper by Dennis vanEngelsdorp et al and published in the Journal of Invertebrate Pathology (2009). In that paper, the authors described cells of stored pollen that were covered by propolis and/or wax cappings. Since pollen is not normally coated in this way, the researchers performed chemical analyses on the pollen samples to discover why they may have been capped.

What they found were cells of pollen containing elevated levels of certain pesticides. The original researchers found especially high levels of the fungicide chlorothalonil in the capped cells. They also reported that the pollen in these cells was brick red.

VanEngelsdorp and his group theorized that the worker bees sensed the pollen in these cells was not good and subsequently covered it so it would not be consumed. Bees often coat offensive items with propolis—such as dead mice or snakes—to keep them from contaminating the interior of the hive. So coating contaminated pollen is consistent with other well-documented bee behavior.

In the months since the original paper was published, other beekeepers have reported the presence of entombed pollen. Entombed cells have been found to contain various colors of pollen and various types of chemicals, including those chemicals used to combat Varroa mites. It has also been documented that colonies containing entombed pollen are usually in the process of dying. Entombing contaminated pollen may be a last-ditch effort made by a colony trying to save itself.

Many questions remain to be answered, such as why the pollen gets collected in the first place. Current theories suggest that the pesticides may undergo chemical changes while stored in the hive, or that the accumulation of pesticides in a confined space is more apparent to bees than the same pesticide in an open field.

While entombed pollen by itself does not answer the larger question of bee die-offs, it does add an intriguing element to the pesticide puzzle.


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  • This is both bothersome and fascinating. Bothersome because I recall seeing several pollen frames with brick red pollen in some of my hives that died this past winter. What is intriguing and fascinating is why the pollen is brick red. Pollen color, to my knowledge, is determined by the plant it comes from. If so, shouldn’t these scientists be able to identify the source of the pollen? The fungicide chlorothalonil is used in many places from home owners to agriculture. Is it the fungicide which turns the pollen brick red or is it a particular pollen? Since chlorothalonil is used so pervasively, shouldn’t we see pollens of many colors having the same effects. Is it chlorothalonil’s interaction with a particular pollen? Let’s hope we can get some answers on this one. And let’s hope that there are some preventative measures we can take.

    • Jim,

      The authors of the original paper speculated that the metabolites (breakdown products) of chlorothalonil may have caused the brick red color. Apparently, these metabolites are highly reactive, and when they combine with certain other chemicals, they can produce this color. This speculation was based on work performed by a soil scientist who was looking at the breakdown of chlorothalonil in soils. (I’m straining my memory here, so don’t quote me on this. I can look it up if you want.)

      One of the complications here is that brick red pollen also occurs in nature (horse chestnut, I believe) but if the brick red pollen in covered with propolis, this is a bad sign.

      One of the recurring themes I found when doing my research on pesticides was that fungicides–although not particularly harmful to bees all by themselves–tend to be highly reactive with other agrochemicals. Therein may lie their real danger.

  • Thanks Rusty. Brushy Mountain Bee Farm’s web site has some fantastic webinars archived that you may be interested in. One is by Dr. Maryann Frazier of Penn State University regarding pesticides in beehives. Here is a link to the page that contains that webinar along with many others from equally prominent beekeepers. In this episode she does talk a little bit about the synergistic results of mixed pesticides.

  • That absolutely makes sense. Much the same as metal will turn red with moisture and oxygen. I would imagine that bee pollen will change with the various elements included such as the soil, water and the source of the pollen.

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