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.