As I understand it this is the propolis gateway through which bees ‘de-bug’ on their way into (and out of) the hive. Brushing past the propolis which has anti-microbial and fungal properties, helps them keep clean.
The great propolis story is how they can ‘embalm’ an intruder that is too big to be physically removed from the hive. For example a mouse. The bees can sting the mouse to death but the warmth of the hive would soon lead to its (extremely unhygienic) decomposition. By covering the mouse completely with propolis it is effectively mummified and decomposition microbes cannot spread to the rest of the hive. The Egyptians, the first beekeepers, are said to have learnt their embalming techniques from the bees.
Propolis is made from tree (and other plant) resins, mixed with other secretions such as saliva and beeswax by the bees. Enzymes in the mixture transform the propolis.
Resin is the tree’s ‘immune response’ to damage and protection against harmful UV radiation among other functions. It contains a highly complex set of chemicals specific to the tree species.
I would be delighted to hear of others’ experiences of the propolis entrance… only one of my two colonies has created it. I tend to lean a piece of wood across two thirds of the entrance to help them defend the hive. The gateway was made in the remaining third. I bought the colony (in a National brood box) last spring from a commercial beekeeper. It was strong and ready to swarm. I nadir-ed with a second brood box, no extra frames (exactly what you’re told not to do!) I haven’t looked, fed or medicated.
Here’s a link to an article about the health giving properties of propolis being discovered