Well, from the beekeeper's point of view, it's a very sticky, general purpose brown "gunk" that bees use. And it looks like this:
Propolis has a couple of important jobs in the hive:
Job one is as a disinfectant for brood cells. One of the properties of propolis is that it inhibits the growth of pathogens. Bees make use of this - every time a bee finishes the process of metamorphosis, and emerges for the first time as an adult bee, it leaves an empty cell ready to be re-used. But before the queen lays a new egg in the cell, it needs to be "cleaned". One of the worker bees (specifically a house bee - one of the younger bees which has not yet started foraging) enters the cell, and lines it with a thin layer of propolis. This (partially) inhibits any pathogens passing through the wax into the cell, so after the queen has laid an egg and it hatches and starts to grow, it reduces the chances of the larva picking up any infections.
The second job is a general-purpose filler. Bees don't like gaps in the hive walls, and they notice the small cracks between the stacked boxes that make up the hive. So they tend to stick them together with propolis. They also like to stick the frames together where they touch. Basically, bees will fill any gap between two non-wax surfaces, that is less than 4mm, with propolis. This is particularly important for the hive walls, as it prevents draughts and also prevents small intruders (such as wax moth) from entering the hive.
The third job is, again, related to hygiene. Normally, if bees find something inside the hive that they don't like (for example, a dead wasp - quite possibly one that they've just killed) they'll carry it out through the hive entrance and dump it away from the hive. This prevents mould and diseases, if it starts to decompose. But sometimes, the thing will be too big to remove. The best example I heard was a mouse that had got inside a hive (not one of mine) to try to hibernate over the winter. For some reason, the mouse had died during the winter. When the beekeeper came to open the hive in spring, the bees had been unable to remove the mouse from the hive because, of course, it was just too big. So the bees had instead encased the dead mouse in a layer of propolis, and mummified it...!
So, where does propolis come from? It's basically a mix of resins and wax. Bees will use various sources of resins, but the most common are tree sap and the sticky stuff that is secreted by some leaf buds.
I've taken a couple of photographs so that you can get a sense of what propolis actually looks like when the bees are using it:
This is a strip of propolis that Miriam's bees added to the bottom corner of one of the brood frames. They did this to a number of brood frames last summer. I must admit, I'm not entirely sure what the function of this is, though I suspect it's related to airflow/ventilation in the hive.
This photo was taken from a solid board that I was using as a temporary hive floor last summer while I was doing some work on the hive stand. I'm almost certain it's a wax-moth caterpillar which has been encased in propolis and mummified. Normally, I'd expect the bees to kill the caterpillar and then remove it. However, it may be that it was pupating and the cocoon was too difficult for the bees to remove, so they just covered it in propolis instead.
I was very lucky to get this photo - and very pleased! It's a worker bee who has recently been out to collect resin for making propolis. You can see that she has attached a blob of resin to each of her back legs (in the same place where she would carry pollen) so that she can bring it back to the hive. After I've re-assembled the hive, she will mix the resin with wax to make propolis, and then probably use it to fill a crack between the boxes.
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