The above is a photo of an entrance in one of my Bee Tree Hives. You can see that the bees have coated the inside with propolis, and even reduced the entrance with propolis so that they have to crawl across it to enter the hive.
From a post by Bill Wood to the Treatment-Free Beekeepers facebook group.
"Propolis is plant resins that honey bees collect and deposit in the hive cavity. Feral bees coat the inside of their cavities with a rather thick layer of propolis. Propolis is difficult for the bees to harvest — they have to scrape the resin off the leaves, pack it on their hind legs, and then, back at the hive, get help from other bees to pull it off of their legs. But they have a very good reason for going to so much trouble.
Dr. Marla Spivak of the University of Minnesota writes that propolis has very amazing health benefits to bees. One of Dr. Spivak’s graduate students found '… that bees exposed to a propolis envelope for just seven days had lower bacterial loads in and on their bodies, and had ‘quieter’ immune systems compared with bees in a colony with no propolis envelope. In other words, the propolis in the colony was killing harmful microbes in the nest, so that the bees’ immune systems did not have to gear up and make peptides and cellular responses that fight off infection.'
How can beekeepers use this information? Dr. Spivak recommends that we should be encouraging our bees to gather propolis by using hive boxes that have rough surfaces on the inside. The bees will be more likely to create a propolis envelope on a rough surface."
This is exactly why I have such rough surfaces on the inside of the Bee Tree Hives hive boxes. You can learn all about it here:
Additional scientific reading: