Why do they use mud?

Mason bees use mud to pack the holes where they lay their offspring. They go to the back of the hole in the nesting block and place a ball of pollen and nectar that we call “bee bread.” They lay their egg on top of the food source, and then build a small mud wall, creating a small cell. They will repeat this process until they have reached the front of the hole, and then plug it with a ¼” of mud that they carry with their mouths. At the end of the springtime, in each hole of the nesting block there will be between 5-10 mason bee offspring behind a plug of mud. When you host mason bees in your yard, you will need to make sure that you mud hole stays wet throughout the season.