Mason bees have a variety of pest and diseases mainly parasitic wasps, small mites, and chalk brood fungus. Harvesting the cocoons interrupts and breaks the disease/pest life cycle. You can use simple holes drilled into wood but after 2 or 3 years your population will crash. Some people use paper or cardboard tubes to line the hole so the bees can be managed and cared for.
RE: I Made This! - Mason Bee Nest