I recognized a need (and opportunity ) for a mason bee box a few years ago, when on a warm spring day I saw a small bee land next to an electrical outlet, then march straight into the opening for the three-pronged plug. I watched for a while, but it didn't come back out.
Mason bees, sometimes called blue orchard bees, are very dark iridescent blue or green, about two-thirds the size of a honey bee, and covered in tiny hairs. They are solitary bees, do not build a hive with a single queen and many workers. What I had witnessed was a female entering a cavity to lay an egg, or deposit nectar and pollen as food for an egg yet to be laid. The next day I opened the casing and found no trace of bee, egg or food, suggesting that the bee had realized her mistake and departed after I tired of waiting.
A natural nest site for a mason bee would be a deep but narrow hole about 7-8 mm in diameter, such as a beetle tunnel in a tree trunk. The female first deposits a plug of mud, then brings in up to twenty loads of pollen and nectar, then lays an egg, then seals the egg in with another plug of mud, forming a cell. She repeats this sequence until the tube is almost full. The entrance is covered with a thicker mud plug. This takes place in the spring, with the female adding only one or two cells per day. Her lifespan is only about a month. Larvae hatch from the eggs a few days after laying and start devouring the food stores. Once these are depleted the larvae pupate until the following spring. Often many females will nest close together on the same tree.
The cells nearest the entrance of the nest contain males, which emerge first by chewing through the entrance plug with their strong mandibles. They hang around the opening, waiting for the girls to appear. They mate, and on it goes...
Mason bees are important pollinators of early blooming shrubs and trees, including cherries and apples. In urban gardens they favour the lily-of-the-valley shrub, Pieris japonica. Mason bee “condos” can be purchased at some local garden or bird-watching supply stores, and several versions are available online.
A very simple design is a block of wood (but not cedar) with rows of 7-8 mm holes drilled about 10 cm deep. Now is the time to set one up. For best luck, place the condo on a south-facing wall near a food plant, and provide a source of dampened earth as a mud supply.
Creative solution to overcrowding at the condo.
Even with a bee box available, some mavericks choose other sites. Last spring there was a steady stream of bees into the small rectangular holes at the corners of our aluminum window frames. I have no idea how those eggs or larvae fared. At least now I know to keep the electrical outlet covers closed.