ourtesy of Exzellente Bilder.
Homeowners sometimes become concerned when they see the bee entering cavities under shake siding or investigating nail holes or other cavities in wood during March through early June. These are not destructive insects, since they do not excavate holes in the wood, though they will clean out loose debris. No controls are recommended, since no damage is done. To prevent the bee from nesting, holes may be filled with caulking.
The females use existing holes in wood for a nest. They choose holes slightly larger than their body, usually 1/4 to 3/8 inches in diameter. The bees first place a mud plug at the bottom of the hole, then brings in 15 to 20 loads of nectar and pollen which they collects from spring flowers, including apples and other fruits. If you watch the bees closely as they enter the nest, you can see the pollen on the underside of the abdomen.
When the female has provided a sufficient supply of food for the larva, she lays an egg and then seals the cell with a thin mud plug. She then provisions another cell, and continues in this fashion until the hole is nearly full. Finally the bee plasters a thick mud plug at the entrance. Some wasps and leaf-cutter bees also build nests in such holes but their nests can be distinguished from the orchard mason bee nests by characteristics of the plug. The plug of the mason bee is always rough while the wasp prepares a smooth plug. Leaf-cutters seal the holes with chewed-up leaves.
Mason Bee Egg in Pollen Mass. Image Courtesy of Red58bill
The female mason bee lives for about a month and can produce one or two eggs each day. The larva hatches from the egg after a few days and begins to eat its provisions. When the pollen-nectar mass is completely eaten in about 10 days, the larva spins a cocoon and pupates within the cell.
Near the end of the summer the bee transforms to the adult stage but remains in the cocoon throughout the winter. In the spring, when the weather has warmed up sufficiently, the males begin to emerge by chewing their way 