Mason bees ( Osmia sp. ) typically use the abandoned tunnels of wood-boring beetles for their nest. These small bees are not social. Mason bees mate immediately after hatching in the spring. The female then searches for an appropriate hole or crevice to build her nest. After preparing a brood chamber, she gathers pollen and nectar until she has enough to feed a larva to adulthood. Then she lays an egg and closes the chamber with mud. She repeats the process until the tunnel is completely filled and caps the tunnel with an extra-thick plug of mud. She will repeat this process until she dies in early summer. The mature larvae pupate and overwinter in their nursery cells. Mason bees are closely related to the Leaf-cutting bees. To gather pollen, they both use a brush of hairs on the underside of the abdomen (called a scopa ) instead of pollen baskets on their legs. There are 140 species in North America. You can find out more about Mason Bees from the North Carolina Extension service and the Wikipedia entry for the species.