There is an old white pine stump by my house left from a tree cut down about 40 years ago. The bark has long since fallen off and wood is soft and weathered. Soon after the tree was cut long-horned beetles found the stump and laid eggs in the bark. These hatched into flattened white grubs that ate the inner bark and sapwood for two years before pupating and transforming into adult beetles. The grubs have hard powerful jaws to chew through the wood and in their quest for food they left many spacious tunnels with large openings to the outside. Now that the stump is no longer food for the grubs the empty tunnels are used as brood chambers by bees.
Many species of small bees are efficient pollinators of wild plants and cultivated plants. Some nest in underground tunnels but others nest in wood or hollow plant stems. Having a few old stumps nearby and leaving some large dry snags along field edges are good ways to provide nesting sites for wood-nesting and cavity-nesting bees. Bee species that will benefit include large carpenter bees (Xylocopa), mason bees (Osmia), and small carpenter bees (Ceratina). You can mimic stumps and snags by placing large logs upright into the soil. Use logs with a beetle grub tunnels or drill new holes for the bees to use. Logs and stumps with hollow interiors will also make good nesting sites for some bee species.
For more information on promoting wood-nesting and other bees check the Xerces Society’s fact sheet Invertebrate Conservation Fact Sheet Nests for Native Bees.