About thirty percent of our region’s 450 or so bees nest above ground. Many do so in dead trees, borer beetle burrows, in stems of plants with soft or hollow stems and piths like elderberry, sunflower, sumac, and blackberries. This is one more reason to leave garden plants standing through the winter, as many are housing insects in various parts of their life cycle, including pupating or adult overwintering bees.
Ecologically minded gardeners don’t tidy up (unless removing diseased plants) until spring, along with planting non-cultivar native plants. Even better is to cut the tops off about a foot high, leaving the stems for bees and other insects to use for the following season. Since many bees need help in getting into a stem, the open tops are perfect for this. Native plants not only provide nesting and nectar sources, but 35% our native bees are oligolectic, needing the specific pollen of the plants they evolved with in order to be able to reproduce.
These mostly solitary bees include Small Carpenter Bees (Genus Ceratina, about 24 species in North America, 4 locally), Large Carpenter Bees (Genus Xylocopa, only the Eastern Carpenter Bee, X. virginica, is common around the DC region, though the Southern Carpenter Bee, X. micans, has been found in southern Virginia), Leafcutters (Genus Megachile, about 130 species in NA), Mason Bees (Genus Osmia, about 150 species in NA, 27 east of the Mississippi, though several other Genera such as Hoplitis and Chelostroma are also often called mason bees) and a few other types which use cavities or take advantage of above ground structures.