Substrates and Materials Used for Nesting by North American Osmia Bees (Hymenoptera: Apiformes: Megachilidae)

Nesting substrates and construction materials are compared for 65 of North America’s 139 described native species of Osmia bees. Most accounts report Osmia bees nesting in preexisting cavities in dead wood or pithy stems such as elderberry (Sambucus spp.), with cell partitions and plugs made from a pulp of finely masticated leaf tissue. Mud is widely used by species constructing free-form clumps of nest cells against stone surfaces. Some Osmia bees adopt abandoned nests of other Hymenoptera, particularly those of mud dauber wasps (Sceliphron spp.) and larger ground-nesting bees (e.g., Anthophora spp.). Reports of subterranean nesting by Osmia species are uncommon but possibly under-represent the habit, because subterranean nests are obscure and likely to be scattered. Ground- or surface-nesting habits are suspected for species that are absent from intensive trap-nesting programs in their native ranges but that otherwise have been commonly taken at flowers. The range of nesting habits and materials of European species are largely comparable, although records indicate that far more European species may nest in empty snail shells.