The Wondrous Habits of Ground-Nesting Bees: Ubiquitous, Diverse, but Inscrutable.

Bees are ubiquitous. Most species are non-social and most nest in the soil (85% of fauna of the eastern U.S.), unlike the few hundred highly social species (e.g., honey bee) many of which nest aboveground. Ground-nesters are found in all ice-free terrestrial biomes, reaching their greatest diversity in those arid regions with rich floras. Preferred soil textures range from sand to clayey soils; at the extremes, some species bear overt adaptations to handle a soil texture. A few excavate sandstone, some others build with dirt. Extensive surveys of preferred soils are rarely possible or available. Tunnel depths range from a few cms down to 3m, often ending in a reliably damp soil layer. Most species paint the walls of underground nest cells with a waterproof secretion, which seems to maintain humid nest cell atmospheres while retarding inundation and soil-borne fungi and bacteria that might kill developing larvae or spoil their cache of nectar-moistened pollen. Ground-nesters sometimes aggregate. Managed alkali bees have the largest reported aggregations (up to 5 million nests and now 60 years old). They depend on soil hydraulic conductivity for shallow soil moisture in their arid habitats. Where managed, 20 million bees displaced about as much soil as less productive earthworm habitats, with less bioturbation. With little exception, we have not learned how to prepare soil sites that bees will reliably colonize, a hindrance to conservation efforts and their use for crop pollination.