This specious genus is notoriously challenging to even the most accomplished bee researchers. Active through the whole season in every habitat, these are some of the most frequently encountered bees. While a few species are distinctive, the vast majority are impossible to identify past sub-genus level from photos. Included in this genus are obligate social species, cleptoparasites, flower specialists, log nesting, and ground nesting species. They are the hosts for most species of Blood Bees (Sphecodes).
Though species-level ID is very challenging, many individuals can be quickly assigned to a subgenus, or group of subgenera. The following 3 categories split out relatively well.
Specimen photos with dark gray background are courtesy of Margarita Miklasevskaja at PCYU with funding from NSERC-CANPOLIN.