Blood Bees, also known as Cuckoo Sweat Bees, are all cleptoparasites, primarily of Lasioglossum species. They are easiest to find as they search for nests of their hosts by flying low over the ground, often near bare soil.
Genus level ID: As a genus, most are easily recognized by their shiny red abdomens and black bodies. In a few uncommon species, the males are all black and thus very similar to some Lasioglossum. Species-level ID is impossible from photographs for the vast majority of individuals.
Example Blood Bees
Unless otherwise specified, photos are courtesy of Margarita Miklasevskaja at PCYU with funding from NSERC-CANPOLIN.