Sweat Bee (Tribe Augochlorini)
Sweat Bee is the common name given to a large family (Halictidae) of small bees. Generalizations about life cycles and degrees of social development among sweat bees is difficult because they vary widely between species. Most species nest in the ground, but some nest in wood. Nests usually consist of a single main tunnel having one or more clumped cells arising from lateral branches. In some species, the bees constantly guard the nest entrances.
Many species are solitary -- that is, the female builds and occupies its nest alone. In some sweat bee species, females nest communally, sharing a common nest entrance but constructing cells individually. Many specie show varying levels of sociality. In these species, there may be several egg-laying "queens" with the other nest mates functioning as workers. Sometimes generations of these bees overlap and live together and there may be a division of labor among nest mates.
Tribe Augochlorini contains three genera of metallic sweat bees that according to BugGuide are "very difficult to tell apart from only a photograph." The are impossible for me to tell apart -- period.