They are normally black or metallic colored, and some are brilliant green or brassy yellow. Sweat bees are among the most common bees wherever bees are found, except in Australia, where they are relatively uncommon. There are about 1000 species in the U.S., Canada, and Central America.
Sweat bees are in particular numerous in North America. Although their small size makes them relatively inconspicuous, hundreds may swarm over flowers in gardens or meadows. The different species are often difficult to distinguish. Most sweat bees visit a variety of flowers. They sting only if handled.
The life cycles of sweat bees vary considerablely among species. Most species nest in the ground, but some nest in wood. Nests generally consist of a single primary tunnel having one or more clumped cells arising from lateral branches. In some species, the bees constantly guard the nest entrances.
A broad range of social development is displayed in the family. Some species, like the common eastern sweat bee, have been intensively studied. Many insights into the evolution of social behavior among insects have been learned from sweat bees.
Many species are solitary-that is, the female builds and occupies its nest alone. This is considered the basic, most primitive nesting behavior. In some sweat bee species, females nest communally, sharing a common nest entrance but constructing cells individually.
Many species, including the common eastern sweat bee, show varying levels of sociality, passing through a continuum of stages in social development. In these species, there might be a few egg-laying “queens” with the other nest mates functioning as workers.
Occasionally generations of these bees overlap and live together and there could be a division of labor among nest mates. Notwithstanding, unlike many social bees, such as Honey Bees, there is little communication between adults.
The alkali bee is a somewhat unusual ground-nesting sweat bee native to arid regions of the western U.S.. It’s black with pearly bands on the abdomen. This solitary, gregarious bee nests in alkali soils where water leaches from below to the surface, leaving a salty crust.
The bee forages on a broad variety of flowers but is in particular attracted to alfalfa and pollinates it very efficiently. Commercial alfalfa seed growers use the alkali bee for pollination in parts of the West. They maintain or construct special alkali bee beds to cultivate thousands of bees.
In some parasitic species of sweat bees, females invade the nests of other bees and lay their eggs in the food stored by the host bee. The sweat bee larva kills the host larva and consumes the food. Hosts are typically other species of sweat bees.