There are over 72 species of these small, black flies in North America. The larvae of most kinds feed on decaying plant material and feces and for this reason are important decomposers in a natural environment. Their presence in a structure in numbers suggests that a food resource of this kind may be present that is supporting the larvae, and finding this and removing it is an important part of control. The adult flies often feed on nectar and will be found on flowers. Because of their common association with excrement the potential exists for them to vector pathogens onto humans or our food.
Finding and removing the source of the larvae is needed. Outdoors this may be decaying vegetation in the landscape, thatch in turf, or thick layers of decomposing compost in the garden. Indoors it may be accumulations of animal feces from bats, pigeons, or rodents.