As many as four generations develop each summer. So if earthworms are abundant, a very substantial fly population can build. In the fall, adult flies seek shelter where they spend the winter months, then emerge the following summer to begin the cycle again. These small flies enter houses or other shelters in the fall. Unlike house flies, they do not spread disease.
Problems are common around well cared for lawns that support a healthy earthworm population.
They can also be a nuisance in buildings surrounded by rich pastureland and turf. The best course of action is to prevent entry of flies indoors by sealing around windows and doors, and repairing screens. Follow label instructions carefully.