Drain flies are also sometimes called sewer flies or filter flies. They are very small flies about 1/16” long with light bodies. The hold their leaf shaped wings roof like over their backs when at rest. Both the body and their wings are covered with long antennae with 13-15 bead-like segments.
The larvae are aquatic and are long and cylindrical with the fore end of the body somewhat flattened on the lower side where eight suckers are found. They have dark, hardened patches on the back of each segment and they breathe through a hardened, stalk-like siphon tube at the end of the body.
Drain flies may become an annoying problem in some homes where they can be found breeding in the liquids found in drains, dirty garbage containers, or septic tanks. They are also frequently a problem when they breed in large numbers in the filter beds of sewage treatment plants. The adult flies are poor fliers and are found in great numbers on walls or flying weakly in the area where they developed.
The females lay 20-100 eggs in a Jell-O like mass in any suitable breeding material. The eggs hatch in two days and the larvae finish development in 9-15 days. The larvae feed on algae, fungi, bacteria, and sludge. The pupal stage lasts about a day and a half. The adults feed on nectar or polluted water.
When these flies are a persistent problem in a home it is evident that they are either developing in the home or in a nearby source such as a sewage treatment plant. Whenever practical the source of infestation should be found and eliminated mechanically. The use of non-residual insecticides can knock down the adult population but will not eliminate infestation. A drain cleaning maintenance program may be needed, with the use of an enzyme, to destroy the breeding site.