Moth flies have many names including drain flies, filter flies and sewage flies. They get these many names from their appearance and habits. Their fuzzy wings make them look like little moths and they are usually found around drains or sewer lines. This information sheet will help you recognize and control moth flies.
There are many types of moth flies. Adults range in size from 1/16 to 1/4 inch. They may be yellow to brown to black in color. The adults are hairy and hold their wings roof-like over their bodies when resting. During daylight, they will often be found resting on walls near drains. They are most active during the evening and may be attracted to light.
The immature or larval stage is seldom seen since they live in drain areas or other protected locations. Most larvae are long, skinny and about 1/8 to 3/8 inch long. They have no legs. They often have a pale underside with a darker head, tail and dark bands across their back.
Adult female moth flies most often lay their eggs in masses on the surface of gelatinous film found in drains and sewers. They may also breed in moist shady areas outdoors such as under potted plants, in bird feeders and baths, in moss, in clogged roof gutters, under air conditioners, in thick mulch, or on wet ground areas. The larvae feed on decaying material that collects in drains. In natural settings, moth fly larvae feed on decaying plants and animals. They breathe through a tube that helps them survive even when their environment is very wet.
Most moth flies are harmless to humans, though they may transmit bacteria and other microorganisms from their breeding sites to areas where people are. Moth flies do not bite. Adults live about two weeks.
Moth flies are not strong fliers and cannot fly long distances, though they have been known to be carried by the wind up to 300 feet. Most infestations originate in the building where the adults are found. Any aerosol or spray insecticide will easily kill the adults. However, for long term control, the breeding sites must be found and removed, altered or treated. Inspect drain areas first, but consider all the potential breeding sites mentioned previously. Drains can often be cleaned with over-the-counter cleaners followed by very hot water. If this is not successful, mechanical cleaning with a stiff brush may be required to remove any film (breeding sites) lining the drain.
If moth fly control is difficult, consider consulting a pest control operator or plumber. You may have broken pipes or moisture problems around your house that you cannot easily find or repair. Before hiring any professional, consider getting quotes from two or three firms and understand any contracts you sign.
Originally published 10/98