Drain flies, also called filter flies, moth flies, and sewage gnats, are nuisance pests. We are most concerned with these flies when they appear in our houses or buildings, creating annoyance. The larvae breed in moist organic matter and feed principally on algae. The muck of gelatinous material that accumulates on the sides of drains and overflow pipes in houses may provide suitable breeding sites. Some species are able to survive hot water and soap. Where they are a problem, the adult flies may be seen resting on walls in kitchens, bathrooms and basements. Usually only a few are present at any one time; some die off, and others emerge, but occasionally, they may occur in large numbers.
The adults are small, thickly haired, broad-bodied flies, usually less than 5 mm (1/4 inch) in length. The wings are often clothed with hairs or scales, giving the flies a "fuzzy" appearance. Wings are held roof-like or tent-like over the body when at rest. They are not strong fliers and often move by crawling on the walls or other surfaces. When they do fly, they move only a few feet at a time and fly in a jerky line. The adults may be attracted to lights at night. Outdoors, these flies are common in shady places in the vicinity of water, and they are often found in large numbers on dense foliage in swampland.
Drain flies may go through the life cycle in 1 to 3 weeks, and the adults can live for about 2 weeks after emerging. Eggs are laid in irregular masses in such places as water traps in plumbing fixtures, around built in sinks, garbage disposals, or anywhere moist decaying organic matter occurs. The larvae and pupae are aquatic or semi-aquatic, living in the decomposing film of organic matter.
Further sources: IDL INSECT DIAGNOSTIC LABORATORY
Cornell University, Dept. of Entomology, 2144 Comstock Hall, Ithaca NY 14853-2601
Prepared 1978 by Carolyn Klass, Senior Extension Associate, Dept. of Entomology, Cornell University