Drain Fly

Drain flies belong to the family Psychodidae and are found worldwide, wherever a thin film of organic material builds up in standing moisture. Their larvae thrive in the gelatinous coating that develops inside drain pipes, sewage filters, and septic systems, feeding on the bacteria and organic matter within it.

Adults are weak fliers, rarely traveling far from the site where they developed, and are more often noticed resting quietly on nearby walls than actually flying. Their dense covering of hair and broad, rounded wings give them a moth-like appearance that accounts for their alternate common name.

While largely a nuisance rather than a structural concern, large indoor populations can build up quickly because of their short life cycle and the constant availability of organic film in poorly maintained drains.

• Small body, 1.5-5 mm long, densely covered in fine hairs
• Broad, rounded wings held tent- or roof-like over the body at rest
• Fuzzy, moth-like overall appearance
• Weak, fluttering, short-distance flight rather than sustained flying
• Tan to gray coloring
• Lookalikes: fruit flies are smoother-bodied with narrower, clear wings and red eyes, lacking the fuzzy moth-like look of drain flies

Drain flies occur worldwide wherever moisture and organic film accumulate, including drain pipes, sink overflow openings, sewage treatment filters, septic tanks, and occasionally overwatered potted plants. They are especially common indoors in bathrooms and kitchens near infrequently used drains.

Adults rest motionless on walls near their breeding site during the day and become more active at night, feeding very little as adults. Larvae are semi-aquatic filter and scrape feeders, consuming bacteria, algae, and organic film that coats the wetted surfaces of drains and similar habitats, playing a minor decomposer role within these systems.

Eggs are laid directly onto the gelatinous organic film found in drains and similar habitats. Larvae feed within this film for one to two weeks, passing through several instars, before pupating near the water's surface. Adults emerge shortly after, and the entire life cycle can be completed in as little as seven to fifteen days under warm, moist conditions, allowing indoor populations to build rapidly if breeding sites are not disturbed.