**Drain Flies** (family Psychodidae), also known as moth flies or sewage flies, are small, fuzzy, gnat-like insects that are common nuisance pests in homes and businesses, particularly around bathrooms, kitchens, and utility sinks. They breed in the gelatinous slime layer found lining drains, sewer pipes, and plumbing fixtures. While not known to transmit disease, their presence indicates poor sanitation or a plumbing leak and is highly irritating.
Taxonomy and Classification
Drain Flies belong to the family Psychodidae in the order Diptera (true flies). They undergo complete metamorphosis (egg, larva, pupa, adult). The genus *Psychoda* contains many of the most common pest species. They are generally weak, erratic fliers and are nocturnal, often resting on walls near the breeding site during the day.
Physical Description
Adult Drain Flies are tiny, $1/16$ to $1/4$ inch long, and typically black or brownish-gray. They have a characteristic fuzzy, moth-like appearance due to dense hair covering their wings and bodies. When resting, their wings are held tent-like over the body. They are often mistaken for tiny moths.
The **larvae** are small, grayish-tan, legless, worm-like organisms (up to 9 mm long) that live completely submerged in the thick, slimy film lining drains. They possess a breathing tube that they use to pierce the surface film of the water or slime.
Distribution and Habitat
Drain Flies are found worldwide, wherever moisture and decaying organic material accumulate. Their habitat is specifically the layer of gelatinous organic slime—composed of bacteria, fungi, algae, and decaying matter—that lines drainpipes, overflow drains, septics, and condensation pans. They emerge into kitchens, bathrooms, and laundry rooms.
Behavior and Life Cycle
The life cycle is rapid, typically completed in 1–3 weeks, leading to continuous overlapping generations. Females lay clusters of 30–100 eggs in the gelatinous film. The larvae live entirely within this slime layer, feeding on the microorganisms and decaying material. The pupal stage occurs in the slime, and the adult fly emerges from the drain opening.
The adults live for only about two weeks, focusing primarily on mating. Their weak flight means they tend to remain close to their breeding source, often seen crawling or hopping rather than flying.
Feeding and Damage
Adult Drain Flies feed on nectar and polluted liquids and are not considered dangerous. The larvae feed on the organic matter in the slime layer. They are not pests of food, fabric, or wood. Their primary negative impact is as a **nuisance pest**, often appearing suddenly and in large numbers, signaling unsanitary conditions within the plumbing system. In healthcare facilities, their ability to transfer microorganisms from the sewer to surfaces is a minor concern.
Management and Prevention
Control focuses entirely on **eliminating the breeding source**—the slime layer. Insecticides are ineffective because they cannot penetrate the slime where the larvae live.
- **Mechanical Cleaning:** The most effective method is scrubbing the interior sides of the affected drainpipes (including the overflow drain and P-trap) with a pipe brush to manually dislodge the slime layer.
- **Biological Cleaners:** Pouring **biological drain cleaners** (containing bacteria/enzymes) down the drain can help dissolve the organic film without damaging pipes. **Boiling water, bleach, or chemical drain openers are ineffective** against the larvae and may damage plumbing.
- **Isolation:** Use duct tape over the drain at night to confirm the breeding source (adults will stick to the tape upon emergence).
Conservation and Research
Drain Flies are nuisance pests. Research focuses on optimizing non-chemical and biological cleaning agents for pipe systems to prevent the buildup of the breeding habitat and to manage pest populations in large commercial and municipal systems.