Drain flies, also known as moth flies, filter flies, or sewer gnats, are small insects commonly found near plumbing fixtures in homes and commercial buildings. These pests belong to the family Psychodidae, and the species Psychoda alternata is frequently encountered as a household nuisance. Measuring only about 2 to 5 millimeters in length, they have a distinctive fuzzy appearance due to a dense covering of hair on their bodies and wings. Their flight is weak and erratic, often appearing more like a short, clumsy hop, which is why they are usually seen resting on walls near their breeding sites.
The Complete Drain Fly Life Cycle
The life cycle of a drain fly is relatively short, typically completing in 21 to 27 days, though this duration can be reduced to as little as seven days under warmer conditions. The cycle begins when the female deposits an egg mass, containing between 30 and 100 eggs, directly onto the moist, organic film of the breeding medium. These eggs hatch quickly, usually within 30 to 48 hours.
The larval stage is the longest phase, spent entirely within the sludge layer of the drain. Depending on temperature and food availability, this legless larva will feed and grow for 8 to 24 days. The pupal stage follows, lasting a brief 20 to 48 hours before the adult fly emerges.
The lifespan of the adult winged insect is quite limited. Males typically live for only a few days after emergence. Females may survive for up to two weeks. The adult’s primary purpose is to mate and lay eggs, perpetuating the cycle within the drain’s organic film.
Understanding Their Preferred Habitat
Drain flies earn their common name because their larval stage is entirely dependent on the gelatinous, organic film that adheres to the inside surfaces of plumbing and sewer pipes. This slimy buildup, known as biofilm or sludge, consists of decomposing organic matter, fats, hair, bacteria, fungi, and other microorganisms. This constantly moist, nutrient-rich environment provides the perfect nursery for the developing larvae.
The larvae feed directly on the fungi and bacteria within the biofilm, which serves as a consistent and protected food source. They thrive in the film, extending small breathing tubes to the surface to draw in necessary oxygen. This habitat is commonly found in infrequently used drains, leaky pipes, sump pump pits, and beneath shower pans where continuous moisture and decaying material are present.
Eliminating the Infestation Source
Effective control of a drain fly problem must focus on removing the larval breeding source rather than simply killing the short-lived adults. Since aerosol sprays only target the adults, they fail to address the eggs and larvae hidden deep within the drain’s organic sludge, allowing the infestation to continue. The most direct method is the physical removal of the biofilm using a stiff pipe brush or drain snake to scrub the interior walls of the pipe.
Following the physical cleaning, a treatment is necessary to break down any remaining residue. Pouring boiling water down the affected drain several times can help to loosen and flush out some of the organic material. A more targeted approach involves the use of enzymatic or microbial drain cleaners, which contain specialized organisms that consume the organic matter, effectively eliminating the larvae’s food source and habitat. Regular maintenance, such as routinely cleaning drains and ensuring no stagnant water remains, is necessary to prevent the re-establishment of the breeding film.