The drain fly, a tiny insect belonging to the family Psychodidae, is often found fluttering erratically near sinks, showers, and toilets. Commonly known as moth flies or sewer gnats, these pests are frequently mistaken for fruit flies or fungus gnats, prompting homeowners to wonder if standard light traps will solve the problem. Determining their true behavioral drivers is the first step toward effective control.
Understanding Drain Fly Phototaxis
Drain flies exhibit a behavior known as phototaxis, which is a movement response to light, but their attraction is weak compared to many other flying insects. Their primary navigational cue is not bright illumination; they favor low-light, humid environments, often resting on bathroom walls or near floor drains during the day. Their flight and mating activities occur during the evening, but standard household lighting is not a reliable method for controlling an infestation. Electronic fly killers that use ultraviolet (UV) light are typically ineffective because the flies are not drawn to these traps with enough intensity to eliminate the population.
The Breeding Ground: Where Drain Flies Live
The true reason for the drain fly’s indoor presence is not light but the availability of a specific, nutrient-rich habitat. Drain flies have a complete life cycle, progressing from egg to larva, pupa, and finally, the adult fly. The adult female lays between 30 and 100 eggs directly into the gelatinous film that lines the inside of drains and sewage pipes.
This slimy layer, known as biofilm, is composed of decomposing organic matter, providing the perfect food source for the larvae. The larvae thrive within this protective sludge for approximately 8 to 24 days, with the full life cycle often completing in less than two weeks. Finding these small, moth-like insects confirms a breeding site is nearby.
Eliminating the Infestation Source
Since light attraction is not the issue, solving a drain fly problem requires addressing the source of the infestation: the biofilm. The most effective control strategy involves the mechanical removal of this gelatinous sludge lining the pipe walls. Pouring boiling water down the drain can provide short-term relief by killing some surface larvae, but it fails to penetrate the thick film where the bulk of the population resides.
The best approach is to use a stiff pipe brush to physically scrub the inside walls of the drain and overflow areas, dislodging the accumulated organic matter. Following mechanical cleaning, an enzyme or bacterial drain cleaner can be applied; these products contain specialized microbes that digest the remaining organic film, effectively eliminating the larvae’s food source and habitat. Chemical products like bleach are largely ineffective because they cannot permeate the thick biofilm layer. Long-term control depends entirely on consistent maintenance and the physical destruction of the moist breeding habitat.