Drain flies (moth flies or sewer flies) are small insects belonging to the family Psychodidae. These tiny pests are common household nuisances that signal a problem with the plumbing system. Identifying the immature stage, the larvae, is the most direct way to confirm an active breeding site. An infestation indicates a consistent buildup of organic material in the pipes, which provides the perfect environment for their life cycle.
Detailed Larval Appearance
Drain fly larvae are small, legless, and have a cylindrical, worm-like appearance. They typically measure 4 to 10 millimeters long when fully mature, roughly the size of a grain of rice. The body is segmented and generally appears translucent white, gray, or pale creamy brown.
Their coloration often includes a slightly darker appearance toward the ends of the body. A distinguishing feature is the presence of a breathing tube, or siphon, located at the posterior end. This siphon allows the larva to remain submerged deep within the organic sludge while extending the tube to access air. Unlike many other fly larvae, they present as a simple maggot-like form without a distinct, hardened head capsule.
Location and Detection
Drain fly larvae thrive exclusively within the gelatinous, nutrient-rich film that adheres to the inner walls of drainpipes. This slimy buildup, composed of hair, soap scum, grease, and decaying organic matter, serves as their shelter and primary food source. The larvae feed on the bacteria, fungi, and microorganisms that colonize this sludge.
Confirming a breeding site is best done by performing a simple overnight test, rather than looking for the larvae directly. Place a piece of clear adhesive tape, sticky side down, over the drain opening, ensuring it does not completely seal the pipe. Alternatively, an inverted plastic cup coated with petroleum jelly can be placed over the suspected drain. If adult flies are trapped by morning, it confirms that the drain is the source of the infestation.
The Complete Life Cycle
The life cycle of the drain fly includes four distinct stages: egg, larva, pupa, and adult. Females lay masses of 30 to 100 tiny eggs on the organic film inside the drain; these typically hatch within 30 to 48 hours.
The larval stage is the longest part of development, usually lasting 9 to 15 days, but potentially extending up to 24 days depending on temperature. Following this, the insect enters the pupal stage, which is very short, lasting only about 20 to 48 hours. The entire process, from egg to adult, can be completed in as little as one week under optimal warm conditions.
Elimination Strategies
Effective elimination strategies must focus on removing the larval habitat and food source—the gelatinous biofilm—rather than simply killing the adult flies. Standard chemical drain cleaners are often ineffective because they flow down the center of the pipe, passing over the biofilm. This leaves the larvae and eggs protected within the sludge.
The most successful approach involves physical scrubbing of the pipe walls using a stiff bristled brush or a drain snake. Following physical removal, an enzyme or microbial-based drain cleaner should be applied, as these products contain specialized organisms that digest the organic matter. Pouring boiling water down the drain multiple times can also help break up the organic matter and kill the larvae and pupae. Consistency is necessary for several days to ensure all newly hatched larvae are eliminated before they mature.