What Causes Sewer Flies in a Basement?

Sewer flies (Psychodidae family) are tiny — about 1/8 inch long — with fuzzy bodies and wings that give them a moth-like appearance. They breed in the gelatinous organic film (biofilm) that coats the interior of drains, particularly in floor drains, rarely-used sinks, and basement drains. They don't bite, don't transmit disease, and don't infest food. But their presence indicates a sanitation issue in your drain system.

Basement floor drains are the most common breeding ground for sewer flies. Floor drains have a trap — a U-shaped section that holds water to block sewer gases. In rarely-used drains, this trap water evaporates, allowing sewer gases and flies to enter from the sewer system. The organic buildup in the drain itself also provides a breeding surface. Regularly pouring water into floor drains (monthly) keeps the trap full and eliminates this entry point.

Any drain that isn't used regularly can develop a dry trap and organic buildup. Basement utility sinks, rarely-used bathrooms, and laundry drains are common culprits. The fix is simple: run water through all drains monthly, and pour a cup of vegetable oil into floor drains to slow evaporation of the trap water.

If you're maintaining your drains and still have persistent sewer fly infestations, the source may be a cracked or leaking sewer line beneath the basement floor. A broken sewer line creates a direct pathway for flies and sewer gases to enter the basement. This is more serious — it requires a sewer camera inspection to locate the break and determine the repair approach. Signs that point to a sewer line source include sewer odors in addition to flies, multiple drain problems throughout the house, and flies that persist despite thorough drain cleaning.

To eliminate sewer flies:

Insecticides treat the symptom, not the cause. The flies will return until the breeding source is eliminated.