Why Do I Have Sewer Flies? (And How to Get Rid of Them)

Sewer flies (Psychodidae) are 1.5–5mm long, fuzzy, moth-like insects. They breed in the gelatinous organic film that coats the inside of drain pipes — a mixture of soap scum, hair, grease, and bacteria. They're not dangerous, but they're persistent and indicate a sanitation problem in your plumbing.

The most common sources in residential plumbing: floor drains with dry or missing traps (the water seal has evaporated, allowing flies to emerge from the sewer), slow or partially blocked drains with heavy organic buildup, cracked sewer pipes with organic material seeping into surrounding soil, and rarely-used drains (guest bathrooms, utility sinks) where the trap has dried out.

When sewer flies persist despite cleaning all visible drains, the source is often a cracked or offset sewer pipe joint. Organic material seeps into the crack and provides a breeding ground that no amount of drain cleaning will reach. A camera inspection of the sewer line is the only way to find this. We've resolved persistent sewer fly infestations that homeowners had been fighting for months by finding and repairing a single cracked joint.

Step 1: Identify all floor drains and infrequently used drains. Pour water into each to refill the trap. Step 2: Clean all drain lines with a drain brush and enzyme cleaner — not bleach, which doesn't break down the organic film. Step 3: If flies persist after 2 weeks, schedule a sewer camera inspection to look for cracked pipes or offset joints. Step 4: Repair any identified pipe defects. The flies will disappear once their breeding source is eliminated.