How To Get Rid Of Drain Flies (And Keep Them Gone)
Those small, fuzzy moth-like flies near sinks and floor drains are Psychodidae — drain flies, also called sewer flies or sewer gnats. They lay eggs in the gelatinous film coating the inside of the drain throat, just below the strainer. Spraying the surface does nothing because the larvae are protected inside the film, which is why a kitchen or restroom can be spotless and still have flies every morning.
Short answer
To get rid of drain flies, find the drain they are breeding in with an overnight tape test, scrub the drain throat mechanically, then apply a foaming biological treatment weekly for three weeks so the foam contacts the full circumference of the pipe where the larvae live. Sprays, bleach, and boiling water fail because they run straight down the center of the pipe and never touch the film the eggs are in. Once the cycle is broken, ongoing biofilm control is what stops them coming back.
Step by step
- 1
Find the exact drain with a tape test
Tape clear packing tape sticky-side-down across each suspect drain overnight, leaving a small gap for airflow. In the morning, the drain with flies stuck to the underside is the breeding site. Test every floor drain, mop sink, dish-pit drain, and rarely used fixture — in commercial buildings it is often a floor drain nobody thinks about.
- 2
Check for dry traps and standing water
A dry trap lets sewer air and adult flies in from the line; a slow or ponding drain gives larvae a permanent nursery. Pour water into unused traps and fix any drain that holds water on the floor.
- 3
Scrub the drain throat mechanically
Remove the strainer and run a long-bristle drain brush around the inside wall of the throat, 6–12 inches down. This breaks the film's grip so treatment can reach it. Skipping this step is why chemical-only attempts stall.
- 4
Foam the drain weekly for three weeks
Apply a foaming biological or enzymatic treatment that expands to fill the pipe and contact the film 360 degrees. Three weekly applications cover the full egg-to-adult cycle (roughly 8–24 days depending on temperature), so you break reproduction rather than just killing visible adults.
- 5
Stop using bleach and insecticide sprays
Bleach runs down the center and dissipates in seconds. Aerosol sprays kill adults you can see while the next generation is already developing inside the pipe. Neither changes the outcome.
- 6
Keep the biofilm down so they can't return
Drain flies come back wherever film re-forms — typically 4–8 weeks in a busy kitchen or restroom. Continuous metered dosing of the drain lines keeps organic load low, which is the only long-term prevention. In multi-drain facilities this is where a monthly serviced program replaces a recurring staff chore.
How to confirm the source
Tape clear plastic over the drain overnight. If flies appear stuck to the underside in the morning, that drain is the breeding site. Do this on every drain at once — chasing one drain at a time is the most common reason infestations drag on for months.
The fix is mechanical plus chemical
Scrub the drain throat with a long-bristle brush, then apply a foaming enzymatic cleanser that expands to fill the pipe and contact the film 360 degrees around. Repeat weekly for three weeks to break the breeding cycle.
Why bleach fails
Bleach runs straight down without contacting the film. Foam clings. That's the difference — and it's also why boiling water, vinegar, and baking soda produce a day or two of relief and no lasting change.
Drain flies vs fruit flies vs phorid flies
Drain flies are fuzzy and moth-like and rest on walls near drains. Fruit flies hover around produce and bar mats with red eyes and clear wings. Phorid flies run rather than fly and often indicate a broken line or sewage under a slab. Misidentifying them means treating the wrong place — a phorid infestation usually needs a plumbing investigation, not drain treatment.
When flies signal a bigger problem
A heavy, recurring population usually means significant grease and organic buildup, a broken or bellied line, or a trap that has never been serviced. If three weeks of correct foam treatment doesn't clear it, camera the line before repeating the program.
How this is handled as a serviced program
Luften does monthly service, not product sales: a technician treats and doses the drain lines, foams the throats, and logs each drain on every visit, so film never re-accumulates to breeding levels across a restaurant group, hotel, or residential portfolio.
Frequently asked questions
- How do I get rid of drain flies fast?
- Identify the breeding drain with an overnight tape test, scrub the drain throat, and foam it with a biological treatment. Adults die off within days; you need three weekly treatments to end reproduction.
- How long does it take to get rid of drain flies?
- Two to three weeks of weekly foam treatment breaks the cycle. Visible adults usually drop sharply after the first treatment.
- Are drain flies dangerous?
- Not directly, but they indicate organic buildup that will eventually clog the line, and they are a sanitation concern in kitchens and healthcare settings.
- Does bleach kill drain flies?
- No. Bleach flows down the center of the pipe and does not contact the film where the eggs and larvae live.
- Why do drain flies keep coming back?
- Because the biofilm re-forms. Without ongoing drain-line dosing, film returns to breeding thickness in roughly 4–8 weeks in a high-use kitchen or restroom.
- Where do drain flies come from in a commercial building?
- Floor drains, mop sinks, dish-pit drains, dry traps, ejector pits, and rarely used fixtures — anywhere organic film accumulates and stays wet.
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