How to get rid of drain flies
Those tiny fuzzy flies near your sink are breeding inside the drain. Find the source, clear the film they live in, and they disappear for good.
Written by Illyrian Plumber
Expert ReviewedLicensed Master Plumbers
NJ Licensed Master Plumber | 10+ Years Experience | Serving Middlesex County, NJ
Drain flies, also called moth flies or sewer gnats, are the small fuzzy insects that hover around a sink or shower and seem to appear from nowhere. They breed in the organic film inside a drain, so killing the adults does nothing until you remove what they are laying eggs in. The good news is this is a do-it-yourself fix, and the rare case where flies keep coming back can point to a hidden moisture problem that our water leak detection team can track down. This guide shows how to find the breeding drain, clean it out, and keep the flies gone.
Note up front: Illyrian Plumber does not offer drain cleaning or sewer cleaning, so the steps here are written so you can solve a drain-fly problem yourself with common household tools.
About Illyrian Plumber
Licensed master plumbers serving Middlesex County, NJ since 2010. We offer water leak detection, plumbing safety inspections, kitchen plumbing, and 24/7 emergency plumbing across East Brunswick, Edison, Sayreville, Old Bridge, Monroe Township, South Brunswick, and North Brunswick.
Step 1: Find the breeding drain
Drain flies are weak fliers and stay close to where they hatch, so the source is almost always a drain in the same room. Test each one overnight.
- Place a clear cup upside down over a drain, or lay a strip of clear tape sticky-side down loosely across the opening. Do not seal it completely, since you want to trap flies as they emerge.
- Leave it overnight. The next morning, the drain with flies stuck to the tape or caught under the cup is your breeding site.
- Check every drain in the room, including shower drains, floor drains, and sink overflow holes, because more than one can be active.
Common culprits are guest bathrooms, basement floor drains, and utility sinks that rarely run, because the standing film never gets flushed away.
Step 2: Clean the drain and kill the larvae
The larvae live in the gel-like biofilm coating the pipe walls. Removing that film is the whole game.
Flush with boiling water
Pour a kettle of boiling water down the drain two or three times a day for several days to soften and wash away film. Skip this on older, fragile drain lines if you are unsure, and use very hot tap water instead.
Scrub the pipe walls
Run a long, stiff drain brush or a straightened wire brush down the drain and scrub the walls just below the opening, where most film collects. Mechanical scrubbing removes what hot water alone leaves behind.
Use an enzyme drain cleaner
Pour a bacterial or enzyme drain cleaner in at night, when water use is lowest, so it sits and digests the organic film. Enzyme products are safe for pipes and septic systems, unlike caustic chemical cleaners. Repeat per the label for a week.
To knock down the adults flying around, a simple trap of apple cider vinegar with a drop of dish soap in a covered cup with small holes works well while the drain treatment takes effect.
Flies that will not quit?
A hidden leak or cracked line keeps the moisture coming. We can find it.
Step 3: Keep drain flies from coming back
Drain flies return wherever film rebuilds, so a little routine maintenance keeps them away.
- Run water through rarely used drains, like guest baths and floor drains, at least weekly to flush film and keep the trap full.
- Wipe sink and shower drains and clean pop-up stoppers, where hair and soap collect, every week or two.
- Pour a little water into floor drains monthly so the trap does not dry out and let sewer gas and flies up.
- Treat drains with an enzyme cleaner monthly as preventive maintenance.
Kitchen drains build film fastest from grease and food. Our guide to unclogging a kitchen sink covers habits that keep that line clean.
Frequently asked questions
What causes drain flies in the house?
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Drain flies, also called moth flies or sewer gnats, breed in the slimy organic film that coats the inside of slow or rarely used drains. They lay eggs in the gel-like buildup of hair, soap, grease, and food, and the larvae feed on it. Any drain that stays moist with that film can host them, which is why they cluster around sinks, showers, floor drains, and basements.
How do I find which drain the flies are coming from?
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Tape a clear plastic cup or a piece of tape sticky-side down loosely over each suspect drain overnight without sealing it. In the morning, the drain with flies stuck to the tape or trapped in the cup is the breeding source. Check every drain in the room, including floor drains and overflow holes, because more than one can be active at once.
Will bleach kill drain flies?
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Bleach is not the best choice. It flushes through too fast to kill larvae embedded in the film and can splash dangerously. A boiling water flush followed by mechanical scrubbing with a drain brush removes the biofilm the larvae live in, and an enzyme drain cleaner digests what is left. Removing the breeding film is what actually ends the cycle, not a quick chemical pour.
How long does it take to get rid of drain flies?
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Adult drain flies live about two weeks, so once you remove the breeding film and stop new eggs from hatching, the population dies off within one to three weeks. Clean the affected drain daily for the first week, then weekly to keep film from rebuilding. If flies keep returning after thorough cleaning, the moisture source is likely hidden and needs a plumber to find.
Recurring moisture or odor problem?
If drain flies or sewer smells keep coming back, our licensed plumbers can find the hidden source. Serving East Brunswick, Edison, and all of Middlesex County, NJ.