Moth flies, also called drain flies, breed in the slimy organic buildup inside drains and pipes. Getting rid of them requires removing that buildup, not just killing the adults you see on walls and ceilings. Their full life cycle takes 21 to 27 days, so a thorough cleanup followed by a couple weeks of maintenance will break the breeding cycle and clear out an infestation.
Find the Source First
Before you start scrubbing, you need to confirm which drain is the problem. Moth flies can breed in any drain with standing moisture and organic film: bathroom sinks, shower drains, floor drains in basements or laundry rooms, and even seldom-used toilets. The small, fuzzy, moth-shaped adults tend to rest on walls near their breeding site, which is your first clue.
To confirm, place a strip of clear adhesive tape sticky-side down over each suspect drain at night. Leave gaps around the edges so air can still flow. Check the tape each morning for tiny flies stuck to it. Do this for a few consecutive nights, since not every adult emerges on the same day. Whichever drain catches flies is your target. You can also tape a clear plastic bag loosely over the drain opening for the same purpose.
Why Bleach Doesn’t Work
Pouring bleach down the drain is the most common first instinct, and it rarely solves the problem. Bleach kills adult flies on contact, but moth fly larvae live embedded in a layer of biofilm, a thick, slimy coating of bacteria and organic matter lining the inside of your pipes. Biofilm has a protective structure that resists chemical penetration. Bleach may kill organisms on the surface, but the larvae deeper in the film survive, and new adults emerge within days.
How to Clean the Drain Properly
The only reliable fix is physically removing the biofilm where larvae feed and develop. This is a mechanical job more than a chemical one.
Start by removing the drain cover and using a stiff, long-handled drain brush (sometimes called a pipe brush) to scrub the inside walls of the pipe as far down as you can reach. You’ll pull out dark, slimy gunk. That’s the breeding ground. Scrub thoroughly, then flush with hot water.
After brushing, follow up with an enzyme-based drain cleaner. These products contain bacteria or enzymes that digest organic matter, gradually breaking down the remaining biofilm you couldn’t reach with the brush. They work more slowly than chemical drain cleaners but target exactly the material moth flies depend on. One application usually isn’t enough. Plan to treat the drain every few days for at least two to three weeks. This timeframe matters because it covers the full life cycle, ensuring you catch larvae that were too deep in the pipe during your first cleaning.
Boiling water poured slowly down the drain can supplement your cleaning routine. It won’t penetrate deep biofilm on its own, but it helps loosen buildup between brush sessions.
Kill the Adults You See
While you’re working on the drain, you’ll still have adult flies around the house. They’re harmless (they don’t bite or spread disease), but they’re a nuisance. Adults only live a few days to about a week, so once you’ve disrupted the breeding site, the visible population drops quickly.
In the meantime, a simple spray bottle with diluted dish soap kills them on contact. You can also use a standard fly swatter. Sticky traps placed near the infested drain will catch emerging adults and help you monitor whether the population is declining. If you’re still catching new flies on tape after two to three weeks of cleaning, you’ve either missed a breeding site or the biofilm hasn’t been fully removed.
Check Beyond the Obvious Drains
If you’ve cleaned every visible drain and the flies persist, look for less obvious breeding sites. Moth flies need moisture and decaying organic matter, and drains aren’t the only place those conditions exist. Common overlooked spots include:
- Floor drains in garages, basements, or utility rooms that rarely get flushed with water
- Air conditioning drip pans that collect condensation and organic residue
- Leaking pipes behind walls or under sinks where moisture soaks into wood or drywall
- Sump pump pits with stagnant water
- Dirty garbage disposals with food residue coating the splash guard
- Potted plant saucers or trays with standing water and decomposing roots
Any area where water sits and organic material accumulates can support a moth fly population. The larvae stage lasts 9 to 15 days at typical room temperature and is the longest part of their life cycle, so even a small pocket of wet sludge gives them enough time to develop.
Preventing Them From Coming Back
Once you’ve eliminated the current infestation, keeping drains clean is the simplest form of prevention. Run water through every drain in your home regularly, especially ones you don’t use often. A guest bathroom drain or basement floor drain that sits dry for weeks develops exactly the stagnant film moth flies look for.
Use an enzyme drain cleaner once or twice a month as routine maintenance in problem-prone drains like showers and kitchen sinks. This keeps biofilm from rebuilding to the point where it can support larvae. Fixing leaky pipes and eliminating any sources of standing water in your home removes the moisture these flies need. Good ventilation in bathrooms and laundry rooms also helps keep surfaces dry enough to discourage breeding.
If your infestation is in a floor drain you rarely use, consider installing a drain trap seal or cover that allows water to pass through but blocks flies from entering or exiting. These are inexpensive and effective for drains that don’t see daily use.