Drain moths, more commonly called drain flies, breed in the slimy biofilm coating the inside of your pipes. Killing the adults you see fluttering near sinks and showers won’t solve the problem. The only way to get rid of them for good is to destroy the biofilm where their larvae live, then prevent it from building back up.
Why They’re in Your Drains
Drain flies lay their eggs directly on moist organic matter, specifically the slimy layer of bacteria, fungi, grease, and decomposing material that coats the inside of pipes that see regular water flow. Females deposit clusters of 15 to 40 eggs at a time, and those eggs hatch in less than two days at room temperature. The larvae feed on the biofilm itself, breathing through tiny tubes that poke through the slime’s surface. A single generation completes its full lifecycle in 21 to 27 days, so a small problem becomes a visible infestation fast.
The most common breeding sites are bathroom sink drains, shower drains, kitchen drains, and laundry room drains. Overflow holes in sinks and bathtubs are easy to overlook but often trap enough moisture and organic matter to support larvae. Floor drains in basements or garages are another frequent source, especially if they don’t get regular water flow. In some cases, the source is a leaking pipe behind a wall or under a slab, where wet organic debris quietly accumulates out of sight.
Find the Source First
Before you start cleaning, confirm which drains are actually producing flies. Place a strip of clear tape or plastic wrap loosely over each suspected drain opening before bed. Any flies emerging overnight will get stuck on the covering. Check in the morning and repeat for two or three nights, since not every fly emerges on the same schedule. This tells you exactly which drains need treatment and keeps you from wasting effort on the wrong ones.
If one drain in an area has drain flies, adjacent drains likely harbor biofilm too. Test all of them, not just the one closest to where you see the most adults.
Scrub the Biofilm Out
This is the step that actually works, and there’s no shortcut around it. Use a stiff drain brush (a long, narrow bottle brush works) to physically scrub the inside walls of the drain body. Push the brush down into the pipe and twist it against the sides to break up the slime coating. The biofilm is a structured microbial community with a protective outer layer that resists chemical penetration, so mechanical removal is far more effective than pouring something down the drain and hoping for the best.
After brushing, flush the drain with plenty of hot water to wash loosened material further down the pipe. For drains with removable covers or accessible P-traps (the U-shaped section of pipe under a sink), take them apart and clean them separately. The larvae tend to concentrate in the top inch or so of accumulated slime, so even a thorough surface scrub makes a big difference.
Why Bleach and Chemical Cleaners Don’t Work
Pouring bleach down the drain is one of the most common first attempts, and one of the least effective. Bleach kills adult drain flies on contact, but it does not penetrate the biofilm where larvae are embedded. The slime layer acts as a physical shield. You may see fewer adults for a day or two, then a fresh batch emerges as if nothing happened.
Enzyme-based drain cleaners are a better idea in theory. They’re designed to digest organic matter in the biofilm. In practice, they work slowly, require repeated applications, and rarely eliminate the entire biofilm layer. The slime extends for feet along the pipe interior, and a single treatment poured from the drain opening can’t reach all of it. Enzyme cleaners can be a useful supplement after mechanical cleaning, but they’re not a standalone fix.
Trap the Adults While You Clean
Scrubbing the drains eliminates the breeding site, but you’ll still have adult flies buzzing around for a few days. A simple vinegar trap helps reduce their numbers in the meantime. Fill a small jar with about a quarter cup of apple cider vinegar and add a few drops of liquid dish soap. The vinegar attracts the flies, and the soap breaks the surface tension so they sink and drown. Punch a few small holes in the lid or cover the opening with plastic wrap poked with holes to let flies in without letting them easily escape.
Place traps near the problem drains and anywhere you see adults congregating. These traps catch flies that are already in your living space but do nothing about the larvae still developing in the pipes, so don’t rely on them as your primary strategy.
Prevent Them From Coming Back
Drain flies return when biofilm rebuilds, and it will rebuild eventually in any drain that sees regular use. A few habits keep the problem from recurring.
- Run water through unused drains weekly. Floor drains, guest bathroom sinks, and basement drains that don’t get regular flow are prime targets. The water trap (the U-shaped bend in the pipe) dries out in two to three weeks without use. Once the water is gone, the drain is essentially an open pipe, giving flies a clear path in.
- Brush drains monthly. A quick scrub of bathroom and kitchen drains once a month prevents biofilm from building up to the point where it can support a new generation of larvae.
- Clean overflow holes. The small holes near the top of sinks and bathtubs connect to hidden channels that trap moisture and organic matter. A small brush or pipe cleaner pushed into these openings removes buildup you’d otherwise never see.
- Fix leaks promptly. Any source of standing moisture combined with organic debris is a potential breeding site. A slow leak under a sink or behind a wall can sustain a drain fly population indefinitely.
- Consider a drain seal for floor drains. Waterless trap seals, essentially one-way silicone valves that drop into a floor drain, create a permanent physical barrier. Water flows down normally, but nothing comes back up. Unlike a water trap, a mechanical seal never evaporates, so it works through vacations, building closures, and long stretches of disuse.
How Long Until They’re Gone
After a thorough cleaning, you should see a sharp drop in adult flies within a few days. But because the full lifecycle runs 21 to 27 days, any eggs or larvae you missed can still produce new adults for up to a month. Plan on re-scrubbing the affected drains at least once about a week after the initial cleaning, then monitoring with the tape test for another two to three weeks. If you’re still catching flies on the tape after a month, there’s likely a breeding site you haven’t found yet: an overlooked floor drain, a cracked pipe, or a hidden leak feeding organic buildup somewhere in the plumbing.
The larvae stage is the longest part of the lifecycle, lasting nine to 15 days at room temperature and as few as eight days in warmer conditions. Warmer months accelerate the entire cycle, which is why infestations tend to peak in summer. Persistence matters more than any single product or technique. Remove the biofilm, keep the drains clean, and the flies lose the only habitat they can reproduce in.