Drain flies are small, fuzzy, moth-like insects that breed in the slimy buildup inside drainpipes. If you’re seeing them in your bathroom or kitchen, the problem isn’t the flies themselves but the layer of organic gunk coating the inside of a drain somewhere nearby. Getting rid of them means finding and cleaning that source.
What Drain Flies Look Like
Drain flies are tiny, usually 2 to 5 millimeters long, with broad, leaf-shaped wings covered in fine hairs that give them a fuzzy or furry appearance. They’re sometimes called moth flies because they look more like small moths than typical flies. They’re weak fliers, tending to make short, hopping flights and rest on walls near sinks, showers, or toilets. You’ll most often spot them in the evening or at night.
They don’t bite. They don’t buzz around your food like house flies. Their main nuisance is simply being present, sometimes in large numbers, and the unsettling implication that something is decaying inside your plumbing.
Why They Show Up
Drain flies breed wherever moisture and decomposing organic matter meet. Their larvae feed on bacteria, fungi, grease, and microorganisms living in the slimy biofilm that coats the inside of wet drainpipes. That film, the slippery sludge you’d feel if you ran a finger inside a drain opening, is their entire habitat.
The most common breeding sites inside a home include:
- Bathroom, kitchen, and laundry sink drains, especially where bio-slime has built up in the trap or along the pipe walls
- Shower and bathtub drains, where soap scum and hair create ideal conditions
- Overflow holes in sinks and tubs, hidden channels that trap moisture and organic matter
- Toilet bases or tanks, particularly if there’s a slow leak
- Leaking pipes behind walls or under concrete slabs, where wet organic debris collects out of sight
- Miscellaneous damp spots like dirty mop heads, clogged gutters, condensation drip pans, or wet garbage cans
Outdoors, they breed in septic tanks, sewage systems, moist compost, and anywhere water pools long enough for algae to grow. Floor drains in basements or garages are especially common culprits because they often go unused. When a drain’s water trap evaporates, which happens in about two to three weeks without water flow, flies can travel up from the sewer line with nothing stopping them.
Their Life Cycle Is Fast
Drain flies complete their entire life cycle in 21 to 27 days under normal conditions, faster in warm weather. A female lays eggs in masses of 15 to 40 (sometimes up to 100) directly on wet organic surfaces like the inside of a drain. The eggs are tiny, under a millimeter, and transparent. At around 70°F, they hatch in less than two days.
The larval stage is the longest part of the cycle, lasting 9 to 15 days at 70°F and as few as eight days at 85°F. The larvae are slender, white to creamy brown, and live embedded in the biofilm lining the pipe. After pupating, adults emerge. Males live only a few days. Females can survive up to a week if they haven’t laid eggs yet. This fast turnover means a small problem can become a visible infestation within a few weeks if the breeding site isn’t addressed.
Larvae can also enter a dormant state to survive cold temperatures, which means they can overwinter in pipes and reappear when conditions warm up.
Are They a Health Concern?
Drain flies don’t bite or sting, and they aren’t known to directly transmit diseases. However, because they originate in filthy conditions, there is a possibility they physically carry microbes on their bodies. The more meaningful health risk is respiratory: inhaling fragments and dust from dead drain flies can trigger bronchial asthma in sensitive individuals. In a large, long-standing infestation, this is worth taking seriously, especially in households with people who have existing respiratory conditions.
How to Find the Source
Before you can eliminate drain flies, you need to confirm which drain they’re coming from. The simplest method is the tape test. Place a strip of clear tape or plastic wrap loosely over each suspected drain opening before you go to bed. Don’t seal it completely; leave gaps so air can still flow. Flies emerging overnight will get stuck on the tape. Check in the morning. Do this for several consecutive nights, because not every fly emerges every night. Whichever drain catches flies is your target.
Test every drain in the affected area, not just the obvious ones. Overflow holes, floor drains, and rarely used utility sinks are easy to overlook. If one drain in a room has drain flies, adjacent drains likely harbor biofilm too.
How to Get Rid of Them
The only reliable way to eliminate drain flies is to remove the biofilm they breed in. Chemical sprays that kill adult flies are a temporary fix. New adults will keep emerging from larvae living in the pipe slime until that slime is gone.
Start by mechanically cleaning the drain. Use a stiff, long-handled brush (a pipe brush or drain brush) to physically scrub the inside walls of the drain body and as far down the pipe as you can reach. This strips away the biofilm layer where eggs and larvae live. Follow up with an enzyme-based drain cleaner, which digests organic matter, rather than a caustic chemical drain opener, which won’t effectively dissolve the biological film clinging to pipe walls.
You’ll likely need to repeat this cleaning every few days for two to three weeks to outlast the full life cycle. Eggs already laid deeper in the pipe will continue hatching, so a single cleaning rarely solves the problem entirely.
What About Boiling Water?
Pouring boiling water down the drain is a popular home remedy, and it can kill adult flies and some larvae on contact. But it doesn’t reliably penetrate the biofilm layer where eggs are embedded, which is why the flies often return within days. Some people report success with pouring boiling water daily for a full week, which may work by catching newly hatched larvae before they mature.
There’s a practical concern worth noting: pouring boiling water directly into PVC pipes can soften or warp the plastic, especially at joints. If your plumbing uses PVC (most modern residential plumbing does), running cold water simultaneously or letting the boiled water cool for a minute before pouring reduces the risk. It’s a supplementary tactic at best, not a replacement for physically removing the biofilm.
Preventing Them From Coming Back
Once you’ve cleared an infestation, prevention comes down to two things: keeping drains clean and keeping unused drains sealed.
For drains you use regularly, running water through them daily is often enough to prevent biofilm from building up to the thickness drain flies need. Periodic cleaning with a brush or enzyme cleaner, roughly once a month for drains prone to buildup, keeps conditions inhospitable. Removing hair from shower drain covers and keeping kitchen drains free of grease also slows biofilm formation.
For floor drains, basement drains, or any drain that doesn’t receive regular water flow, the water in the P-trap (the curved pipe section that holds a small pool of water as a barrier) will evaporate in two to three weeks. Once it’s dry, there’s a direct path from the sewer line into your home. The simplest fix is to pour a cup of water into these drains every couple of weeks. A more permanent option is installing a waterless trap seal, a one-way silicone valve that drops into the drain body and lets water flow down while blocking anything from coming back up, including flies, sewer gas, and odors.
If you’ve cleaned every visible drain and the flies persist, the breeding site may be somewhere less accessible: a leaking pipe behind a wall, under a slab, or in a crawl space. Persistent infestations that don’t respond to drain cleaning often point to a hidden moisture problem that needs professional plumbing inspection.