Plumbing guide

Drain Flies: Where They Come From and How to Stop Them

Small, fuzzy flies that hover around a sink, shower or floor drain are almost always breeding inside the pipe. Killing the adults does nothing until the slime they breed in is gone.

By the Urban Comfort Plumbing team · October 27, 2026

What drain flies are

Drain flies, also called moth flies or sewer flies, are small, dark, fuzzy-winged insects about the size of a pinhead. They rest on walls near drains and fly in short, clumsy hops. They do not bite, and they are not a sign of a dirty house. They are a sign of a drain with a layer of organic film. Extension specialists at UF/IFAS describe the adults as mottled, because of dense hair-like scales on the wing veins, and put the full life cycle at 21 to 27 days, so a few stragglers can still emerge after a drain is cleaned.

The female lays eggs in the moist film that coats the inside of a drain, in a trap, or in the damp debris around an overflow. The larvae feed on that film and grow, then emerge as adults. The cycle from egg to adult often takes somewhere between one and three weeks, depending on warmth. In a warm Florida house, it runs at the quick end of that range, which is why a few flies can become dozens in a short time.

How to confirm the flies are coming from a drain

Put a strip of clear tape, sticky side down, over the drain opening in the evening. Check it in the morning. Flies that land or try to come out will be stuck to it, and the drain that collects them is your source. A cup placed upside down over a drain, with a little petroleum jelly around the rim, does the same job. UF/IFAS Extension suggests the same check and leaves the tape in place for 24 hours.

Check every drain, not only the one where you saw flies. Kitchen sinks, bathroom sinks, tubs, showers, laundry drains, floor drains in the garage and the condensate drain from an air conditioner all qualify. Guest bathrooms and drains that are rarely used are the most common culprits, since the water seal in the trap can evaporate and the film builds up undisturbed.

Why pouring bleach rarely works

Many people pour bleach down the drain and are surprised when the flies come back. Bleach runs straight through the pipe and contacts the film for seconds. It does not scrub it off, and it does not reach the upper walls of the pipe or the underside of the drain flange where larvae sit. UF/IFAS Extension's main advice is to remove the organic buildup itself: scrape the inside of the drain with a pipe brush, then flush it with boiling water.

A better approach is mechanical. A stiff drain brush or a flexible scrubbing tool removes the film from the first foot or so of pipe and from under the strainer. After scrubbing, flush with hot water. Enzyme-based drain cleaners, which use bacteria to digest organic matter, can help keep the film from re-forming. They work slowly and are best used at night when the drain sits unused for hours. Skip heavy chemical openers, which can damage older pipe and do not remove the film from the walls.

Floor drains, dry traps and sewer connections

Floor drains that rarely see water are a classic breeding spot. The trap under a floor drain holds a plug of water that blocks sewer gas, and that water evaporates in a few weeks. Without it, flies and odors can come up from the line. Pouring a quart of water down the drain every month refills the trap. A thin layer of mineral oil on top slows evaporation, though some codes prefer a trap primer, which refills the trap automatically.

If the flies keep coming back no matter how clean the drain is, the problem may be deeper. A cracked or broken drain line under the slab, a missing cleanout cap or a sewer lateral with a leak gives flies a place to breed outside the house and enter through the plumbing. A slow drain along with the flies is a hint. So is a sewer smell. In those cases, a sewer camera inspection shows the condition of the line and where it is damaged.

When a drain needs real cleaning

A drain with a thick, long-standing film may need more than a brush. A plumber can run a cable through the line to remove buildup, or use hydro jetting to scour the pipe wall. That removes the film from the full pipe, not only from the first few inches.

Routine drain cleaning is a good idea if drains have been slow for a while, since the same film that feeds drain flies also catches hair, grease and soap scum, and it eventually narrows the pipe. If a kitchen drain smells of rot, grease may be coating the line. Running hot water after each use and keeping grease out of the drain slows the buildup.

Drain flies, fruit flies and fungus gnats

Not every small fly near a sink is a drain fly. Fruit flies are tan or brown, have red eyes and hover around ripe fruit, trash and open drinks. Fungus gnats are slim, dark and weak fliers that hover around overwatered houseplants. Drain flies are fuzzy, gray or tan, look like tiny moths and rest with their wings folded like a roof.

The difference matters because each needs a different fix. Fruit flies need the food source removed, and fungus gnats need the potting soil to dry out. If the flies are fuzzy and appear near a drain, scrubbing and flushing the pipe is the right move, and a trap that has lost its water needs a refill before anything else.

Prevention that works

Use every drain at least weekly, and run water in unused sinks and tubs. Clean the strainer and the stopper in bathroom sinks, where hair and soap collect. Wipe the rim of garbage disposals, and grind a few ice cubes with a slice of lemon to loosen film. Keep the dishwasher drain and the air conditioner condensate line clear, because both stay damp.

Fix leaks promptly. A slow leak under a sink keeps the cabinet and the pipe wet, and that moisture feeds the flies as well as mold. In a house with a history of drain flies, a quick monthly routine of scrub, flush and refill the traps beats any spray.

Common questions

Drain fly questions

Are drain flies dangerous?

They do not bite and are mainly a nuisance. They can carry bacteria from the film on their bodies, so cleaning the source is better than ignoring them.

Why do I see drain flies in my bathroom at night?

Adults are most active in the evening and rest near drains during the day. A strip of tape over the drain overnight shows whether they are coming from there.

Will boiling water kill drain fly larvae?

It can kill the ones it touches, but it does not remove the film they feed on. Pair it with scrubbing, and use caution with PVC and older pipe.

Could drain flies mean a sewer problem?

Sometimes. If scrubbing and cleaning do not stop them and you also smell sewage or see slow drains, a camera inspection can check the line.

Drains that smell or keep clogging?

Call and describe what you see. A dispatcher will connect you with a local contractor.

Call (727) 555-0100