Drain Flies: Why They Keep Coming Back and How to Stop Them

Is it a drain fly?

Drain flies are easy to name once you know the two giveaways. First, they look fuzzy rather than sleek, because the body and the wings are both covered in fine hairs. Second, they sit still. A drain fly will rest on a tile or a mirror for a long stretch, then make one short clumsy flight of a few feet and settle again.

The family is Psychodidae, which holds more than 2,600 species worldwide. In homes you are usually looking at Clogmia albipunctata, the larger spotted-wing species, or at Psychoda species, which are smaller and plainer. People also call them moth flies, filter flies, sink flies, sewer flies and sewer gnats, so those names all describe the same insect.

Why drain flies keep coming back

This is the part almost every guide skips, and it explains the whole problem. Inside your drainpipe, above the standing water, there is a layer of biofilm. It is a soft rubbery mat of bacteria bound together with hair, grease and soap scum, and it clings to the pipe wall. In a neglected drain it can reach roughly an eighth of an inch thick.

That film is not a side effect of the infestation. It is the infestation. Female drain flies lay their eggs directly into it, in masses of anywhere from 10 to 200. The larvae then live inside the mat and eat the bacteria, algae and fungi growing in it. They never need to leave the pipe.

So think about what happens when you pour a liquid down that pipe. Bleach, vinegar and boiling water are all thin, and they run down the middle and out. They barely touch a rubbery layer stuck to the wall above the waterline. As a result the adults in the room die, the nursery survives, and a fresh batch emerges a few days later. That is the three-day return so many people report.

Ohio State University Extension puts it bluntly: bleach does not seem to rid pipes of the scum, and drain fly larvae are surprisingly resistant to bleach. Until the film comes off the wall, nothing else matters.

How to get rid of drain flies

The method below is mechanical first and chemical second, which is the order that actually works. Set aside three weeks rather than one weekend, because that is what breaking the life cycle takes.

One detail is worth repeating, since it saves most of the labor. Run the tape test before you clean anything. Homes often have six or eight drains and only one breeding site, so testing first means you scrub one pipe properly instead of eight pipes badly.

What does not work, and what is unsafe

A lot of popular advice targets the wrong stage of the insect. None of the following is a cure, though some of it has a limited role.

Are drain flies harmful?

For nearly everyone, no. Drain flies are a nuisance pest rather than a health threat, and the short answer to the common worries is reassuring.

• They do not bite. Drain flies have no biting mouthparts at all. Adults take nectar and water, and some species do not feed as adults.
• They do not sting. Nothing in this family stings.
• They are not established disease carriers. Unlike house flies, they are not linked to the spread of specific human illnesses.
• They do not damage the house. They eat microbial film, not wood, fabric or food.

Two small caveats are worth stating honestly. Because the larvae develop in drains and sewage lines, an adult landing on a worktop can transfer microbes mechanically, so it is still sensible to wipe surfaces. And in large, long-running infestations, inhaled fragments and dust from dead flies have been linked to bronchial asthma. Both risks scale with how long the problem is left, which is another reason to deal with the drain.

Can drain flies lay eggs in a person?

No, and the claim deserves a clear answer because it circulates widely. A 2022 review in the Journal of Medical Entomology went through all 21 published cases that blamed drain flies for myiasis, which means fly larvae developing in living tissue. The authors concluded the evidence does not hold up. Larvae were almost never recovered from an actual lesion, imaging and clinical confirmation were usually missing, and the insect feeds on decaying matter rather than living tissue. Contamination of urine or stool samples in a bathroom explains the reports better than infection does.

Baby drain flies, larvae and drain worms

People search for baby drain flies expecting a miniature version of the adult. In fact the young stage looks nothing like one, which is why it often gets reported as a separate pest called a drain worm.

Larvae are pale grey, legless and thin, roughly 4 to 10 mm long, with a darker head and tail end and faint bands across the back. They live buried in the pipe film. Cleverly, they breathe through a tube that reaches up to the surface, so they survive in shallow, dirty, low-oxygen water that would kill most insects. Finding them confirms the breeding drain immediately.

Egg to adult can finish in as little as one to three weeks depending on temperature, and warm pipes speed it up. Note what that timing means in practice. Clean once and you remove the current generation, yet eggs laid in the days before keep hatching afterwards. Arkansas Cooperative Extension therefore recommends treating for at least three weeks to break the cycle.

What causes drain flies, and where they come from

Drain flies come from inside your own plumbing far more often than from outdoors. They need two things together: an organic film to breed in, and an opening into the room. Anywhere those overlap, a population can establish quietly over a few weeks.

The film builds fastest where water moves slowly or rarely. That is why the usual sources are so predictable:

• A guest bathroom or spare shower that goes weeks without use, so nothing scours the pipe.
• Basement and garage floor drains where the trap has dried out, which also lets sewer odor in.
• Slow kitchen drains lined with grease and food residue.
• Utility and laundry sinks that collect lint and detergent scum.
• Septic lines and sewage beds, which is why several homes on one street sometimes get them at once.

Seasonality plays a part too. Searches for drain flies roughly quadruple between February and July every year, because warmth speeds the life cycle up and holiday homes and spare bathrooms sit unused over summer. If they turned up suddenly in June, nothing unusual has happened to your house.

When it is not the drain

Sometimes every drain tests clean and the flies persist. In that case the breeding site is another patch of wet organic matter, and these get overlooked constantly:

• A failed toilet floor seal. Water seeping under the base feeds a hidden film. Rocking the toilet or seeing staining at the foot is the clue.
• An air conditioner condensate pan or drip line. Permanently damp, rarely inspected.
• The saucer under a potted plant, or chronically waterlogged soil.
• Clogged roof gutters holding wet leaf sludge near a window.
• A rain barrel, bird bath or unused bucket outside a door.
• Any slow leak under a cabinet that keeps the base damp and dark.

Track the source the same way as before. Watch where the flies rest and cluster, because they are weak fliers and rarely travel far from where they emerged. A wall covered in them is usually within a few feet of the breeding site.

Drain flies, fruit flies and fungus gnats

Three small flies turn up indoors constantly, and they need completely different treatments. Getting the identification right first saves weeks of work on the wrong drain.

A fourth insect fools people regularly as well. Phorid flies also breed in drains and in sewage under a cracked pipe, yet they run across surfaces in quick jerky bursts instead of flying. If the flies are humped at the shoulder and scuttle rather than rest, a phorid infestation can point to a broken sewer line, so it is worth investigating properly.

Still not sure what you are looking at? Our guide to small black bugs in the house works through the usual suspects room by room, and the free insect identifier gives an answer from a photo. For anything else indoors, start at what bug is this.

Pest control disclaimer. Photo-based identification can help, but it is not always perfect. For dangerous spiders, large infestations, or anything you are unsure about, contact a licensed pest control professional.