Drain flies breed in the slimy film that coats the inside of a drain, not in the water. Everything that works removes that film: a stiff drain brush, boiling water, and a bacterial or enzyme drain cleaner that digests what the brush cannot reach. Do that for every drain that fails the tape test below, repeat it for two weeks, and the flies stop. Vinegar, essential oils and bleach poured down the pipe do not, because they run straight past the film.
The adults you see are harmless and short-lived. Killing them changes nothing; the next generation is already in the pipe.
What a drain fly is, and what it is not
Drain flies go by several names: moth flies, sewer flies, filter flies, sink flies. They are true flies in the family Psychodidae, and the one you will meet indoors across the Carolinas and Virginia is almost always the same species, Clogmia albipunctata. Penn State puts the adult at about a tenth of an inch, roughly a third the size of a house fly; Minnesota Extension says an eighth. Either way it is small, dark gray, and so densely covered in hairs that the body and wings look furry. The wings are shaped like leaves and held either flat or tented over the back at rest.
People confuse them with two other small flies, and the fix is different for each.
| Fly | How to tell | Where it breeds | What fixes it |
|---|---|---|---|
| Drain fly Clogmia albipunctata | Fuzzy gray, leaf-shaped hairy wings, hops or flies a few feet then lands on a wall | The organic film inside drains, overflow pipes, floor drains, sump pits, sewer breaks | Scrub and digest the film in the infested drain |
| Fruit fly Drosophila | Tan with red eyes, hovers over fruit and drinks, flies well | Ripe or rotting fruit, wine and beer residue, recycling bins | Remove the fermenting food, clean the bin |
| Fungus gnat Sciaridae | Slender, black, mosquito-like, weak flier around houseplants | Overwatered potting soil | Let the soil dry between waterings |
Why they keep coming back
A drain that carries hair, soap, grease and skin builds up a gelatinous layer on the inside of the pipe and around the trap. Penn State describes that gelatinous organic deposit as the ideal breeding site. Females lay their eggs on it, the larvae feed in it on the bacteria and fungi growing there, and under warm conditions a new adult emerges in about two weeks. The larvae are small, slender and legless, and you will not see them; they stay inside the film.
That is why the flies survive everything poured down the drain. Water, vinegar, bleach and hot tap water run down the center of the pipe and leave the film on the walls untouched. The eggs and larvae are in the film. Whatever you do has to remove or dissolve it.
Two situations are worth knowing about because they are not the drain you think. A guest bathroom or a floor drain that has not run in weeks lets the trap dry and the film ripen with nobody flushing it. And drain flies turning up where there is no obvious drain, or in large numbers in a basement, can mean a broken sewer line or a leak under a slab, which is a plumbing job rather than a pest job. Minnesota Extension lists sewer-line breaks as a recognised source.
Find the drain first: the tape test
Adults do not travel far, so the breeding drain is in the same room or the next one. Both Penn State and Minnesota Extension recommend the same simple test rather than guessing. Dry the area around each suspect drain, then lay a strip of clear tape across the opening, sticky side down, leaving a gap so air can move. Penn State's version is a jar over the drain or a plastic bag taped over the overflow outlet. Leave it overnight or for a day or two. Adults emerging from that drain get stuck on the tape or collect in the jar. Check kitchen and bathroom sinks, tubs, showers, floor drains, the washing machine standpipe and any drain that is rarely used. Treat the drains that catch flies; leave the ones that do not.
How to get rid of drain flies naturally
Natural here means mechanical and biological rather than chemical, and it is also what the extension services recommend first. Penn State calls removing the organic material the most economical and sanitary method. Work through these in order on each drain the tape test flagged.
- Scrub the pipe. Pull the strainer or stopper and run a stiff, long-handled drain brush as far down as it will go, twisting to scrape the walls, then work the overflow hole on a sink or tub the same way. This is the single most effective step because it physically removes the film with the eggs and larvae in it. Rinse the brush in hot soapy water afterwards so you are not moving larvae to the next drain.
- Flush with boiling water. A full kettle or pot poured slowly so it runs down the walls, not just the center, dislodges loosened film and kills larvae it reaches. Repeat daily for a week. On its own it rarely finishes the job, which is why it is step two and not step one.
- Add a bacterial or enzyme drain cleaner. These are the products Penn State singles out: cultures of bacteria or enzymes that digest the gelatinous scum the brush cannot reach around the trap and in the pipe joints. Pour or foam it in last thing at night so it sits undisturbed, and repeat as the label directs for a week or two. Brands sold as "drain gel," "bio-drain" or "enzyme drain treatment" at hardware stores are this category. They are not caustic and are safe for septic systems.
- Baking soda and vinegar, if you want to. Half a cup of baking soda followed by a cup of vinegar foams up and can loosen surface residue, and it is harmless. It does not digest film or kill larvae the way an enzyme product does, so treat it as a scrub aid rather than the treatment.
- Catch the adults while the drain clears. A shallow dish of apple cider vinegar with a few drops of dish soap, or a sticky trap on the wall behind the sink, takes out the adults that are already flying. This is cosmetic. It shortens the annoyance while steps one to three stop the breeding.
- Keep every drain wet and clean. Run water in rarely used sinks, showers and floor drains once a week so the trap stays full and the film never ripens. Keep hair catchers in tubs and scrape plates before rinsing so less builds up.
What does not work
| Remedy | Why it fails | Do this instead |
|---|---|---|
| Bleach down the drain | Runs past the film, damages pipes and septic bacteria, and can react with other cleaners | Brush, then enzyme cleaner |
| Essential oils or vinegar alone | No contact with larvae inside the film; scent evaporates in hours | Brush, then enzyme cleaner |
| Spraying the adults | Adults live about a week and the next generation is already in the pipe | Tape test, then treat the drain |
| Caustic chemical drain openers | Made for clogs, not film; hard on pipes; not needed | Brush, then enzyme cleaner |
| Foggers or bug bombs | Nothing reaches inside a drain | Tape test, then treat the drain |
Do drain flies bite or spread disease?
They do not bite; their mouthparts are not built for it. The health question is less clean. Minnesota Extension notes they can potentially carry disease because of what they breed in, and the same is true of any fly that walks through sewage film and then lands on a toothbrush. In an ordinary home the risk is small and the problem is a nuisance one. The exception is a sewer break under a house, where the flies are the messenger and the sewage is the problem.
When to call someone
- You have treated every drain that fails the tape test for two weeks and the flies continue, which usually means a drain you have not found: a floor drain under an appliance, a disused stack, a condensate line.
- Flies are appearing in a basement or crawl space with no drain nearby, or in numbers that a sink could not produce. Have the sewer line and any slab plumbing checked for a break or leak.
- The building is a restaurant, food plant or care facility, where the same flies around a floor drain are a health-inspection issue rather than an irritation.
Professional treatment of a drain fly problem is mostly the same work done thoroughly: finding the drain, mechanical cleaning, and a commercial-strength foaming biological treatment that coats the whole pipe. Penn State notes most pest management companies offer it as a service. If the source turns out to be structural, the fix is a plumber. Either way, drain flies are covered under our residential pest control plans across South Carolina, North Carolina and Virginia, and the same film they breed in is what silverfish and small roaches feed on, so cleaning it out pays twice.
The short version
- They breed in the film on the pipe wall, not in the water. Anything poured down the center misses it.
- Adult in about two weeks, lives about a week, does not bite, barely flies. Killing adults is cosmetic.
- Tape test every drain before treating; the source is in the room or next door.
- Brush, boiling water, enzyme cleaner, in that order, for two weeks, on the drains that caught flies.
- Bleach, vinegar and oils alone do not work. Baking soda and vinegar is a harmless scrub aid at best.
- Flies with no drain nearby mean a sewer break or slab leak. Call a plumber.
Sources: Penn State Extension, Moth Flies in the Home (size, breeding site, two-week development, brush and steam cleaning, bacterial and enzyme products, the jar and bag test); University of Minnesota Extension, Flies: Moth fly (appearance, drain film, sewer breaks, tape test, bite and disease status).
Frequently Asked Questions
From the gelatinous organic film inside a drain, overflow pipe, floor drain or sump, or from a broken sewer line. Females lay eggs on the film and the larvae feed in it. Adults barely fly, so the breeding drain is in the same room or the one next to it. Rarely used drains whose traps have dried out are the usual culprit.
Dry the rim of each suspect drain and lay clear tape across the opening, sticky side down, with a gap for air. Or set a jar over the drain, or tape a plastic bag over the overflow hole. Leave it a day or two. Flies emerging from that drain stick to the tape or collect in the jar. Treat only the drains that catch flies.
It helps once the film has been loosened, but on its own it runs past most of it. Scrub the pipe with a stiff drain brush first, then pour boiling water slowly so it runs down the walls, then follow with a bacterial or enzyme drain cleaner. Repeat for a week or two.
Not in a drain. Both run down the center of the pipe and leave the film where the eggs and larvae live. Bleach also damages pipes and septic bacteria. Baking soda and vinegar can loosen surface residue as a scrub aid, and a dish of vinegar with dish soap catches flying adults, but neither stops breeding. Use a brush and an enzyme cleaner for that.
About two weeks of daily treatment on the right drain. A drain fly develops from egg to adult in roughly two weeks in warm conditions, so you need to keep the film from re-forming for one full cycle. Adults live about a week and will disappear on their own once nothing new emerges.
They do not bite. Because they breed in drain and sewage film they can carry bacteria on their bodies, so they are a hygiene nuisance rather than a health threat in most homes. Large numbers, or flies where there is no drain, can point to a sewer break, and that is the real health issue.
When two weeks of treating every drain that failed the tape test has not stopped them, when flies appear in a basement or crawl space with no drain nearby, or in a commercial kitchen where floor-drain flies are an inspection issue. A technician can find the hidden drain and apply a foaming biological treatment; a plumber handles a suspected sewer break.
Photo credits: xpda, CC BY-SA 4.0, via Wikimedia Commons; Sanjay Acharya, CC BY-SA 3.0, via Wikimedia Commons.