Identify Your Pest Flying insects Drain fly
Drain fly
Psychodidae · Also called: Moth fly, filter fly, sewage fly
Drain flies are tiny, fuzzy flies that look like miniature moths and rest on bathroom and basement walls. They breed in the slimy film inside drains, so seeing them usually points to a plumbing spot that needs cleaning.
Size Up to about 5 mm long
Colour Grey, tan or dark brown, covered in fine hairs
Key feature Fuzzy body and broad, hairy wings that make it look like a tiny moth
Season Year round indoors, wherever drains stay wet
Where Sinks, tubs, floor drains, sump pits, basements and commercial kitchens
What it looks like
Drain flies are small, up to about 5 mm long. The body and wings are covered in fine hairs, which gives them a fuzzy, moth-like look. They are grey, tan or dark brown. At rest, the wings are held flat or roof-like over the body, often forming a rounded or heart shape.
They are weak fliers. They make short, hopping flights and often sit still on walls near a tub, sink or floor drain. If crushed, they tend to leave a powdery smudge. The larvae are thin, greyish, worm-like grubs up to about 10 mm long that live in the dark slime lining drains and pipes.
Often confused with
Fruit fly
Fungus gnat
Indian meal moth
Signs to look for
Fuzzy flies on bathroom walls
Flies near floor drains
Slow or smelly drains
Flies caught on tape
Risks and damage
Drain flies do not bite or sting and are mostly a nuisance. Because they breed in drain and sewer slime, they may carry bacteria onto surfaces, but they are not known as a major cause of illness. In large numbers, dust from dead flies can bother people with allergies or asthma. A lasting problem often points to a plumbing issue, such as a slow drain, a cracked pipe or a dry floor drain trap.
In Alberta
Drain flies can turn up in any Alberta building with plumbing, in any season, because the drains they breed in stay wet all year. They are often noticed in basement bathrooms, rarely used floor drains and busy commercial kitchens in Edmonton, Calgary and smaller centres.
Drain fly larvae have a useful side. In some sewage treatment systems, they live in the filter beds and feed on the slimy film, which helps keep it from building up.
How to keep them out
Scrub drains regularly
Keep drain traps full
Fix leaks and slow drains
Clean hidden wet spots
When it is time to bring in a professional
Call a professional if drain flies return after drains are cleaned, show up in several rooms, or affect a restaurant or food plant. A hidden source, such as a broken pipe under a floor, may need an inspection. Technica’s licensed technicians can help find it, and a plumber may be needed for repairs.
Frequently asked questions
Are drain flies harmful?
They do not bite or sting and are mostly a nuisance. Because they come from drain slime, keep them away from food areas. A large or lasting problem is worth fixing, since it can point to a plumbing issue.
Will pouring hot water down the drain get rid of them?
Hot water alone usually is not enough. The larvae live in a sticky film that clings to the inside of the pipe. That film has to be scrubbed off with a stiff brush. Once the slime is gone, that drain can no longer feed the larvae.
Why do I only see them in my basement bathroom?
Drains that are rarely used can build up slime, and their traps can dry out. That gives flies a quiet place to breed or a path up from the sewer line. Running water in those drains and scrubbing them often helps.