Identify a pest
Cluster fly and drain fly identification
Slow, dark flies gathering at upstairs windows from autumn are cluster flies, which shelter in your loft over winter. Tiny fluffy moth-like flies around a sink or shower are drain flies. Neither bites, and each has a different fix.
Size
8-10 (cluster); 2-5 (drain) mmadult lengthWhere
lofts, upstairs windows, sinks, showers, floor drainsindoorsHarmful?
Noto people or the house
- How to identify cluster flies
- How to identify drain flies, and other lookalikes
- Why they are indoors, and are they harmful?
- How to get rid of them
- When a professional is worth it
How to identify cluster flies
Cluster flies (*Pollenia*) look like a slightly larger, darker house fly, around 8 to 10 mm, with a matt olive-grey body and short golden hairs on the thorax. They are sluggish, bump into windows and, when disturbed, tend to drop rather than fly off cleanly.
They are not breeding or feeding indoors. From late August to November, adults look for a sheltered place to spend the winter, such as a loft, wall void or window frame. They leave scent markers, so flies often return to the same house every autumn.
How to identify drain flies, and other lookalikes
Drain flies (moth flies, *Psychodidae*) are 2 to 5 mm, grey-brown and furry, with wings held in a tent shape and a short, jerky flight. They breed in the slimy film inside sink, shower and floor drains.
- Fruit flies are tan, with red eyes, and gather around ripe fruit and bins.
- House flies are more active and live around food and waste.
- Bluebottles are noisy, metallic and fast. If many appear with a smell, check for a dead animal.
Why they are indoors, and are they harmful?
Cluster flies are looking for winter shelter. They may buzz about on sunny winter days when the loft warms up. Their larvae live in the soil outside, not in your home.
Drain flies come up from a drain that has a build-up of slime and organic matter. A single drain can produce them continuously.
Neither is harmful to people. They do not bite, sting or damage the building. Large numbers of dead cluster flies can leave stains and smears and are a nuisance.
How to get rid of them
Cluster flies: vacuum them up, then empty the bag outside. Do not swat them on curtains or walls, as the smears stain. Seal gaps around windows, eaves, soffits and loft hatches during summer to stop them entering. Fly screens help.
Drain flies: find the drain by taping a clear sheet over each plughole overnight. Scrub the inside of the pipe with a stiff drain brush to remove the slimy film. Pour hot (not boiling) water down it, and keep the traps full. Repeat for several days. Fix leaks and clear any wet debris from under appliances.
When a professional is worth it
For drain flies, rarely: a thorough scrub fixes most. If they keep coming back, a plumber to check for a broken or leaking pipe is more useful than an insecticide. For cluster flies, a professional is worth considering if you have hundreds in the loft or rooms every year. A treatment in late summer, before they settle, works better than one in midwinter. Cluster flies in small numbers do not need one.
Questions people ask
Why do I have so many flies in my loft?
They are probably cluster flies seeking somewhere warm to overwinter. They use scent markers, so they tend to return to the same house each year.
Do cluster flies bite?
No. They do not bite, do not feed indoors and do not breed there.
Where do flies from the drain come from?
From slimy build-up inside the drain, where drain flies lay their eggs. Scrubbing the pipe removes the breeding site.
Are drain flies harmful?
Not usually. They do not bite, but they signal a dirty drain and can be a hygiene nuisance in kitchens.