Drain Flies and Fruit Flies in London: How to Tell Them Apart and Clear Them

Few things are more annoying than a cloud of tiny flies around your kitchen sink or fruit bowl. They're not dangerous, but they signal a hygiene issue that needs fixing — and in London's older properties with Victorian drainage and smaller kitchens, they can persist for months if the source isn't found.

The first step is knowing which fly you have. Drain flies and fruit flies look similar to the untrained eye, but they breed in completely different places and need different fixes.

Drain flies vs fruit flies — how to tell

These two are constantly confused, but a 10-second look tells you which you're dealing with.

Drain fly (Clogmia / Psychoda): fuzzy, moth-like body, wings held roof-like over the back, poor fliers — they tend to hop and flutter in short bursts. Grey or tan colour.
Fruit fly (Drosophila): sleek, reddish-brown body, bright red eyes, strong straight flight. Often seen circling fruit bowls, wine glasses, and compost caddies.
If you cover the suspect drain with tape overnight and find flies stuck to the underside in the morning, it's drain flies. If they're around the fruit bowl but not the drain, it's fruit flies.

Where drain flies breed (and why London is prone)

Drain flies lay eggs in the gelatinous biofilm that builds up inside drains, overflow pipes, and condensate trays — especially in slow-draining or partially blocked pipes.

Kitchen sink drains with a buildup of organic matter and grease.
Bathroom sink and shower drains — hair and soap scum create perfect biofilm.
Toilet overflow pipes and cistern inlets in older properties.
Condensate trays under fridges and freezers.
Unused drains and standpipes that have dried out — the stagnant water line breeds bacteria and flies.

Where fruit flies breed

Fruit flies need fermenting organic matter — they don't breed in drains unless the drain is severely blocked with rotting debris.

Overripe or damaged fruit left on the counter.
Recycling and compost caddies — especially those with residue from wine bottles, juice cartons, or fruit peel.
Bin bags with food waste — even a small tear in the bag creates a breeding site.
Spills under appliances — particularly under fridges, dishwashers and cookers where fruit juice or wine has been missed.
Drains with rotting food debris trapped in the trap — less common but possible.

How to clear drain flies properly

Killing the adults with a spray does nothing — the larvae are in the biofilm. You need to destroy the breeding site.

Pour boiling water down the drain twice daily for 3 days — this loosens biofilm.
Scrub the drain with a long-handled brush and a biofilm-breaking enzyme cleaner.
Use a drain unblocker if flow is slow — partial blockages are the main cause.
Cover unused drains with a plug or mesh to prevent adults from entering and laying eggs.
For persistent cases, a professional drain camera survey may reveal a cracked pipe or broken seal beneath the floor — common in Victorian London properties.

How to clear fruit flies

Fruit flies are easier to clear than drain flies because the source is usually above ground and visible.

Remove all overripe fruit and veg — store fresh produce in the fridge during an active infestation.
Empty and clean all bins, recycling boxes and compost caddies.
Check under appliances for forgotten spills and clean thoroughly.
A simple vinegar trap (apple cider vinegar, a drop of washing-up liquid, covered with cling film pierced with holes) catches adults and helps monitor numbers.
Rinse wine glasses and juice bottles before they go in the recycling.

When to call a pest controller

Most fly issues are resolved with thorough cleaning. Call a professional if: drain flies persist after 2 weeks of cleaning (suggests a hidden pipe problem), you're in a flat and suspect a shared drainage issue, or you need a camera survey to rule out cracked pipework beneath the floor.

Frequently asked questions

Are drain flies a sign of a blocked drain?

Often yes, but not always. Slow-draining water creates biofilm faster. Even a drain that appears to flow normally can have a biofilm coating the pipe walls deep below. A CCTV drain survey is the only way to be certain.

Can fruit flies make you sick?

They don't directly transmit disease to humans in the way houseflies do, but they carry bacteria from rotting food surfaces onto fresh food. In commercial food premises they're an EHO issue. At home, the bigger concern is what their presence says about hygiene standards.

How long until they're gone after cleaning?

Fruit flies usually disappear within 3–7 days of removing breeding sources. Drain flies can take 1–2 weeks because the lifecycle includes time underground in the biofilm. Be patient and consistent with cleaning.