Drain flies — also called moth flies or filter flies — are small, fuzzy-winged insects about 1.5–5mm long. They look like tiny moths and move in short, erratic hops rather than flying smoothly. If you have them, they’ll typically be clustered near drains, on bathroom tiles, or around waste pipes.
They’re not dangerous, they don’t bite, and they don’t carry disease in the way other flies do. But their presence almost always indicates a problem with your drainage system that needs addressing.
What Are Drain Flies?
Drain flies (family Psychodidae) breed in the gelatinous film of decomposing organic matter that builds up inside drain pipes and traps. This film — a mix of soap scum, skin cells, hair, grease, and food waste — provides both a breeding site and a food source.
The adult flies emerge from this film and congregate near the drain opening. One female lays 30–100 eggs, which hatch in 32–48 hours and develop to adults in 8–24 days. In warm conditions, a small infestation can become a significant problem within weeks.
Where They Come From
The most common breeding sites are:
- Sink traps: The U-bend under sinks collects debris and is a classic breeding site if not cleaned regularly
- Shower trays and bath traps: Hair and soap scum accumulate quickly
- Slow-draining floor gullies: Particularly in utility rooms and downstairs bathrooms
- Rarely used drains: Dry traps or stagnant water in infrequently used fixtures
- Partially blocked pipes: Any section of pipe where flow is restricted and debris can accumulate
How to Confirm You Have Drain Flies
Place a piece of sticky tape over the drain opening — clear tape works best. Leave it overnight. In the morning, check for small fuzzy-winged insects on the tape. If you find them, that drain is active.
Test all drains in the affected room, and any drains in adjacent rooms, to find all breeding sites.
DIY Treatment
Step 1: Clean the Trap Manually
Unscrew the trap under the sink — it’s a U-shaped section of pipe, usually plastic, that unscrews by hand or with a wrench. Remove and clean it thoroughly: scrub off all the black gelatinous film inside. This is unpleasant but essential.
Step 2: Clean the Drain Opening
Use an old bottle brush or pipe cleaner to scrub the inside of the drain pipe as far as you can reach. Remove all visible debris.
Step 3: Flush with Boiling Water and Cleaning Agent
Pour a kettle of boiling water down the drain. Follow with a cup of baking soda and a cup of white vinegar, wait 15 minutes, then flush with more boiling water. This breaks down the organic film further down the pipe.
Step 4: Repeat for 5–7 Days
Adult drain flies may have already emerged before you treated the drain. Continue the hot water and cleaning treatment daily for a week to kill emerging adults and prevent re-infestation.
Step 5: Fix Any Dry Traps
If a trap has dried out — letting flies enter from the sewer — run the fixture regularly to keep it full, or pour a small amount of cooking oil down to slow evaporation.
When DIY Doesn’t Work
If drain flies return after thorough cleaning, the problem is usually one of:
Deeper blockage or debris accumulation: DIY methods only reach the trap and the first metre or so of pipe. A professional drain jetting clears the full pipe run and removes the biofilm throughout.
Cracked or damaged pipe: A crack in a drain pipe creates a site where organic matter accumulates outside the normal flow path — an ideal breeding site that you cannot access without a CCTV survey.
Inaccessible breeding site: Some older Manchester properties have floor-level gullies or drainage channels that are difficult to access and easy to overlook.
Call us on 0161 282 8080 if DIY treatment hasn’t resolved the problem within two weeks. We can jet the affected drains and survey them to identify any structural issues providing ongoing breeding sites.
Prevention
- Clean sink and bath traps every three to six months
- Run all infrequently used drains weekly
- Never pour cooking fat or oil down drains
- Use a drain strainer to catch hair and debris before it enters the pipe
- Arrange annual drain jetting for older properties as a maintenance measure
We cover Manchester, Salford, Stockport, and all Greater Manchester postcodes.
Frequently Asked Questions
Are drain flies harmful?
Drain flies are not known to transmit diseases to humans and they don’t bite. However, their presence indicates poor drainage hygiene, and in large numbers they can be a nuisance. The underlying cause — decomposing organic matter in pipes — should be addressed regardless of the flies.
I’ve treated the drain but still see flies. Why?
Adult flies can survive outside the drain for several days after hatching. Continue treatment for a full week and vacuum up any adults you see. If the infestation persists beyond two weeks of daily treatment, the breeding site is deeper in the system than DIY methods can reach.
Can drain flies come from outside?
Occasionally — they breed in moist soil, compost heaps, and any standing water with organic matter. But if they’re inside a bathroom or kitchen, the drain is almost always the source.