How do I get rid of drain flies?

Clean the drain, don't spray the air. Drain flies breed in the gelatinous film coating the inside of drains, so the fix is to scrub that film out with a stiff drain brush and flush with hot water. Killing the adults you see does nothing while larvae keep developing in the slime. Once the film is gone and the drain stays clean, the flies disappear within a couple of weeks.

Where do drain flies come from?

From the organic film inside drains, sewers, and any standing wet organic matter. Females lay eggs on the gelatinous scum lining sink, shower, and floor drains, and the larvae feed on it. Less-used drains — a basement floor drain, a spare bathroom, a bar sink — are common sources because the film builds up undisturbed. Outdoors they breed in wet mulch, gutters, and birdbaths.

How do I know if it's the drain that's the source?

Tape a clear plastic cup or a piece of tape (sticky-side down) loosely over a suspect drain overnight. If drain flies are breeding there, adults emerging from the drain get caught underneath. Test each drain this way to find the exact source before cleaning — including floor drains, overflow holes, and rarely used sinks that are easy to overlook.

Are drain flies harmful?

Not seriously. Drain flies don't bite and aren't strong disease vectors, but because they breed in drain scum and sewage film they can carry bacteria onto surfaces, and in large numbers they're a genuine nuisance. In hospitals or food premises they're taken more seriously. For most homes the concern is comfort and cleanliness rather than health.

Will bleach get rid of drain flies?

Bleach usually doesn't work well on its own. Poured down a drain it runs straight through without dwelling long enough to break up the thick film where the larvae live, and it can flush past them entirely. Mechanical cleaning — a stiff drain brush to physically scrub the film off the pipe walls — plus hot water and an enzyme drain cleaner is far more reliable.

What is the difference between drain flies and fruit flies?

Drain flies are fuzzy and moth-like with rounded grey wings, rest flat on walls near sinks, and fly weakly in short hops. Fruit flies are smooth, tan-brown, often red-eyed, and hover briskly around fruit and open drinks. Drain flies breed in drain film; fruit flies breed in fermenting produce. If they're on the bathroom wall, they're drain flies; if they're on the fruit bowl, they're fruit flies.

Drain Flies in Ontario: Identification & Removal

Overview

Drain flies — also called moth flies or filter flies — are the small, fuzzy, moth-like flies that appear on bathroom and kitchen walls, usually near a sink or floor drain. They’re weak fliers that seem to materialize out of nowhere and rest flat on nearby surfaces. The reason they feel impossible to swat away for good is that the adults you see aren’t the problem: they hatch from larvae living in the gelatinous film coating the inside of your drains. Spray the air and they come back; the fix is always the drain, not the fly. Drain flies are more of a comfort-and-cleanliness nuisance than a health threat, but a steady supply of them is a reliable sign that a drain or wet spot somewhere is quietly feeding a breeding colony.

Identification

Drain flies are tiny — 1.5 to 5 mm — grey to tan, with a fuzzy, hairy body and rounded wings held roof-like over the back, giving them a distinctly moth-like look. When disturbed they fly in short, weak hops rather than sustained flight, and they rest flat against walls near their breeding drain. Under magnification the body and wings are covered in fine hairs that flake off as a powder when crushed.

Feature	Drain Fly	Fruit Fly
Body	Fuzzy, moth-like	Smooth
Wings	Rounded, held roof-like	Narrow, held flat
Colour	Grey to tan	Tan-brown, often red eyes
Flight	Weak, short hops	Brisk hovering
Rests on	Walls near sinks/drains	Fruit, drinks, recycling
Breeds in	Drain film	Fermenting produce

The other common mix-up is with fungus gnats, which are slender, dark, and mosquito-like and breed in damp houseplant soil rather than drains — check the fruit fly page for the full three-way comparison.

Life Cycle

Drain flies develop fast: the complete cycle from egg to adult can finish in about two weeks in warm indoor conditions. Females lay eggs in irregular masses on the surface of the gelatinous film inside drains and sewers. The eggs hatch into thin, legless, banded larvae — 3 to 9 mm long — that feed on the organic scum, bacteria, and decaying matter in the film. After several days the larvae pupate, and adults emerge to mate and lay the next batch. Because the cycle is short and continuous indoors, a single neglected drain can sustain a rolling population year-round, with new adults emerging daily.

Habitat & Behaviour

Drain flies live wherever a thin layer of wet organic matter collects and sits undisturbed. Indoors that means the inside walls of sink, shower, tub, and floor drains; the overflow holes in sinks; the traps of rarely used fixtures; sump pits; and the gunk under a slow drain. Adults stay close to their breeding site, resting on nearby walls and ceilings by day and becoming more active in the evening. They don’t travel far, which is actually helpful — the flies cluster near their source, so where you see them points you toward the drain to clean. Outdoors they breed in wet mulch, clogged gutters, compost, and birdbaths.

Diet

The larvae are the feeders, grazing on the bacteria, fungi, and decaying organic material in drain film and wet organic matter. Adult drain flies barely feed at all — they may sip water or nectar and live only a short time, focused on mating and egg-laying. This is why food traps and kitchen sanitation don’t touch them: the problem is the larval food source in the drain, not anything on your counter.

Signs of Infestation

Fuzzy grey flies resting on walls near a sink, tub, or floor drain — the clearest sign, and a pointer to the source.
Flies emerging when a drain is run after sitting unused.
A positive drain test — adults trapped under tape or a cup placed over a drain overnight.
Weak, hopping flight when the flies are disturbed, distinguishing them from brisk fruit flies.
Larvae in the drain film — thin, banded, legless, visible if you scrape the scum from a pipe wall.

Damage Caused

Drain flies cause no structural or material damage. They don’t chew, bore, or contaminate stored goods. The only real downside is the nuisance of their presence and the fact that a persistent population signals a drain that needs cleaning. Their appearance can, however, be a useful diagnostic: a sudden bloom of drain flies sometimes reveals a hidden plumbing problem, such as a cracked or leaking drain line under a floor slab that’s feeding a colony out of sight.

Health Risks

Drain flies are a low health risk. They don’t bite, and they’re not efficient disease vectors, but because they develop in drain and sewage film they can pick up and carry bacteria onto surfaces, and in very large numbers their shed body hairs have been linked to respiratory irritation and asthma in sensitive people. In hospitals and food premises they’re treated more seriously as a sanitation indicator. For a typical home, the honest read is nuisance first, health a distant second.

Seasonal Activity in Ontario

Because drain flies breed in heated indoor plumbing, they can appear year-round in Ontario, even in mid-winter, wherever a drain stays warm and filmy. That said, numbers climb in the warm months when the short life cycle runs fastest and when summer heat and humidity raise indoor drain temperatures. Seasonal cottages and homes with a floor drain or spare bathroom that sits unused for weeks are prime cases — the standing water and undisturbed film in a neglected drain build the perfect nursery. Basement floor drains that dry out and refill are a classic recurring source.

Where They Hide

The larvae live inside drains: kitchen and bathroom sink drains, shower and tub drains, basement and garage floor drains, sink overflow holes, sump pits, and the traps of any rarely used fixture. Adults hide on walls, ceilings, and in the shadowed areas immediately around those drains. Slow or partially clogged drains, where film accumulates fastest, are the most productive breeding sites.

How They Enter Homes

Unlike most flies, drain flies usually don’t fly in from outside — they emerge from within the plumbing. A colony establishes when a drain accumulates enough organic film to feed larvae, often in a fixture that’s used infrequently. Occasionally adults enter from outdoors through open windows or up from municipal sewer connections and floor drains, then find an indoor drain to colonize. The common thread is always standing wet organic matter, indoors or in the drain system.

Prevention Tips

Clean drains regularly — scrub the inside walls with a stiff drain brush to remove the film larvae feed on.
Flush with very hot or boiling water, followed by an enzyme-based drain cleaner, not just bleach.
Run water in rarely used sinks, tubs, and floor drains weekly to keep traps full and film from building.
Fix slow or clogged drains promptly — the slower the drain, the faster film accumulates.
Repair leaks and eliminate standing water under and behind fixtures.
Keep floor drains and sump pits clean, and pour water into floor-drain traps that tend to dry out.
Clear gutters and remove wet mulch or debris against the foundation to cut outdoor breeding.

Cleaning the drain is the whole game — do it thoroughly and the flies have nowhere to breed.

DIY vs. Professional Treatment

Most drain-fly problems are a homeowner fix: find the source with a drain test, scrub out the film, and keep the drain clean. No spray is needed, and honestly, a spray is the wrong tool — it kills adults while larvae keep developing in the slime you haven’t cleaned. Professional help is worth it when you’ve cleaned the obvious drains and the flies persist, which often means the source is hidden — a cracked drain line under a slab, a colony in a sewer connection, or a wet void feeding larvae out of sight. Sani IQ traces the true breeding site, treats it, and advises on the plumbing fix, backed by our Pest-Free-Or-It’s-Free guarantee. For recurring or commercial cases, book an inspection or explore our commercial program.

References

Last updated: July 16, 2026 · Reviewed by Sani IQ licensed technicians

Drain Flies in Ontario