Drain Flies: Why They're in Your Bathroom and How to Actually Get Rid of Them

Quick answer: Drain flies breed in the film of organic sludge — biofilm — coating the inside of a drain pipe, not in the water. That’s why bleach or boiling water fails: it flushes the pipe without removing the film the larvae live in. The fix is mechanical. Brush the pipe walls, follow with an enzyme drain cleaner, repeat for a week, and the flies stop appearing because there’s nothing left to breed in. If they keep coming back after that, you likely have a cracked pipe, a dry trap or a leak under a floor — a plumbing problem wearing a pest costume.

Every September the same call comes in. Small fuzzy flies in the bathroom, sitting motionless on the wall above the sink. Not many — five, ten — but they never go away, and the homeowner has already emptied a bottle of bleach down every drain in the house.

The bleach was the wrong tool, applied correctly, to the wrong target. Understanding why is most of the solution.

If they’ve spread past the one bathroom, or you’re dealing with them in a rental unit or commercial kitchen, our teams cover this across Innisfil, Orillia and the rest of central Ontario — but read this first, because a lot of drain fly problems are solvable without us.

First: are they actually drain flies?

September is peak indoor-fly confusion in Ontario: three completely different insects appear at once and get treated as one problem. Getting the ID right takes thirty seconds and changes everything.

Drain flies (moth flies). Two to five millimetres. Fuzzy, grey or tan, broad wings held roof-like over the body — they look like tiny moths. They make short, weak, hopping flights and sit still on tile near a sink, shower or floor drain, in the same spot daily. Source: the drain.

Fruit flies. Two to four millimetres. Tan, smooth-bodied, red eyes, no fuzz. They fly properly and hover around fruit bowls, recycling bins and wine glasses. Drain flies never hover over a banana. Source: fermenting organic material.

Cluster flies. Six to ten millimetres — house-fly size. Dark grey with golden hairs on the thorax, sluggish, appearing in numbers on sunny walls, in attics and around upstairs windows in fall. They come from outside and overwinter in the structure; Health Canada’s page on clustering flies covers the pattern. Source: outdoors.

If it’s fuzzy, small and sitting on the bathroom wall, it’s a drain fly. Our Ontario Pest Library entry on common flies has photos of all six species we see most often.

The biofilm truth: why bleach doesn’t work

The inside wall of a drain pipe accumulates a gelatinous layer of organic material — soap scum, hair, skin cells, toothpaste, food residue, and the bacteria feeding on it. This is biofilm: a few millimetres thick, clinging to the pipe wall above the water in the trap, and a perfect nursery — warm, wet, undisturbed, made entirely of food.

Drain fly larvae live in that film. They aren’t swimming in the water; they’re embedded in a layer stuck to the pipe wall, breathing through a siphon at the surface.

Now consider what bleach does. It runs down the middle of the pipe, contacts the film for perhaps two seconds, and leaves through the trap. It doesn’t dwell, doesn’t penetrate a gel matrix, and removes nothing. Boiling water has the same problem. Both kill whatever adults are in the pipe at that moment — which is why people report “it worked for two days.” The larvae are untouched, and a new generation emerges within one to two weeks.

NC State Extension’s write-up on drain flies makes the same point: control depends on eliminating the breeding medium, not treating the water.

Confirm the source before you treat anything

Before you scrub four drains, find out which one is producing flies.

The tape test. Cover each suspect drain opening overnight with clear packing tape, sticky side down, leaving a small gap for airflow. Emerging adults stick to it, and in the morning the drain with flies on the tape is your source. Do every drain in the room, plus floor drains, and don’t skip the overflow hole in a sink or tub — a frequent culprit almost nobody checks. For a large floor drain, invert a drinking glass instead. Run it two nights; emergence isn’t even night to night.

Sources people miss: the sink overflow channel, a guest bathroom with a dried-out trap, a basement floor drain, a condensate line from an air conditioner or dehumidifier, the pan under a fridge with an ice maker, and — in older homes — a leaking drain line under a basement slab, where the “drain” is the wet soil beneath your floor.

The protocol that actually works

Once you know the drain, treat it mechanically. About ten minutes of work spread over a week.

1. Brush the pipe walls. Use a stiff, long drain brush — the kind sold for bottles or dryer vents. Insert it as far as it goes and scrub with a twisting motion on the way out, several times. You are physically stripping the film.

2. Flush with hot water. Not boiling. A few litres of hot tap water to carry off what you dislodged.

3. Apply an enzyme or bacterial drain cleaner. The step that matters most and the one people skip. Enzyme cleaners digest the remaining film over hours — what a two-second bleach flush cannot do. Apply at night, after the fixture’s last use, for undisturbed dwell time.

4. Repeat nightly for five to seven days. Eggs and larvae are staged at different depths and ages. One treatment misses a cohort; a week does not.

5. Deal with dry traps. If the source is a floor drain or unused fixture, the trap has likely dried out, letting sewer gas and flies pass freely. Pour in a litre of water, then a splash of cooking oil — it floats and slows evaporation so the seal holds for months.

6. Don’t use a chemical drain opener as a fly treatment. Caustic openers are for clogs and won’t clear biofilm from the full pipe wall.

Adults you can see are a rounding error — they live about two weeks and can’t re-establish without a breeding site. Kill the site and the visible flies disappear within a generation.

When drain flies mean a plumbing problem

If you’ve brushed and enzyme-treated the correct drain for a full week and flies still emerge, stop treating drains. Something is feeding them outside the pipe you can reach. Signs the problem is structural rather than sanitary:

• Flies still emerging from a drain you’ve cleaned thoroughly, repeatedly
• Flies appearing at a wall or floor junction rather than at a fixture
• A sewer or musty odour that comes and goes
• A soft, discoloured or damp patch of floor or wall nearby
• Drain flies in a basement room with no plumbing fixture
• Numbers increasing rather than tapering after treatment

These point at a cracked drain line, a failed wax seal under a toilet, a leaking sanitary stack inside a wall, or a broken line under a slab creating a sewage-saturated pocket of soil. The flies are the symptom; no pest treatment fixes the cause. What we can do is confirm they’re drain flies breeding in a concealed source and tell you where to look — worth doing before you open a floor.

Commercial kitchens, restaurants and multi-unit buildings

In a commercial setting drain flies aren’t a nuisance, they’re an audit finding.

Floor drains under a dish pit, mop sinks, grease interceptor lids, condensate lines from walk-in coolers, and the joint where a floor drain meets cracked epoxy — the standing sources in almost every restaurant with a drain fly problem, and precisely where a health inspector looks. Drain flies also travel between units via shared stacks, so one unit’s neglected floor drain becomes six units’ complaint.

The commercial fix is a scheduled biological drain program rather than a one-time scrub: enzyme foam on a fixed cadence to every drain in the building, with high-risk drains documented. Unglamorous, effective, and it produces the paperwork that makes an inspection go smoothly. That’s the core of our commercial pest management program for restaurants, hotels and multi-unit residential.

Year-round protection is the honest recommendation

A one-time visit will find your source and clear it. What it can’t do is stop biofilm from re-forming, because biofilm re-forms in every drain that gets used.

So the one-time service is the entry point, and a year-round protection plan is what keeps the problem gone. Scheduled visits mean drains get treated on a cadence rather than after flies appear — and the same visits catch the fall pests arriving in the same weeks: mice, cluster flies, spiders and boxelder bugs.

• Insect Control — $845/year
• Mice Protection — $895/year
• Essentials — $1,195/year
• VIP Protection — from $1,395/year
• ELITE — from $2,795/year

For a one-time visit, General Insect Control starts at $395 exterior or $475 interior and exterior. Every service starts with a $95 professional inspection, credited when you book the treatment, and one-time services carry a 60-day guarantee. Call (705) 302-1887 or take the free quote quiz.

Frequently asked questions

How long does it take to get rid of drain flies? Adults disappear within about two weeks of the breeding site being eliminated — roughly their lifespan. Still seeing new flies after three weeks of correct treatment means you haven’t found the real source.

Are drain flies dangerous? They don’t bite or sting and don’t transmit disease the way house flies do. The concern is what they indicate — a sanitation issue, a dry trap or a plumbing breach.

Will bleach kill drain flies? It kills adults in the pipe at that moment and does essentially nothing to larvae in the biofilm, which is why the problem returns in days. Mechanical brushing plus an enzyme cleaner is the combination that works.

Why do I only get them in the guest bathroom? Almost certainly a dried-out trap. Fixtures that go weeks unused lose their water seal, opening a path from the drain line. Run the fixture monthly, or add water and a film of oil.

Can drain flies come up from the sewer? Yes — through a dry trap, cracked line or unsealed floor drain. If they appear in a room with no fixtures, look for the breach rather than treating a drain.

They came back a month later. Why? Either the biofilm re-established — normal in a heavily used drain that’s never maintained — or you treated the wrong drain. Re-run the tape test on every drain in the room, including the sink overflow.

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