How to Get Rid of Drain Flies for Good

Updated July 2026 · 7 min read · Geelong Emergency Plumbing

Drain flies — those tiny, fuzzy, moth-looking loiterers on your bathroom tiles — are one of nature's more passive-aggressive houseguests. They don't bite, they barely fly, and yet they multiply with the quiet confidence of creatures who know you'll blame the wrong thing. Spray the adults all you like: unless you evict where they're breeding, reinforcements arrive within days. Here's the method that actually ends them, plus the important bit most guides skip — when drain flies are a symptom of a plumbing problem rather than a housekeeping one.

Know Your Enemy (Briefly)

Drain flies (also called moth flies or sewer gnats) are 2–5mm, grey-brown, fuzzy-winged, and terrible at flying — they hop and flutter near sinks, showers and floor wastes, mostly at night. The adults are irrelevant; the operation runs on their larvae, which live in the biofilm — the slimy organic coating of grease, soap scum, hair and general regret lining the inside of drains. Adults live about two weeks but lay up to 300 eggs at a time, hatching within 48 hours. That maths is why swatting is a hobby, not a strategy.

Step 1: Find the Breeding Drain

Don't assume it's the drain nearest the flies. Test every suspect — bathroom sinks, showers, floor wastes, laundry troughs, the kitchen sink — with the tape test: dry the drain surrounds, stick clear tape sticky-side-down across the opening (leave gaps for airflow), and check after 24 hours. Flies stuck to the tape = maternity ward located. Infrequently used drains are prime suspects, because a dried-out water trap is also an open door for the sewer smells covered in our drain causes guide.

Step 2: Destroy the Biofilm (The Actual Fix)

This is the part sprays can't do — larvae live inside the slime layer, protected from anything that merely washes past. The eviction sequence:

1. Scrub mechanically. Remove the grate and scrub the drain walls with a stiff bottle-brush or a flexible drain brush as far as you can reach. Physical removal beats every chemical.
2. Flush hot. Kettle-hot water (not boiling into porcelain) in stages, dissolving what the brush loosened.
3. Treat with an enzyme or bio drain gel. Enzyme cleaners digest the organic film larvae feed on; used per label over several nights, they clean where brushes can't reach. Baking soda + vinegar (the classic from our unblocking guide) helps agitate but enzymes do the sustained work.
4. Repeat for a week. Eggs already laid will hatch into a suddenly uninhabitable drain. One treatment kills a generation; a week ends the dynasty.

Step 3: Handle the Stragglers

Adults die out within about two weeks once breeding stops. Speed it up with a simple trap — a small dish of apple cider vinegar with a drop of dishwashing liquid near the drain — and normal fly spray for the ones that offend you personally. But remember the hierarchy: traps are for morale, biofilm removal is the war.

Step 4: Make Your Drains Boring Forever

• Weekly hot-water flushes on kitchen and bathroom drains keep film from re-establishing.
• Run water in unused drains (spare bathrooms, floor wastes) every week or two to keep traps full — a cup of water with a teaspoon of vegetable oil slows evaporation in rarely-used floor wastes.
• Fix the food supply: hair catchers in showers, no grease down the kitchen sink — the same commandments that prevent blockages prevent flies, because both feed on the same slime.

When Drain Flies Are a Plumbing Symptom

Here's the part worth taking seriously: a fly population that survives proper treatment, or appears in multiple rooms at once, is often breeding somewhere you can't brush — a cracked pipe leaking under the slab, a broken sewer junction, a disused wet area with standing water, or an overflow gully with a drainage fault. Persistent drain flies are a recognised field sign of hidden pipe damage; they're essentially tiny inspectors reporting moisture where moisture shouldn't be. If two weeks of the method above changes nothing, the next step is a CCTV drain inspection to find the wet crack they've colonised — annoying insects occasionally do you the favour of announcing a sewer problem before it announces itself.

The One-Week Battle Plan, Summarised

Day 1: tape-test every drain. Day 2: identify the breeder, scrub it thoroughly, hot flush, first enzyme treatment, set the vinegar trap. Days 3–7: enzyme treatment nightly, keep the trap out, run water in every unused drain in the house. Day 8: re-tape-test — a clean tape means you've won; flies on the tape mean either a second breeding drain (test wider) or a hidden moisture source that deserves a camera down the pipe. Total cost of the campaign: about $20 and some resolve. Total cost of spraying adults for six months instead: your sanity, in instalments.

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Related guides: Why does my drain smell · Signs of a blocked sewer · Blocked drains Geelong