How to get rid of drain flies: scrub out the slime they breed in

Step by step: how to get rid of drain flies

Safety first. Wear gloves; drain slime carries bacteria. Never follow a lye or other caustic drain cleaner with bleach, because the mix can release chlorine gas, and never combine different drain products in the same pipe. Do not pour insecticide down a drain.

1. 1

   Find the drain they come from

   Stick a strip of clear packing tape across each suspect drain overnight, leaving a small gap so air can move. Check it in the morning: flies stuck to the underside show which drain is breeding them. Test sinks, tubs, showers, floor drains and the overflow holes too.

2. 2

   Pull the stopper and clear the opening

   Remove the pop-up stopper, strainer or drain cover and clean off the hair and gray gunk you find on it. Use a flashlight to look down the pipe: a brown or black slimy coating is what you are after.

3. 3

   Scrub the inside of the pipe

   Push a long, stiff drain brush down as far as it will go and scrub with a twisting motion for a few minutes. Do the same in the sink overflow hole. You should pull up dark slime on the bristles; keep going until the brush comes up mostly clean.

4. 4

   Snake or open the trap if it is slow

   If the drain is slow or the brush will not reach, run a plastic barbed drain snake through it to pull out hair and sludge. Under a sink you can also put a bucket below the U-shaped trap, unscrew the slip nuts by hand, and scrub the trap clean before refitting it.

5. 5

   Flush with hot water

   Run hot tap water for two to three minutes, or pour in a kettle of hot water, to carry away the loosened film. Water alone does not remove the slime, which is why the scrubbing comes first.

6. 6

   Treat with an enzyme cleaner

   Pour an enzyme or bacterial drain gel down the clean drain at night, when the drain will sit unused, following the label for the amount. Most need repeat doses over one to several weeks to digest the film that remains deeper in the pipe.

7. 7

   Check the other wet spots

   Drain flies also breed in the refrigerator drip pan, air conditioner condensate pans, sliding door tracks and wet grout under cracked tiles. Clean and dry any of these that stay damp.

8. 8

   Remove adults and keep monitoring

   Swat or vacuum the adults resting on walls; they are weak fliers and easy to catch. Keep a tape strip over the treated drain for two to three weeks. If new flies still get stuck, scrub that drain again.

Are they drain flies, fruit flies or fungus gnats?

Drain flies are fuzzy, gray and moth-shaped and sit still on bathroom walls; fruit flies are tan with red eyes and hover around food; fungus gnats are slim and black and stay near plants. All three can turn up near a sink, so look closely.

Ohio State University’s fact sheet describes adult drain flies as about 1/5 to 1/6 inch long, with the body and wings densely covered in hair and wings held roof-like or out flat, and says they fly in short, jerky hops of a few feet. They are most active in the evening around sinks and drains.

Fruit flies can breed in drains too. The University of Kentucky lists drains and garbage disposals among fruit fly breeding sites and suggests taping a clear plastic bag over the opening overnight to check. The cleanup is the same: scrub out the film. If you see fruit flies mainly around produce and trash, follow how to get rid of fruit flies; if they come from plant pots, see how to get rid of fungus gnats. For a side-by-side table of all three, read our guide to gnats in the house.

Why don’t bleach or boiling water kill drain flies?

Because the larvae live inside a sticky film on the pipe walls, and liquids poured from above run past it in seconds. The film has to be scraped or digested away.

Ohio State notes that bleach does not seem to rid pipes of the scum and that drain fly larvae are surprisingly resistant to it. It recommends enzyme products designed to break the bond the bacterial film forms on pipes, or mechanical cleaning with a stiff brush and a snake, followed by a flush. Texas A&M AgriLife agrees that bleach and drain cleaners are largely ineffective on these films and that bacterial drain products usually need repeat treatments over several weeks.

What if drain flies keep coming back?

Then they are breeding somewhere you have not cleaned, often a drain nobody uses, a floor drain or a plumbing fault. Check these before you scrub the same sink again.

• Basement floor drains and laundry standpipes, which collect lint and stay damp
• The overflow channel of a bathroom sink or tub, which most people never clean
• Guest bathroom drains used once a month, where the film sits undisturbed
• Refrigerator and air conditioner drip pans
• A broken drain or sewer line under a slab, which Texas A&M notes can feed heavy indoor fly problems and needs a plumber with a camera

Do not pour insecticide into drains. Ohio State says no insecticides are registered for use in drains or sewer systems, and doing so can harm sanitation systems. If you use a flying insect spray for adults, check that its label covers indoor use near sinks; the EPA explains that following the label is a legal requirement. Our guides to cleaning a shower drain and cleaning a sink drain cover routine drain care.

Mistakes to avoid

• Pouring bleach down the drain and waiting, while the slime and larvae stay put.
• Mixing bleach with a lye-based drain cleaner, which can release chlorine gas.
• Cleaning only the drain opening and never the sink overflow or the pipe below the stopper.
• Using a single dose of enzyme cleaner and stopping; it usually takes repeat doses.
• Sealing the drain completely with tape for the test, which stops flies from coming up.
• Pouring insecticide down the pipe, which is not a labeled use.