Sewer flies come from drains and need both removal and source control

Sewer flies — also called drain flies or filter flies — breed in the buildup inside your pipes and drains. You will see them as tiny dark insects, usually near bathroom or kitchen sinks, showers, or floor drains. They are not dangerous, but they multiply fast and signal that organic matter is collecting somewhere in your plumbing.

Getting rid of them means two things: killing the ones you see now, and stopping new ones from hatching by cleaning out the drain where they breed. If you only spray the flies, they will return in days because the larvae are still in the pipe. If you only clean the drain once, you may need to repeat it if the buildup returns.

Key Takeaways

  • Sewer flies breed in the sludge inside drains, so killing adult flies without cleaning the drain will not stop them from coming back.
  • A drain brush or plumbing snake can physically remove the buildup where larvae live, and this is often faster than chemical drain cleaners alone.
  • Boiling water poured down the drain kills some larvae and washes away loose debris, but works best after you have already broken up the clog.
  • Enzyme drain cleaners dissolve organic buildup over hours or days and are safer than caustic cleaners, though they work slower.
  • If flies keep returning after you clean the drain, the problem may be in a trap under the sink, a vent pipe, or a floor drain you have not yet found.

Remove the drain buildup where larvae are living

The sludge inside your drain is where sewer fly eggs hatch and larvae feed. You need to break this up and wash it out. Start with a drain brush — a long flexible brush with bristles that scrub the inside of the pipe as you push and pull it. Push it down as far as it will go, then pull it back up with force. Repeat five or six times. This breaks apart the slime layer where flies breed.

If a brush does not reach far enough or the clog is stubborn, use a plumbing snake (also called a drain auger). Feed it down the drain slowly, then turn the handle to spin the tip and break up the blockage. Pull it back out and repeat. You may see dark, foul-smelling material come up — that is the buildup you are after.

After you have scraped the drain with a brush or snake, pour a kettle of boiling water down it to flush away the loosened debris. This also kills some of the larvae. Let the drain sit for 30 minutes, then run hot tap water for a minute.

Use enzyme drain cleaners for ongoing buildup

Enzyme cleaners contain bacteria or enzymes that eat away at organic matter — hair, soap scum, food particles — that accumulates in drains. They work slowly (usually 4 to 12 hours) but are much safer than caustic drain cleaners like lye, which can burn skin and damage old pipes.

Pour the enzyme cleaner down the drain according to the product instructions, usually followed by hot water. Let it sit overnight if possible. Enzyme cleaners work best after you have already removed the worst of the blockage with a brush or snake, because they need time to reach the remaining buildup.

Use an enzyme cleaner once a week for two or three weeks to break down layers of sludge. After that, use it monthly to prevent new buildup from forming. This reduces the food source for sewer fly larvae.

Check the P-trap under the sink

Under your bathroom or kitchen sink is a U-shaped pipe called a P-trap. It holds standing water that blocks sewer gases from coming up through the drain. Sewer flies often breed in the sludge that collects inside this trap, especially if the sink drains slowly.

Place a bucket under the trap, then unscrew the large nut on each end by hand or with a wrench. The trap will drain into the bucket. Use a bottle brush or old toothbrush to scrub the inside of the trap, then rinse it thoroughly with hot water. Screw it back on hand-tight, then tighten with a wrench until snug — do not over-tighten or you may crack it.

If water leaks from the connection after you reassemble it, tighten the nut a quarter turn more. If it still leaks, the rubber washer inside may be worn and needs replacement (a hardware store sells these for a few dollars).

Look for floor drains and unused drains

Sewer flies can breed in any drain that is not used regularly, including floor drains in basements, utility rooms, or garages. If you have not used a drain in weeks, the water inside it may have evaporated, leaving the pipe dry and full of sludge.

Pour a kettle of boiling water down any floor drain you find, then follow it with an enzyme cleaner. If the drain is completely dry and you suspect it is the source, pour a cup of mineral oil down it — this creates a seal that prevents flies from coming up while you work on the main drains.

If you have a basement or crawlspace, look for any drain opening you may have forgotten about. Sewer flies can travel through connected pipes, so treating only the kitchen sink will not help if they are breeding in a floor drain in another part of the house.

Seal or treat drains you do not use

If you have a drain you rarely or never use — a second bathroom sink, a shower in a guest room, a floor drain in a storage area — it can become a breeding ground for sewer flies. The water evaporates, the pipe dries out, and organic matter builds up.

Pour boiling water and enzyme cleaner down the drain once a week for a month. After that, pour a cup of mineral oil down it monthly. The oil floats on top of any remaining water and prevents flies from laying eggs. Alternatively, you can plug the drain with a rubber stopper or cap if you do not need it at all.

Use drain covers and improve ventilation

A straightforward mesh drain cover (a few dollars at a hardware store) catches hair and debris before it goes down the drain, reducing the buildup that attracts sewer flies. Install one in every sink and shower drain in your house.

Sewer flies also need moisture and darkness. If a drain is in a warm, humid area with poor air circulation, flies will thrive there. Run a bathroom exhaust fan during and for 20 minutes after showers to reduce humidity. In a basement, a dehumidifier or a fan can help dry out the air and make the environment less attractive to flies.

Frequently Asked Questions

How long does it take to get rid of sewer flies after I clean the drain?

Adult flies die within a few days once you remove the breeding material, but new flies may hatch from eggs or pupae still in the drain for another week or two. Keep the drain clean and use enzyme cleaner weekly for three weeks to break the cycle completely. If you see flies again after that, the problem is likely in a different drain you have not yet treated.

Can I use bleach or caustic drain cleaner to kill sewer flies?

Bleach and lye-based drain cleaners kill some larvae, but they do not remove the sludge where flies breed, so new flies will hatch. They are also harsh on old pipes and dangerous to handle. A drain brush or enzyme cleaner is more effective and safer. If you do use a chemical cleaner, always follow the label instructions and never mix it with other products.

Why do sewer flies keep coming back after I clean the drain?

You may have missed a drain — check floor drains, unused sinks, and the P-trap under every sink in the house. Sewer flies can also enter through a vent pipe on your roof or through cracks in the foundation. If you have treated all visible drains and flies still return, the problem may be in the main sewer line or a vent pipe, and you should contact a plumber.

Do I need to call a plumber to get rid of sewer flies?

Most sewer fly problems can be solved with a drain brush, boiling water, and enzyme cleaner — no plumber needed. Call a plumber if flies return after you have cleaned all drains thoroughly, if you have a slow or backed-up drain that will not clear, or if you suspect the problem is in the main sewer line or vent pipe.