What sewer flies are and where they come from
Sewer flies are small dark insects, usually about one-eighth of an inch long, that breed in the organic buildup inside your drain pipes. They are not fruit flies or gnats — they are drain flies, also called moth flies, and they live in the sludge and decomposing matter that collects where water sits or moves slowly through your plumbing.
They enter your bathroom through the drain itself. The flies lay eggs in the film that coats the inside of pipes, especially in P-traps (the U-shaped section under your sink), shower drains, and toilet overflow tubes. Once the eggs hatch, the larvae feed on that organic buildup for about two weeks before becoming adult flies that crawl or fly up into your bathroom.
The reason you see them now is that something in your plumbing has changed — a slow drain, a leak that sits, a toilet that runs constantly, or a vent pipe that is blocked. The flies themselves are a symptom of a moisture and food source problem, not the root cause.
Key Takeaways
- Sewer flies breed in the organic film inside drain pipes, so killing the adults does nothing unless you also clean the pipes where the eggs are.
- The fastest way to stop them is to pour boiling water mixed with a drain cleaner down the affected drain, which kills larvae and breaks up the buildup they feed on.
- If boiling water does not work, a drain brush or plumbing snake can physically scrub the inside of the pipe to remove the film where eggs are laid.
- Once the drain is clean, find what is keeping it wet or slow — a leak, a clogged vent, or standing water — and fix that to prevent them from returning.
- Covering the drain with a plug or screen at night can trap emerging flies, but this is a temporary measure while you clean the pipe.
Boiling water and drain cleaner as the first step
Pour one to two gallons of boiling water directly down the drain where you see the flies. Do this at night when the flies are less active and more likely to be in the drain. The heat kills the larvae and softens the buildup they live in.
If boiling water alone does not stop them within a few days, follow it with a commercial drain cleaner. Pour the cleaner down the drain according to the product instructions — most require you to let it sit for 30 minutes to several hours before flushing with hot water. The chemical breaks down the organic film and kills remaining eggs. Repeat this process every three to five days until you stop seeing flies.
Drain cleaners are caustic and can burn skin, so wear gloves and do not mix products. If you have old metal pipes, check the cleaner label first — some products corrode older plumbing.
Using a drain brush or snake to physically clean the pipe
If boiling water and drain cleaner do not work after two weeks, the buildup is too thick to dissolve. A drain brush or plumbing snake can scrub the inside of the pipe and dislodge the film where the flies are breeding.
A drain brush is a long flexible rod with bristles that you push down the drain and twist to scrub the pipe walls. You can buy one at any hardware store for under $15. Push it down as far as it will go, twist it back and forth several times, then pull it out. Repeat this two or three times, then flush the drain with hot water to wash out the loosened debris.
A plumbing snake works the same way but has a coiled metal head instead of bristles. Both tools are effective, but a brush is easier to use for a homeowner. If the drain is severely clogged or the pipe bends sharply, you may need to call a plumber with a motorized snake.
Finding and fixing the source of moisture
Once the drain is clean, the flies will return unless you address why the drain became a breeding ground in the first place. The most common causes are a slow or clogged drain, a leak under the sink, a toilet that runs constantly, or a blocked vent pipe.
Check under the sink for standing water or dampness. If you find a leak, tighten the connection or replace the P-trap seal. Run water down the drain — it should flow quickly. If it drains slowly, use a plunger or snake to clear the clog. For a toilet that runs, the fill valve or flapper may need adjustment or replacement; most hardware stores sell repair kits for under $20.
A blocked vent pipe is less obvious. Vent pipes run from your drains up through the roof to let air in and sewer gas out. If a vent is blocked by debris or a bird's nest, water drains slowly and sits in the pipe, creating the perfect breeding ground. You may need a plumber to clear a vent, but you can also try running a hose up from the roof opening if you can safely access it.
Trapping emerging flies while you work
While you are cleaning the drain, you can trap the adult flies that emerge at night. Cover the drain opening with a piece of plastic wrap or a drain plug, leaving a small gap. The flies will crawl up and get trapped under the plastic. In the morning, remove the plastic and flush the flies down the toilet or dispose of them outside.
This method does not solve the problem — it only removes the adults you can see — but it reduces the number of flies in your bathroom while the drain treatment works. Do this every night for a week or two while you are treating the drain.
Do not rely on this method alone. The eggs in the drain will continue to hatch, and you will see new flies emerge within days if you do not clean the pipe itself.
When to call a plumber
If you have tried boiling water, drain cleaner, and a drain brush for two weeks and still see flies, the problem is likely deeper in the plumbing system or in a part you cannot reach. A plumber can use a motorized snake to clean the main drain line, inspect the pipes with a camera to find the source of the buildup, or clear a blocked vent pipe.
You should also call a plumber if you find a leak under the sink that you cannot fix yourself, if the toilet runs constantly and a repair kit does not stop it, or if multiple drains in your home are slow. These are signs of a larger plumbing issue that will keep bringing the flies back.
A plumber visit typically costs $150 to $300 for a service call and drain cleaning. If the problem is a vent pipe or a main line issue, the cost may be higher, but it will solve the problem permanently.
Preventing sewer flies from returning
Once the flies are gone, keep them from coming back by maintaining your drains. Pour boiling water down each drain once a week to keep the pipes clean and break up any buildup before it accumulates. This takes five minutes and costs nothing.
Fix leaks and clogs as soon as you notice them. Do not let water sit in a sink, shower, or toilet for days. If a drain is slow, use a plunger or snake right away instead of waiting for it to clog completely.
Check that your toilet does not run constantly. If it does, replace the fill valve or flapper — both are cheap and straightforward to replace. Make sure your vent pipes are clear by listening for a gurgling sound when water drains; if you hear it, the vent may be blocked and needs attention.
Frequently Asked Questions
Are sewer flies dangerous or do they carry disease?
Sewer flies do not bite, sting, or carry disease. They are a nuisance because they are unpleasant to see and indicate a plumbing problem, not because they pose a health risk. However, the conditions that allow them to breed — standing water, organic buildup, slow drains — can create other problems, so it is worth fixing the underlying issue.
Can I use bleach to kill sewer flies?
Bleach poured down a drain will not kill the flies or their larvae effectively. The bleach mixes with water and does not stay in contact with the film where the eggs are. Boiling water and drain cleaner are much more effective because they either dissolve the buildup or kill the larvae directly.
How long does it take to get rid of sewer flies?
If you clean the drain with boiling water and drain cleaner, you should see fewer flies within three to five days. The larvae take about two weeks to mature, so you may see new flies emerge for up to two weeks after you start treatment. If you also clean the pipe with a brush or snake, the process is faster — usually three to seven days.
Why do I see sewer flies only at night?
Sewer flies are attracted to moisture and are most active in the dark. They spend the day in the drain and emerge at night to feed and lay eggs. This is why pouring boiling water or drain cleaner at night is most effective — the flies are in the drain and exposed to the treatment.
Can I use a drain cover or screen to stop sewer flies?
A drain cover or screen will trap some flies, but it will not stop them from breeding in the pipe. The eggs will continue to hatch, and new flies will emerge. A cover is a temporary measure while you clean the drain, not a permanent solution.