How to Get Rid of Drain Flies

To get rid of drain flies, clean the drain where they are breeding rather than chasing the adults. Drain flies lay eggs in the slimy layer of organic matter, called biofilm, that coats the inside of slow or seldom-used pipes. Scrub that film out, flush the pipe, and set a simple trap for the stragglers, and the infestation collapses within a couple of weeks.

What are drain flies and where do they come from?

Drain flies (also called moth flies or sewer gnats) are tiny, fuzzy, moth-like insects about the size of a sesame seed. They are weak fliers and tend to sit on walls near sinks, tubs, and floor drains, hopping short distances when disturbed. They are not the same as fruit flies, which hover around ripe produce.

They breed in standing water mixed with organic gunk: kitchen and bathroom drains, overflow holes, basement floor drains, rarely-used guest-bathroom traps, and even the saucers under houseplants. The adults you see are a symptom; the real population lives as larvae inside the pipe.

How do I find the breeding source?

Finding the right drain is the step most people skip. To pinpoint it, dry the area around each suspect drain, then tape a piece of clear plastic wrap loosely over the opening overnight (leave a small gap so flies can emerge but get stuck). Check in the morning:

• Flies stuck to the plastic mean that drain is a breeding site.
• No flies means that drain is probably clear; move on to the next one.

Test every drain in the home, including basement and laundry drains, because more than one can be active at once.

How do I clean the drain to remove the breeding film?

The goal is to physically remove the biofilm, not just pour something down the pipe. Work through these steps:

1. Remove the drain cover or stopper and clear any visible hair and debris.
2. Run a stiff drain brush or pipe brush as far into the pipe as it reaches, scrubbing the walls. This is the single most effective action.
3. Carefully pour a kettle of boiling or very hot water down the drain to loosen remaining slime and kill eggs and larvae clinging to the walls. (Skip boiling water on fragile or PVC fixtures the manufacturer warns against; use hot tap water instead.)
4. Optionally pour in about a cup of baking soda followed by a cup of white vinegar, let it fizz for 30 minutes, then flush with hot water to help break down buildup.
5. For stubborn cases, use an enzyme or bacterial drain cleaner designed to digest organic matter, and follow the product label.

Repeat the scrub-and-flush for several days in a row. One cleaning rarely gets every egg.

How do I trap the adult flies that are left?

While the pipe clears, catch the adults with a cheap homemade trap. Fill a small bowl or jar with apple cider vinegar (or leave a splash of old wine or beer), add a drop or two of dish soap, and set it near the active drain. The scent lures the flies and the soap breaks the surface tension so they sink. A shallow dish covered with plastic wrap poked with a few holes works the same way. Replace it every few days until you stop catching flies.

Which method should I use?

How do I keep drain flies from coming back?

Prevention is mostly about denying them moist organic matter:

• Run water through seldom-used drains (guest bath, floor drains) once a week so traps don’t dry out or stagnate.
• Clean drains and strainers regularly to stop grease, hair, and food scraps from building up.
• Fix slow drains, leaks, and damp spots that give larvae a foothold.
• Empty and rinse houseplant saucers and clean out any standing water nearby.
• Wipe down garbage disposals and run them with cold water and a little dish soap.

How long until the drain flies are gone?

Expect a noticeable drop within a few days of removing the breeding film, and a full clear-out within one to three weeks. Here is why the timeline stretches out: adult drain flies live only about two weeks, but eggs and larvae already in the pipe keep hatching until the biofilm they feed on is gone. That is why a single cleaning rarely finishes the job and why daily scrubbing and flushing for the first week pays off. If you are still seeing new adults after three weeks of diligent cleaning, you have almost certainly missed a breeding site, so repeat the plastic-wrap test on every drain, including ones you assumed were clean. Keeping a vinegar trap running the whole time gives you a simple gauge: when the trap stops catching flies for several days, the infestation is over.

When should I call a professional?

If flies keep returning after you have thoroughly cleaned every drain for a couple of weeks, the breeding source may be somewhere you can’t reach, such as a cracked pipe, a dry or broken sewer vent, a hidden leak under a slab, or a problem in the main line. At that point a licensed plumber can inspect and clear the line, and a licensed pest-control professional can confirm the source and treat it. Follow the label on any pesticide product you use, and call a pro promptly if you also notice sewage odors or signs of a leak.

Frequently asked questions

What causes drain flies in the house?

Drain flies come from the moist organic buildup (biofilm) inside drains, overflow pipes, and traps. Standing water in a seldom-used sink, floor drain, or even a houseplant saucer gives their larvae a place to feed and grow.

How long does it take to get rid of drain flies?

Adult drain flies live only about two weeks, so once you remove the breeding film and keep the drain clean, an infestation usually clears within one to three weeks. Repeated cleaning speeds it up.

Does bleach get rid of drain flies?

Bleach can kill some larvae on contact but often slides past the sticky biofilm without removing it, so flies return. Physically scrubbing the pipe and using an enzyme drain cleaner works better.

Will drain flies go away on their own?

Only if the breeding source dries up or is cleaned out. As long as there is moist organic matter in a drain, new generations keep hatching, so they rarely disappear without action.

How do I find which drain the flies are breeding in?

Dry the area, then tape a piece of clear plastic wrap loosely over each suspect drain overnight, sticky side down. The drain with flies stuck to the plastic by morning is the breeding site.

Are drain flies harmful?

Drain flies do not bite or sting and are not known to spread disease to people, but large numbers are a nuisance and signal a sanitation or moisture problem worth fixing.