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Attack of the Drain Flies

House Doctor | Gary Dymski

January 9, 2003

Q. Since the day we moved into our house two years ago, we have had problems with drain flies - identified as such by our exterminator - in the master bathroom.

We have tried a few things to get rid of this nuisance, including pouring bleach down the shower drain and using chemicals sold over the Internet specifically to fight this problem.

These drain flies are generally confined to our stall shower, but sometimes a few can be found in other parts of the bathroom.

Other than the remedies that we have tried, neither our exterminator nor plumber knows how to treat this condition.

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A. Drain flies, also called moth flies and sewer gnats, are indeed a nuisance, especially in your case because they are originating inside your home. These tiny flies, from about 1/5 to 1/6 of an inch long, accumulate around showers, bathtubs, sinks and floor drains.

They also reproduce outdoors, in shallow standing water, compost bins, overflow pipes and garbage containers.

The flies lay eggs in the soft, moist organic matter that collects in and lines your drains. After hatching, the larvae feed on this matter and mature in 10 to 15 days.

I'm surprised that your exterminator did not know how to rid your drain of these pests. Several gels are made that can kill the flies.

These gels are sprayed around the drain area and are sold only to professionally licensed exterminators. You might want to consult another exterminator.

However, you can try to solve the problem yourself. The first step is to clean buildup from the lining and the surrounding areas of your drains. Use a long, wire drain brush - similar to a bottle- washing brush - to scrub inside the drain.

A bleach solution and nylon scrub brush can help eliminate the buildup around the edge of the drain.

One reason for shower and tub drain clogs is the hair that collects in traps (the curved portion of the drain that holds water). Try bending a thin wire hanger to pull out some of this hair. Put a hook on one end and pull out as much of the debris as possible.

You also can rent a snake, a device that winds through drain clogs using old-fashioned arm strength.

Once you've cleaned the drain of hair and buildup, try using a plunger to push any remaining debris through the trap and down the drain.

Although I don't use caustic drain cleaners because I don't want to upset the enzymes in my septic tank, you might consider using such a product during the initial cleanup. Follow directions, and do not use the cleaner in conjunction with other cleaners, like bleach (or even the household remedy I describe below).

Instead of caustic cleaners, try keeping the lining of your drains clean a couple of different ways. First, I use an organic drain cleaner about three times a year to keep my sink drains and tub drains flowing smoothly. Try Bio-Clean (available online at www.safedraincleaner.com), which uses enzymes to break down the organic matter that lines and clogs drains.

Also, use a drain freshener of salt, baking soda and white vinegar weekly to keep scum from building up in drains. Pour a half- cup of salt, then a half-cup of baking soda followed by a cup of vinegar into the drain. Let the drain foam for as long as possible, even overnight, then flush with boiling water.

There's a chance that the drain flies are coming from another source, too. Moldy, mildewy areas underneath loose floor tiles or behind rotting drywall can be a breeding ground for flies. Examine the areas around your tub or shower floor. Clean and reset all loose tile and rotting wallboard.

For temporary control of flying drain flies, use aerosol insecticides designed for small flying insects. These generally contain pyrethrins or resmethrin and will kill adult flies. The sprays, however, won't kill larvae and should not be sprayed into drains.