Drain flies find slimy pipes ideal neighborhood

Drain flies (aka moth flies or sewer flies) are a common but relatively harmless pest. Even new homes can have them if the house is hooked up to old pipes.

Adult flies (about the size of the tip of a pencil eraser) usually show up in bathrooms and kitchens emerging from drains. Drain flies make masses of from 30 to 100 eggs, which hatch within two hours of being laid. The larvae live in the thin film of slime that lines your pipes. They feed on the sediment that collects there.

The maggots grow for nine to 15 days, pupate for a day and a half and emerge as adults and start the cycle all over again. You've probably never seen any of the young maggots. The adults, however, are most often observed on the walls of basements, bathrooms, kitchens or other sites where sewer drains and plumbing fixtures are found. They are quite poor fliers and most often hang around drains or other areas where they can have good access. The young live in drain traps, pipes, garbage disposals, toilet tanks, or in wet areas around septic tanks, leaking pipes or sewer lines -- even in wet mops.

Fortunately, despite their preferred choice of residence, the moth fly doesn't pose any particular risk for disease transmission. They don't bite. They're just a general nuisance. However, they may be an indicator of septic or sewer problems that needs to be corrected.

To check for possible breeding sites, you'll need to make an X across each drain with masking tape. You can also use an inverted clear plastic cup sprayed with cooking oil set above the drain. Make certain that you don't completely cover the opening. If you do, there will be no air flow and the flies won't try to emerge. Check the traps periodically. If you find adult flies stuck to the tape or inside of the cup, it's probably the source of your infestation or, at least, one of the sources.

The only way to rid yourself of drain flies is to completely eliminate their food, and that means thoroughly cleaning the drain lines and plumbing fixtures where they might live and breed. Pouring bleach or boiling water down a drain may help to knock down the numbers of immatures. An enzyme or bacterial based drain cleaner can be used to eliminate the gelatinous layer.

The adults can be killed by almost any household insecticide or a good flyswatter (they tend to "puff" when swatted). However, to significantly reduce their numbers for any significant period of time will require your hiring a professional plumber to clean out your pipes.

Tim Lockley, a specialist in entomology, is retired from a 30-year career as a research scientist for the U.S. Department of Agriculture. For answers to individual questions, please send a stamped, self-addressed envelope to Tim Lockley, c/o Sun Herald, P.O. Box 4567, Biloxi, MS 39535.

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Drain flies find slimy pipes ideal neighborhood