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What’s the Best Way to Kill Drain Flies?

By Chris Williams on April 12, 2016.

We have an ongoing problem with drain flies in our downstairs bathroom. I’ve tried all kinds of drain cleaners and pouring bleach and boiling water down the drain. The flies disappear for a while but a couple of weeks later, they’re back. Do you have any suggestions? E. G., Boscawen, NH

Drain flies (1/8-inch long) have fuzzy wings and are also called moth flies

Drain flies (1/8-inch long) have fuzzy wings and are also called moth flies

You might want to cut your losses and contact an exterminator for professional help. Drain flies are difficult to get rid of and drain cleaners usually have no effect. Since you say the flies are in a bathroom, we’ll assume that they are coming from the most likely source – a sink or tub/shower drain (see Scummy Drains Can Mean Drain Fly Problems). Drain flies can come from a variety of other sites, however, so if you’re having trouble controlling them, it might be worthwhile to check some other possible sites outside of the bathroom.

Drain Flies Develop in Wet, Slimy Gunk

Drain fly larvae are semiaquatic and feed and develop in damp, dark, poorly ventilated polluted areas with decaying vegetation, sewage, or other organic matter like algae. In homes, the gooey “muck” that collects down inside drains serves as a good food source for the larvae.

You can also find drain flies developing in sites such as: sewer leaks or backups, dirty garbage cans, drip or evaporation pans under refrigerators, water heaters, air conditioners, etc., or scummy potted plant saucers. Outdoors, they will breed in clogged roof gutters, wet compost or mulch, rain barrels, damaged septic lines, and in other wet sites with decaying organic matter.

Biocide Drain Cleaners Eliminate Food for the Larvae

The only way to get rid of drain flies that are developing in drains is to thoroughly remove the gunk from the drains. Regular drain cleaners don’t work. The cleaning can be done with a stiff brush, pressure washer, or steam cleaner, but more often drains are cleaned with professional biocide cleaners. These bacterial or enzyme gels or foams cling to the sides of the drain and slowly eat away at the bacteria. Drain fly larva are killed indirectly as their food source is removed. No more larvae developing = no more adult drain flies flitting around your bathroom.

Actually, new flies may still emerge from the drains for a short time after treatment but they will be unable to successfully lay eggs in the now clean drains and the cycle will end. Bacterial or enzyme drain cleaners are an effective, non-pesticide way to get rid of drain flies. Biocide cleaners generally must sit in the drain for several hours or overnight and should be repeated on a regular basis to keep drains clean. Give us a call at Colonial Pest and let us eliminate your drain fly problem for good!

Photo Credit : Sanjay Acharya| CC BY-SA 3.0 via Wikimedia Commons.

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