By Chris Williams on March 9, 2017.

We have a chronic problem with tiny flies in a bathroom. They’re dark and they look sort of like miniature moths. We seem to always see two or three almost every time we use the downstairs bathroom. I thought they were coming in from outside but we don’t see them in any other rooms. Do you know what they are? O. G., Dracut, MA

I couldn’t tell you what they are without actually seeing a specimen. Two of the most likely flies are the phorid fly (humpbacked fly) or the drain fly (moth fly). Based on your description of the fly, it seems that drain fly is the best guess. Both of these tiny flies can be associated with drains, either floor drains, or sink, tub, or shower drains (see Nuisance Gnats Indoors in the Fall?).


The fly’s eggs are laid inside drains that have a film of gelatinous goo on the inside of the pipe. Worm-like larvae hatch out and feed on microscopic bacteria, algae, and other sludge in the drain before turning into adult flies. If you don’t use this downstairs bathroom as often as upstairs bathrooms, the drains may not get the same attention, making them more suitable for drain flies. A chronic, low-level infestation can continue for a long time, and if it can’t be resolved with professional treatment, it can sometimes mean that a broken pipe is the source.

Drain flies are just as often called moth flies because of their shape and the fact that they have hairy wings and a fuzzy body like moths. Because they are very tiny, 1/8-inch, it’s best to use a magnifying lens to see these features. They also are attracted to lights but are weak fliers and are more often found sitting on walls nearby. The larvae won’t be seen unless you are pulling gobs of gunk out of the drain!


Don’t think that you can kill the larvae inside the drains (you’ll have to determine which drains are breeding the flies) by using caustic drain cleaners, bleach, or hot water. They’re very resilient. The best way to get rid of drain flies is to have a pest control professional do a thorough cleaning of the drains with a long-handled drain brush, then treat the drains with a biocide product (usually a foaming bacterial or enzyme material). These products do not kill drain fly larvae outright, but instead break down and digest the goo that they are feeding on, basically starving them. Call Colonial Pest, we can do this job for you.

Because the drain flies can move on to infest other drains, you will probably want to have all of the drains in your house cleaned and treated. The flies can also develop in other indoor sites with semiliquid organic material such as dirty garbage cans, mop buckets, drip pans, and sump pump pits. In unusual situations, drain flies can be entering homes from nearby sewage or wastewater treatment facilities where the flies are actually beneficial in breaking down sewage. They can be carried by the wind from these sites and can enter through screens.

See also How Can I Get Rid of Drain Flies?

Photo Credit : Marvin Smith | CC BY-SA 2.0



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