By Chris Williams on December 13, 2017.

We keep seeing little gnats in our downstairs bathroom. Sometimes there are several of them at once, other times just one or two, but it’s been going on for months. We don’t know where they’re coming from. We tried using an aerosol bomb in case they were coming from the drains but that didn’t stop them either. K. S., Raymond, NH

First, you need to collect a few of your flies and have them identified. At this time of year, they probably are coming from your drains rather than getting in from outside. Unfortunately, an aerosol bomb isn’t going to reach or affect fly larvae that are living down inside the drains of your sinks, tub, or shower.

There are actually three different types of small flies that can infest drains. Most common is the drain fly, also called a moth fly because it resembles a miniature moth (see Bathroom Flies That Look Like Little Moths?). Sometimes phorid flies and even darkeyed fruit flies can breed in drains. In all cases, the larvae are feeding on the bacterial film that grows on the scum, or goo, or whatever you want to call it that collects down inside drains. After the larvae pupate, adult flies emerge from the drains and are often seen resting on bathroom walls.


You may think it should be easy to get rid of these larvae then by just pouring drain cleaner down the drains. Sorry, but that doesn’t work. They’re tough, even hot water or bleach won’t reliably kill them. In the pest control business, we depend on specialty drain cleaning methods that treat drains with a bioenzyme gel or foam. It works by eliminating the bacteria and the film that the larvae feed on, which in turn eliminates the larvae (see What’s the Best Way to Kill Drain Flies?).

The reason that fly identification is important is that sometimes these tiny bathroom flies, especially if they are phorid flies, can be associated with a sewer line crack or break. In that case, it will take much more than drain treatment to solve the problem.


Give us a call at Colonial Pest. One of our friendly expert technicians can visit your home, identify your flies, inspect the bathroom and other areas, and discuss the best approach for getting rid of them for good…guaranteed!

For more on bathroom flies, see:

Gnats Indoors in Winter?
Why Are There Always Bugs in my Bathroom?
How Can I Get Rid of Drain Flies?

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