By Chris Williams on March 5, 2019.

At the end of last summer we were having a problem with little flies, mostly in our downstairs bathroom. We determined that they were drain flies and thoroughly cleaned the drains in that room and finally got rid of them. But now they’re back! Do we just keep scrubbing the drains forever?

B. B., Revere, MA

Right off the bat, I have a few questions: (1) Who “determined” that they were drain flies? Was a pest management professional involved in the identification? (2) How did you clean the drain? Soap and hot water, Drano, or an enzyme drain cleaner? Did you use a scrub brush? (3) Did you check for any other fly breeding sites besides the bathroom drains? The answers to these questions are important because a wrong turn at any point could explain whey you are still having problems.


There are at least three different species of small flies that can infest slimy, scummy drains; all have similar habits. Drain flies, also called moth flies, are probably the most common (see Bathroom Flies That Look Like Little Moths?) But tiny phorid flies, and dark-eyed fruit flies, will also infest drains as well as many other fermenting or decaying materials. It’s the larvae or maggots of these flies that live inside the drains, feeding on the gelatinous goo that forms on the inside of the pipes. Adult flies emerge from the breeding sites but may remain nearby.

Drain fly control often fails because people don’t realize just how resistant the scum and the larvae are to boiling water, bleach, detergent, and even caustic drain cleaners (see How Can I Get Rid of Drain Flies?). The larvae usually survive the assault. Using a long-handled scrub brush to get the gunk out of the drain helps. But absolutely the best control for drain flies is a pest control professional with a biocide drain cleaning system that uses enzymes or bacteria to digest and eliminate the organic film in the drain. When the drains are squeaky clean and there is nothing left to feed on, the larvae and the flies will die out.


Dirty drain and tiles. Shutterstock.

Besides inadequate control measures, another reason why you have drain flies present again could be because there was a secondary infestation in another site that was not discovered or treated. There may be other bathroom drains, or a floor drain, laundry room drain, or sump pump that are breeding flies.

Besides drains, all of the tiny “wet gunk gnats” can breed in many different sites containing overripe, decaying, or rotting semi-liquid organic materials. Examples are drip pans under appliances, dirty garbage cans or recycling bins, scummy potted plant saucers, or stagnant water in a mop bucket. In unusual situations, and especially if phorid flies are involved, the fly outbreak may not be from the drain itself, but from a sewer line break or crack that will require the services of a plumber.

Dirty floor drain.Shutterstock.

Save yourself a lot of time and frustration by calling a pest control professional. With Colonial Pest, your drain flies will be gone for good with little inconvenience on your part, and usually without the use of insecticides. Even better, our work is guaranteed so if the drain flies come back…so do we!




We’re not satisfied until you are. Learn More