Why do I have drain flies?

"Where do drain flies come from, and what could be causing a drain fly infestation?"

Drain flies, also called sewer flies or moth flies, breed inside your pipes, not outside them. They lay their eggs in the thin layer of organic matter that builds up inside slow or stagnant drains: decomposing hair, grease, soap scum, and food residue. Even a drain that looks clean on the surface can harbor enough buildup to support a breeding population.

Seeing sewer flies in the house usually means they’ve emerged from inside the drain itself, which is why you'll typically spot them resting on nearby walls and ceilings rather than flying in through a window. Bathroom sinks, shower drains, floor drains, and kitchen pipes are the most common sources.

A drain fly infestation tends to build up gradually and go unnoticed until numbers get high enough to be obvious. Warm, humid conditions speed up their breeding cycle, which is why you might see a spike in summer or in bathrooms with poor ventilation.

Cleaning the drain can help reduce numbers, but if the organic buildup is deep in the pipe, surface cleaning often won't reach it. For a persistent problem, Ehrlich's fly control services can identify the source and recommend the right treatment, including drain line maintenance to clear the source of the infestation.