In Praise of the Drain Fly

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I encountered some drain flies when I changed toilet, but I did not know it. I thought it odd that I had fruit flies in my bathroom, and I did not relate the winged insects to having pulled the toilet. They ignored my attempts to lure them with things that fruit flies are drawn to, including vinegar.

I later saw a posting ( that discussed drain flies and read a bit. I have never seen these in the house except for when that closet flange was open. They somehow stay on their own side of the water seals in the traps in my experience. If they did not, such as in the problem described in that thread, I would think differently about them. They seem to have found their niche. I suspect they find their way into plumbing vents and lay eggs in the moist pipes. The larvae eat the sewer waste, and change into more flying drain flies.

I have been surprised that 1/4 inch per foot is enough to move solid waste through a pipe-- especially cast iron. Think of solids in your driveway with a similar slope. I now suspect that the drain fly larvae are very useful to keep things working. I think this is a symbiotic relationship. I wonder if it would be useful to start a drain fly colony where there is new construction too far from other plumbing vents for them to find their way on their own, or for Studor single-pipe systems where there is no open roof vent. says "In small numbers, drain fly larvae are considered beneficial because they break down materials that cause drain clogs. Drain fly larvae have extremely strong jaws and are capable of cutting through layers of dense slime and build-up."
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wow even the drain system has some helpful creatures just like how our intestines have bacteria
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