Drain Flies

Drain flies have a surprising life cycle

When I moved to the country I started noticing little furry triangular flies. I sometimes saw them on the walls of the shower stall. They don't seem to fly much and don't bite.

Wikipedia has a page about drain flies"

tiny drain flydetail tiny drain fly

Drain flies are about an eight of an inch across and have a very characteristic triangular shape. They look very fuzzy and if you look closely have striped wings.

They are not very active and usually don't fly away if you come up close.

Drain flies have many names: sink flies, filter flies, sewer flies, sewer gnats, sewer moth.

These are true flies of the Psychodidae family. Look at the Wikipedia article for a long list of different varieties.

Only people who try to sell you something would call them pests. At the most they are a nuisance. They are an indication that somewhere in your home there is a spot with still water and organic matter. If you get rid of this you get rid of the drain flies.

Where do they come from?

Sewer flies, or drain flies have figured out how to live in the sewer lines and drain of houses. Because they are hard to wet, they are very resistant to soaps and other cleaning products that find their way in drains. In my house I avoid harsh cleaners and they live happily in the shower drain particularly in winter. I don't seem to see them anywhere else, maybe because there is less water flow there. The kitchen drains also get very hot water drained out so maybe the flies can't live there.

This summer, when I had the septic system pumped out, I noticed small worms living in the tanks and I took a sample. That's when I discovered the secret life of the drain fly.

The water was teeming with the little worms and with larger ones. I found out that the smaller worms were just a less mature version of the larger grubs. Sewer flies larva spend all their time in the water. When it becomes time to pupate they are much larger and stop feeding, they don't move much. They stay in the water till they come out as little flies. It takes about 10 days to 2 weeks for the life cycle to complete.

breathing connection

In order to breathe they extend their rear up to the surface of the water. It makes a sort of star pattern on the surface of the water.

If you look at the dark triangular spot in the middle of the picture, you can see a silver coloured spot where a worm has his breathing hole set up.

In fact there are dozens of worms in the picture, but all you can see is their breathing holes. The yellowish stuff is fibrous waste. This mostly digested material.


This photo shows a larger pupa just sort of floating and occasionally putting up a breathing tube. It is just a bit above the center of photo. There is also an immature larva in the lower right. They are very hard to photograph since they are quite small and tend to present their bottoms to the camera.

The photos are difficult but the video shows the larva and pupa quite clearly. If it is not showing up the youtube address is: https://youtu.be/ugyAJ8ICQBg

Are sewer flies dangerous or harmful?

No, for the most part they keep away from food, don't bite, don't usually carry diseases in temperate areas. Unless you have a large population there is no problems associated with a few drain flies walking around. They are not great fliers.

Since the larva develop in still water, eating the slimy organic film that often develops there, making sure there is no slimy still water is a good way of controlling them. This also helps with mosquito control so controlling mosquito spots helps control drain flies.

If you have a lot, then cleaning the drains and covering them when not in use will disrupt the life cycle. Pouring down hot water can also reduce the numbers in the drain. In fact pouring a pail of water in seldom used drains and toilets every once in a while flushes the larva away and goes a long way in reducing adult populations.

They might actually be useful in your septic system since they help break down waste.

They are tough little grubs and hard to eliminate completely. You're better off reducing the access to damp areas that have lots of organic matter.

Since they are quite harmless, pouring insecticides and other nasty chemicals around your home might do YOU more harm than good. You certainly do NOT want to kill whoever is living in your septic system.

Here is a page on drain flies from the Kentucky Entomology department.

Do it yourself pest control has a page on how to get rid of drain flies if you feel you have to. From a health point of view though, you are better off going after mosquitoes, fleas and ticks, I think.

email me if you find mistakes, I'll fix them and we'll all benefit: Christine

Small Print

This information is for general knowledge and entertainment. I don't claim to be an expert in anything. This page is the result of my curiosity. I just checked out some grubs in the septic tank when it got pumped out. I linked the grubs to a tiny fuzzy fly that occasionally shows up in the bathroom.