Get rid of drain flies (sink flies, filter flies, sewage flies)

Question

Since I moved into my house I started noticing drain flies coming into my bathroom and kitchen. Sometimes there are a lot of them, I once counted more than fifty. In my bathroom there is a septic put under the floor that collects the sewage water with a double lid that I had sealed off. I think they live over there. They are very annoying.

The flies keep reappearing, I guess from slits and holes that connect the bathroom to the area behind the walls and fake ceiling. Sometimes the flies are gone for a month, but they always return.
I know little of drain flies and I'm looking for a way to exterminate them, so they won't reappear ever. A second question is, how to identify where they breed?

Notes:
- I will try to completely seal the holes and slits in the the bathroom and kitchen (if that is possible)
- I dont have access to the septic put and I dont want to break open the walls
- I prefer no to use antyhing poisonuous
- there are no windows in the bathroom, but there is a ventilator in the ceiling, so the humidity is above average.

Answer 1

I'm a huge fan of fly strips (a.k.a. fly paper). The strips are cheap and low tech. And one strip can catch a lot of flies.

I would hang them anywhere you see the flies, and anywhere you think the flies might be originating from. Replace the strips whenever there are too many flies on them (and they look gross).

Hopefully, within a few weeks you'll catch all the flies. That's happened in my home a few times: we had an infestation, hung up some fly strips, and caught all the flies. The reason that's happened more than once is not because the original colony came back, it's that we brought more flies in via fresh fruit like peaches or something.

Answer 2

It's possible that the flies are hatching from eggs laid in your pipes, rather than from entering your home. Here's a few things to try:

1. Keep the drains clean and clog free.

2. Make sure that there is no stagnant water in your bathroom or kitchen.

3. Occasionally spray IGR (Insect Growth Regulator) in your drains. IGR spray prevents the drain fly eggs from breaking out of the larvae stage and hence can greatly reduce the chances of any future infestation.

https://www.fliesonly.com/how-to-get-rid-of-drain-flies/

1. Use a metal pipe brush and push it through the pipe back and forth as far as it will allow followed by lots of boiling water.
2. Trap flies by setting a bowl of equal parts sugar, water and white vinegar with a 5-10 drops of liquid dish soap on the counter next to the sink overnight. Flies will be attracted to the fragrant liquid and drown.
3. Boil a pot of water and pour it down the drain 1-2 times daily for a week.
4. Pour 1/2 cup of salt, 1/2 cup of baking soda and 1 cup of vinegar down the drain and allow to sit overnight. Follow with a pot of boiling water in the morning.
5. Pour 1/4 cup of apple cider vinegar into a glass and cover tightly with plastic wrap. Poke holes in the plastic wrap with a fork and place the glass next to the sink. Flies will be attracted to the cider vinegar and work their way inside and drown.
6. A natural product called Bio-Clean is a nontoxic, environmentally friendly drain cleaner that eats away at organic matter blocking your drain. Once the drain is cleaned, the flies will start to disappear.

https://www.mnn.com/your-home/at-home/stories/how-get-rid-drain-flies-naturally

Answer 3

Find the source where the drain flies spawn and seal it air tight with a water lock. In my case it was where the washer drain entered the water lock pipe: it was put in too deep so when it flushed the water lock was emptied and the flies could escape that way.

Answer 4

I'm adding another answer because it's completely different than my first answer. I've used it in the past and it works surprisingly well!

1) Get a small bowl or dish.

2) Pour in some vinegar. It can be apple cider vinegar, white vinegar, or whatever you have available. I've used apple cider vinegar in the past. You only need about 1/4 cup.

3) Add a few drops of liquid dish soap. Stir it up just a little.

4) Do that several times, in several bowls.

5) Leave the bowls around your home where you've seen the flies. (Keep curious pets away from them!)

The flies will get attracted to the scent of the vinegar, and will attempt to drink it. The soap eliminates the surface tension of the vinegar, so the flies get sucked into the vinegar and sink to the bottom. You'll notice them in the bowls after a few hours or days.