I have a problem with flies suddenly. I have been living in my house for almost 20 years and never had a problem with flies. How can I find out where they are coming from and eliminate the source?

They are almost all on the first floor, so that would seem to rule out a source in the basement or upper floors. For example, I have a cat litter box on the second floor, but there are no flies there, so I know the flies are not coming from anywhere near there.

That would seem to leave two possibilities:

(1) They are breeding outside the house and somehow finding a way in, or

(2) There is some hidden source on the first floor

I have carefully tried to seal off any crack or opening to get into the house, yet the flies are just as numerous as ever. I do see flies outside on the windows, but in small numbers, so they could either be coming from outside, or just be escapees coming from inside the house.

I have searched the grounds very carefully for animal bodies or other organic matter and found nothing.

How can I locate the source of the flies?

  • Sometimes the non-lifehack way is a great solution. Have you considered several rolls of fly paper? You might also consider reading this fascinating article from wikipedia. en.wikipedia.org/wiki/Fly-killing_device
    – Sidney
    Jun 15, 2017 at 15:20

1 Answer 1


The issue might be caused by a dead animal in a drain pipe or vent stack. A small rodent can even fit into a sink or tub overflow and be unable to get out.

If that is the cause, but you can't find the source, then at least be consoled in knowing that once the remains are devoured, the flies (and any bad odour) will be gone.

  • 3
    The question is how to find it. Jun 12, 2017 at 1:08
  • You can clean every drain, either with a mechanical device (e.g. flexible plastic such as Zip-it) or liquids (e.g. NaOH =lye, dangerous!). Don't forget the overflow on sinks and tubs. It's trial and error, but the effort is not wasted -- it's good to keep drains clean of hair, fats etc. Jun 13, 2017 at 1:07
  • They are not drain flies. Jun 13, 2017 at 2:21
  • What is a drain fly? Jun 13, 2017 at 16:41
  • @DrMoishePippik Nothing like the idea of running a Zip-it through a half decayed rat corpse. 🤢
    – Sidney
    Jun 15, 2017 at 15:14

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