Now is the transition from spring to summer. The house I am now living has some strange things happening.

In the daytime of one day last week, I found many flies gather on the inside face of the window in the front living room. I then opened the window and drove them out. The next day, I found there were still many flies again on that window. Same thing happened again and again in the past week. Besides that window, there are also some flies flying elsewhere in the first floor. At nights they gather around our lights. I remember same thing happened in last year's summer as well.

The windows and doors are either closed or have screen on. I cannot think out of some possible way for the flies to come in. There was an envelop slip on the front door, but it was by default closed, unless people from the outside want to deliver some mails to the inside. There was also a pipe stretching from the basement to the outside, for letting out vapour of heating during winters.

I wonder if this kind of things has happened to you guys as well? Where can so many flies come from?

  • 4
    I'm guessing they are not getting in, they are already inside. There may be a food supply in a wall or ceiling (dead animal, etc), that has become a breeding ground. Call an exterminator, they should be able to locate the source and eliminate it.
    – Tester101
    Apr 18, 2012 at 11:36
  • @Tester101: Is asking help from exterminators free? I am in Maryland.
    – Tim
    Apr 19, 2012 at 0:18
  • An exterminator might come and look for free and offer an estimate, but if they do anything they will charge you. Call a couple different places, they may offer some advice over the phone for free or be able to give you an estimate.
    – Tester101
    Apr 19, 2012 at 11:35
  • When we purchased my last home I had trouble with lady bugs, thousands would show up until I was remodeling and found a hole in the exterior that they were making there way in under the upstairs base board. After sealing the hole never had them inside again in any numbers, this year we purchased 50 lbs of fresh oranges, brought them in the house and before we could get them squeezed into juice we had a bunch of flys came in with the fruit.
    – Ed Beal
    Feb 19, 2018 at 20:17

7 Answers 7


We went away on holidays for 4 weeks and returned to a house full of flies. I sprayed them everyday and kept all the flyscreens shut so they couldn't get into the house, but somehow they just kept multiplying! There was nothing in the house that smelled off or any food scraps or dead animals. After 4 days of this driving me insane I realised that our coffee machine had sat unused for 4 weeks and I had not cleaned it out yet. Bingo - a fly had laid eggs in old coffee grounds that were sitting in the bottom of the machine. I have now cleaned it all out and we have no more flies! So check all your appliances too particularly coffee machines/filters and toasters, ovens, etc. I hope you find out the answer to where they are coming from!


Like others have said, there's probably a dead thing somewhere. However, depending on the fly type, they could be breeding in very dirty water or in pretty much any organic matter. For example, some will breed in rotting vegetation, some will eat wool or fleece if they have too. So looking for pretty much anything out of the way that could be rotting. You may not find it though - attics, basements, even in the walls if a rat died in there or something.

The life cycle of flies is often very temperature dependent - it slows down when it is cold and speeds up when warm. Some of the common flies found in houses can go from egg to adult in about a day in ideal conditions, so even if you kill them or let them out, this is probably how they are replenishing - new ones are hatching.

I looked up the breeding time and so on a while back, when this happened to my Mum - all these flies showed up in the lobby to the house one morning, on the ceiling. It was kind of creepy. We cleaned out the room, and couldn't find any obvious points of entry from the ceiling or floor, so you're not the only one!

What do your flies look like? Different ones eat different things and have different habits.

This interesting looking chart describes them and gives their "favourite foods" as well. http://www.jfoakes.com/fly_identification_chart.htm

This site gives many many pictures of flies. If you click on the headers above each row of pictures, you get more pictures of flies in that subgroup, so you can keep getting more and more specific pictures and ID. Not recommended if you are grossed out by fly pictures. http://bugguide.net/node/view/55/bgpage

  • Thanks! The emergence of those flies in this house are seasonal, about a week in the late spring every year, and after that they disappear completely. I am curious about that.
    – Tim
    Jun 21, 2012 at 10:49

Cluster flies. They might come in through can lights in the ceiling from the attic, or other tiny cracks.

You might try putting up some fly strips in the attic. Add fine screens to the inside of attic vents.

  • A lot of attic's are full of them, you just never see them. Really the best way to get rid of them is call a pest company to come fog your attic and other areas to kill'em!
    – Steven
    Apr 18, 2012 at 13:08

They're called cluster flies. They find a way into your house and leave their scent on the trail so others follow. We had them hanging out in our skylights and would find the dead ones all over the house. We bought some things called 'cluster busters' - that attach to the window where they hang out. They're attracted to the inside and "drown" in a fine powder made from crushed egg shells. These worked OK, but we still had them. Last spring, we had our attic re-insulated and the first thing they did was seal all openings from the house into the attic. Voila - no more flies in the house.


If you have a fireplace open the damper and take a whiff.You may have a bird,racoon or some other animal in the chimney.

  • Thanks, there is no fireplace in this house. There may or may not be one in the basement, I am not sure. But the basement is locked away from the rest of the house.
    – Tim
    Apr 17, 2012 at 20:46
  • @Tim hollow walls, roofspace,... all ways for flies to get in Apr 17, 2012 at 21:27

This happened several times in my last place. Each time it was because a bird had died outside our condo. I'm not sure if they crashed into a window or just thought our back patio was a nice place to retire, but each time I found the bird's remains out back.

I still am not sure how or why the flies got from the bird to inside the house. But once in the building, I think they got into air ducts, and from there followed light sources trying to get out, and we ended up with dozens of flies on an inside window like you said.

I would check your exterior walls, patio, yard, etc for any remains. It might be hard to spot as one time I did this, the remains were quite decayed already, almost just a pile of feathers. Try not to touch the remains directly and of course clean up afterwards. If you can't find anything and the flies persist, then as others have mentioned you could have something in an attic, chimney, flue, basement/crawlspace, roof eaves, etc.


This happened to my place...turns out that the flies breed in the soil in pots that I had moved inside at the beginning of winter. The only solution is to take the pot out and let the eggs freeze.

Not the answer you're looking for? Browse other questions tagged or ask your own question.