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Ive started to find big black house flies in my house that I haven't seen before. Initially I suspected that they came in when the door was open but we have been keeping the door shut all the time now and new ones keep coming in. We also saw a few outside insects in the house that we would normally never see.

I have tried to find whether there is any open window but there is none. Any idea what can i do to find the source of these flies?

  • I find flower pots have been a place that flies have laid eggs, we have horses and are very meticulous about closing doors and keeping the house sealed (at least with screens). A while back we all of a sudden had 6 then 12 that I killed then the next day another 6-10 more, my wife went to water her plants and the boys were coming from the pot. You might look in plants for the source. – Ed Beal May 7 at 19:14
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Get a FLIR attachment for your iphone, crank the heat in the house, look for cold sources on your exterior walls. Assuming the exterior is typically colder than the interior of the house.

https://www.flir.ca/products/flir-one-gen-3/

You probably have makeup air vents, wall caps for fans / fan hoods, chimney. Also insects will go into suspended animation when temperatures are too cold and then wake up when it gets warm enough again so it is possible they were already in the house and the wall / attic / space they were in has now warmed up enough for them to wake.

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Flies are VERY good at getting in through small openings. Take a good look at light fixtures. Many of them collect flies, and you really wonder how.

Favoured overwintering spots include any cracks between boards, under vinyl siding, soffits, electrical box openings.

Ingress: Even casual going in and out can bring flies in. If you have pets and open and shut doors for them...

Not all doors fit tightly, particularly check the weather seal at top and bottom edges.

Check all screens for holes. See that they shut all the way. Bowed screens are a good way for them to enter.

Exhaust fans are another common entry point. We have one that allows about 2 bumble bees a month in.

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