House flies galore is bad enough - but what's killing them??
December 5, 2021 1:31 PM   Subscribe

We just moved into a 1 bedroom adobe house in New Mexico, which in this (unseasonably warm) November seems to be full of dead flies! I'm cleaning up 50+ dead/dying flies on the floor/window sills every day. I hardly ever see a thriving/flying one, which has me spooked. What's going on?

- These definitely aren't fruit or drain flies, but big black buzzing flies.
- We have no garbage disposal or compost or garbage heaps -- though we do have an acre of meadow outside.
- Their corpses are littering the house in every room but the bathroom.
-We have no open windows this time of year. We do have a radon dispersal system and a fireplace, which are the only places they might be getting in.
- I'm cleaning up their bodies multiple times a day -- and then more corpses just appear a few hours later.

What can be causing them to get in/generate, when we nothing but canned food in the pantries here, having just moved in (after the house sat vacant for 6 months) -- and more importantly, WHY ARE THEY ALL DEAD? Given the sheer numbers of them I'm sweeping up every day, you'd think I'd see more than one living one at a time!
posted by egeanin to Home & Garden (12 answers total) 1 user marked this as a favorite
Best answer: My guesses: somewhere in your house — in the walls, roof structure, under your floors, or in your chimney — a critter of some kind died. You're not smelling it thanks to whichever way the air flows past it, taking the smell outside and not inside. This perhaps points to the chimney, where the natural air flow is up. Or, it did its stinking before you moved in, but is still attractive to flies. In any case, flies fed on the critter, laid eggs that became maggots, maggots became flies, which started looking for a place to go. Some of them are finding their way into your house. But you have no food for them in there, so they don't live long. They are drawn to the windows and die there.

You could try taping a sheet of plastic securely over the fireplace opening, to test out the chimney theory. If that stops them, you might need a chimney sweep to remove the critter.
posted by beagle at 1:54 PM on December 5, 2021 [11 favorites]

Best answer: You might have cluster flies which are harmless but very hard to get rid of. They’re not drawn to garbage or dead animals, but they do rely on earthworms for part of their life cycle and having an acre of meadow outside fits with that.
posted by corey flood at 1:58 PM on December 5, 2021 [10 favorites]

Two suggestions:

1) I had a similar problem with bugs that came out of nowhere. Eventually, I realized that there was a box of trash above my refrigerator that spawned the damn things.

2) I have occasional Asian beetles that I encounter in my house. They are scary, because they are big, but also sad because they are usually dead. They are waterbugs and they are harmless. They just come into your house for water and cellulose. I usually just sweep the sad things away; sorry, little guy, better luck in your next life.
posted by SPrintF at 1:58 PM on December 5, 2021

Best answer: Exact same thing happened to me after a rat was poisoned anddied in the wall of my house.
Your description of the flies and their behaviour matches my experience to a tee.
It will stop when the animal decays eventually.
Sorry. I know it was a nightmare for about a week for me.
posted by Lucky Bobo at 3:53 PM on December 5, 2021 [2 favorites]

Response by poster: Re: Cluster flies -- those are usually a problem in the Spring, aren't they? Do Cluster flies come out and die in the Fall?
posted by egeanin at 4:03 PM on December 5, 2021

Best answer: When I worked in an office with a serious cluster fly problem, the dead ones were an issue year round. That may depend on your climate, though. If you talk to local contractors or exterminators they can probably give you a pretty good idea of what’s common in your area.
posted by corey flood at 5:16 PM on December 5, 2021

You sure they aren’t termites swarming?
posted by missmary6 at 9:16 PM on December 5, 2021

Best answer: When this happened to me it was, as beagle suggests, a dead squirrel in the attic rafters.
posted by Wretch729 at 5:43 AM on December 6, 2021

Response by poster: These are definitely juicy black house flies -- I once lived in a house with termites so I'd probably recognize them.

I've searched the rafters and have found nothing dead, but I'll have the chimney sweep check out our (currently-disused) chimney. He had found a dead squirrel in there back in September so it's conceivable there's another one and we're just not smelling it.

But am I right in that having all these dozens of flies be dead every morning (as opposed to buzzing around) isn't a sign that we have a gas leak or radon or carbon monoxide or some such?
posted by egeanin at 6:31 AM on December 6, 2021

Best answer: I had something similar in a very old (1900s) house that was converted to apartments (Denver, CO). These fat, bloated flies would climb out from behind non-brick walls and immediately die. Pretty gross. This would happen during the winter if it was unusually warm and I have a south facing glass room that would get warm (and also freezing cold). I suspect that it was warm enough to hatch, but not enough food or get cold at night and kill them.

We also had overflowing fly traps during the summer. I eventually sprayed the grass yard and cracks around the house. The next few years were bug free. I'm not exactly sure how they got into the house, but my suspicion was that they had a healthy population in the yard during the summer, they would try to winter in our walls and managed to have just enough cycle to keep going through the winter (and possibly lay eggs) if the weather was warm enough.

My experience is exactly what corey flood describes above.
posted by kookywon at 7:51 AM on December 6, 2021

But am I right in that having all these dozens of flies be dead every morning (as opposed to buzzing around) isn't a sign that we have a gas leak or radon or carbon monoxide or some such?

The spaces where we had the problem with flies had no gas appliances or anything that could have leaked carbon monoxide, so in my amateur opinion I think you’re probably ok. Maybe just pick up CO/radon alarms and have the gas company come out and check for gas leaks for your own peace of mind. Usually gas company checks are free and they’re happy to do it. (Apologies if you have all of this covered!)
posted by corey flood at 8:43 AM on December 6, 2021

Best answer: I had a similar problem on my South facing window in colder months. Googling showed that I had what looked like cluster flies. These jerks can fit in gaps between closed sliding windows, and definitely around screens. They are like mice of flies, and seem to be able to flatten out completely.
I found that removing the windows, and cleaning all the grooves really helped to reduce incoming flies. I assume that the seal was tighter with everything clean and WD-40'd (silicone edition).
My theory is that it was the warm side of the house, they bred around there, and loved hanging out on the outside of my windows. The moment it got colder, they'd squeeze their squishy selves into any gap available in my windows. And then, no food, cold panes and lathery got them. I still get the occasional lazy fly coming in, now that winter is starting and temperatures vary through the day. But one or two, not an army, like I had before.
Absolutely gross, and had me in tears several times, as fly cemeteries don't belong in bedrooms.
posted by Yavsy at 9:04 PM on December 6, 2021

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