Large Black Flies
March 26, 2010
For the second time in about five years, we have large black flies in our house. They are not coming in from the outside and they seem to spend most of their time on our windows. In two days, I have swatted at least 50-60, so you can imagine how gross this problem is. We do have cracks where the windows meet walls-our house is on a slab, but it shifts considerably. We have not noticed any foul odors, except under the kitchen sink, but there is not a leak or sign of anything that may have died under there. We do keep rat poison in the attic, but my husband checked and found no sign of anything that died up there. I’m concerned about the health risk and have no idea, other than swatting, how to get rid of them or where they come from. Please, I hope you can help. I’d li ke to know if they bite, spread disease and how long they live, besides how to get rid of them. (I saw Amityville Horror and already told my husband that if the walls begin to bleed, I’m outta here! LOL!)Thanks-Linda
Linda Mendez
Houston Texas

Cluster Flies

Hi Linda,
It is difficult to make out any details in your images, but your description is consistent with an outbreak of Blow Flies.  Many of the Blow Flies pictured on BugGuide have metallic coloration in shades of green and blue, but others are black like your specimens.  It is possible that the poison dispatched a rat and a female Blow Fly was attracted to the rotting flesh where she laid her eggs.  We will check with Eric Eaton to see if he concurs.  If blood begins to seep from the walls, please let us know.

Cluster Fly

Eric Eaton Agrees
Yes, I would bet on blow flies, probably “cluster flies” in the genus Pollenia.  Cluster flies are well known for harboring between walls during the colder months, then emerging in vast numbers as described in the letter.  Still, I’d have to examine actual specimens before I could be certain.

Wow, thanks so much for the speedy reply-I’ll go to your website for answers on health and getting rid of them. Some of these do have the coloration you mentioned. I appreciate your help-Linda

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3 Responses to Cluster Flies appearing in home

  1. For now it might just be best to grin and bear it. Treating the wall voids indeed may kill hundreds of flies but you invite things like carpet beetles who’ll feed on the carcasses and then of course you have a whole new problem. Better to work on sealing up the house for next year. Here’s an article that may help.

  2. Kun says:

    Oh my goodness. I have killed 3 big flies. I came here from searching this problem up on google. I am terrified of flies. My room is pretty clean beside some dust here and there. As I am typing this, there is a fly sitting in the corner of my window right now.

  3. Wayne says:

    We bought our home three year ago, (20 year old home in Georgia). One day when spring was starting, we noticed large, black flies on the windows and ceiling. They are slow and lethargic so it was easy to use a vacuum cleaner and eventually get them. We have been diligent in sealing up everything we can find where they might have been hiding during the winter. Now in our third year, this spring I found and killed less than 20, compared to hundreds during the first year. We hope that we have seen the last of them. Local pest companies didn’t really know what to do about it. Just tackle the problem, and eventually you will get control of it.

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