Cluster Flies: The Lazy Houseguests in Your Attic
Big lazy flies in your house during winter? Those are Cluster Flies. Learn why they are in your attic and why you shouldn't bug bomb them.
It's January. There is snow on the ground. Yet, you have huge, slow-moving flies buzzing around your living room window.
"Did something die in the wall?" you ask.
Probably not. Meet the Cluster Fly.
They aren't eating trash
Unlike House Flies or Blow Flies, these guys don't care about your garbage or your food. They are parasites of earthworms. Yeah, worms.
In the summer, they live outside. But when autumn hits, they seek warmth. They crawl into your attic, wall voids, or siding gaps by the thousands. They cluster together (hence the name) and go dormant.
The "False Spring"
Here is the annoying part. On a sunny winter day, your attic warms up. The flies wake up, thinking it's spring. They crawl towards the light, which usually leads them out of your recessed lighting fixtures and into your bedroom.
They are unbelievably stupid
Have you noticed they are slow? You can easily swat them. They spin on their backs on the windowsill. They are groggy from hibernation.
How to stop them
Spraying them inside is endless whack-a-mole.
The only fix is exterior sealing. You have to caulk the cracks around your windows, eaves, and siding in late summer (August). Once they are in the wall for the winter, you are stuck with them until spring.
Don't use a bug bomb in the attic—you'll just end up with thousands of dead flies that attract carpet beetles.