Cluster Flies in Fall: Why They Keep Picking Your Attic

If you’re finding slow, clumsy flies at the upstairs windows in October, and they seem to come back no matter how many you swat, you’ve probably got cluster flies. They’re a different animal than house flies, and the difference matters for how you handle them.

What makes a cluster fly different

Cluster flies are a bit larger than house flies, dark gray, with short golden hairs on the thorax you can see in good light. They fly like they’re half asleep. The strangest part of their biology: the larvae are parasites of earthworms. All summer they’re out in lawns and fields doing that. Come September, the adults gather on warm south-facing walls and slip into attics and upper wall voids to overwinter. Older two-story homes in St. Paul neighborhoods like Mac-Groveland and Como, with lots of roofline joints and original windows, are prime real estate.

Why they show up in January

They’re not breeding in your house. There’s no dead animal, no rotting food, nothing to find. When your furnace warms a wall void or the sun hits the attic on a mild winter day, some of them wake up and move toward light, which means your windows. That’s why you get a trickle all winter instead of one event.

What we do about them

• Seal the obvious entry points: gaps at fascia boards, around attic vents, where dormers meet the roof.
• Apply an exterior residual treatment to the upper walls and roofline in early September, before the flights.
• Indoors, vacuum them up. A shop vac in the attic works fine.

The limitation nobody mentions

The source is earthworms in the surrounding turf, and no one is going to treat every lawn on the block for earthworms, nor should they. Earthworms are good for the soil. So if your home sits next to a park, golf course, or open field, some cluster fly pressure returns each fall. The goal is to shrink it from hundreds to a handful, and a timed exterior treatment plus sealing gets you there most years.

Timing is everything

Once flies are inside the structure in November, exterior work does almost nothing until next season. Call in August or early September for the best result. If you’re already seeing them indoors, we can still knock down the population and plan the fall barrier for next year.

Ankora Pest Control is licensed, bonded, and insured in Minnesota and inspections are free. Call (651) 556-2666 and we’ll figure out where they’re getting in.