Straight answers from a licensed New York exterminator and Associate Certified Entomologist (ACE) — serving all five boroughs, in English and Spanish.
Get a Free Estimate →⏱ 3 min read
Quick answer: Cluster flies are slightly larger, sluggish relatives of the house fly that overwinter inside walls, attics, and window frames, gathering in large clusters on warm days. They do not breed indoors or contaminate food the way house flies do. The key to control is exclusion — sealing them out before fall — plus vacuuming the ones that emerge.
What do cluster flies look like?
Cluster flies are slightly larger than house flies, dark gray, and noticeably sluggish. Up close they have short golden hairs on the thorax and overlap their wings at rest. They fly slowly and clumsily, often bumping against windows.
Signs of cluster flies
The hallmark is large groups of slow flies on sunny exterior walls in fall, then indoors around windows and light fixtures on warm winter and spring days as overwintering flies wake up.
Why cluster flies get indoors
Unlike house flies, cluster flies do not breed in garbage — their larvae develop in soil, parasitizing earthworms outdoors. In fall the adults simply seek shelter to overwinter, slipping into wall voids and attics through gaps around windows, soffits, and siding.
How to keep cluster flies out
The durable solution is exclusion before fall: seal cracks and gaps around windows, doors, soffits, utility penetrations, and siding, and repair or add screens. For flies already inside, vacuum them up as they appear at windows. Interior sprays give only temporary relief because more flies remain hidden in the voids.
When to call a professional
For homes with recurring fall invasions, a professional can identify entry points, perform exclusion, and time an exterior treatment to the seasonal invasion for the best results.
A Brooklyn-based, NYSDEC-registered company (Reg. #15140) led by Jorge Bedoya, an Associate Certified Entomologist (ACE). For pests in your home or building, NYE provides IPM-based, low-exposure control matched to the exact pest and verified with a follow-up. ACE-led work comes with a client portal of service reports and photos, fully bilingual service, and no long-term contract.
Cluster Fly FAQ
How do I identify cluster flies?
Cluster flies are a bit larger than house flies, dark gray, sluggish, and have short golden hairs on the thorax. They fly slowly and tend to gather on sunny walls and windows, especially in fall and on warm winter days.
Why do cluster flies come indoors?
In fall they seek sheltered spots to overwinter, entering through gaps around windows, soffits, and siding, then clustering in wall voids and attics. Warm days coax them back out toward windows.
How do you get rid of cluster flies?
Vacuum up active flies, and seal exterior gaps and cracks before fall so they cannot enter. Because they overwinter deep in voids, exclusion done ahead of the season is the durable fix.
Are cluster flies harmful?
No. Cluster flies do not bite, breed indoors, or contaminate food like house flies; they are a nuisance because of their numbers and clustering.
Where do cluster flies hide in winter?
In wall voids, attics, around window and door frames, and behind siding and trim, emerging toward light on warm days.
More in our Pest Library · Not sure what you have? Try the NYC Pest Identifier. Reviewed by Jorge Bedoya, ACE.
request a free estimate
- Phone: (347) 210-4646
- Email: info@nyepestcontrol.com
- Address: 1115 East 13th Street, Brooklyn, NY 11230