How To Get Rid Of Cluster Flies In Attic Winter
It is one of those crisp, sunny winter days, the kind you finally open the curtains for, and suddenly your living room fills with dozens of slow, buzzing flies bumping against the windows.
They are not house flies, they are not fruit flies… they are coming down from the attic in waves. You know exactly what they are: cluster flies. And the question hits: “How do I stop this without making a bigger mess?”
I have dealt with this exact invasion more winters than I would like to admit, and I know how unsettling it feels when your home suddenly turns into a fly convention.
The good news is cluster flies are harmless to you and your family; they do not bite, sting, or spread disease, but they are annoying and can leave stains on walls/curtains if crushed.
In this guide, I will show you why they swarm indoors on sunny winter days, why chemical bombs in the attic are a bad idea, and the light-trap strategy that actually gets them out safely and quickly without risking your health or your attic insulation.
Why Cluster Flies Suddenly Swarm from the Attic in Winter
Cluster flies (Pollenia rudis) are not your typical house flies. They spend the warm months outdoors feeding on earthworms and laying eggs in the soil.
When autumn arrives, they look for sheltered overwintering spots, and attics, wall voids, and roof spaces are perfect: warm, dry, and protected from wind and predators.
They squeeze through tiny gaps under eaves, around vents, or in soffits, often hundreds or even thousands at a time.
During winter, they stay dormant in torpor (a light sleep-like state). But on sunny days when the attic warms up (even if it is freezing outside), the heat wakes them. They become active, start flying towards the light, and end up inside your living space looking for a way out.
That is why the swarming is worst on bright, mild winter days, especially on south- and west-facing windows.
Read also: 4 Tips For Effective Cluster Fly Control
Why You Should Never Use Foggers or Bombs in the Attic
It is tempting to grab a bug bomb and “nuke” the attic, but please don’t. Here is why it usually backfires:
- Foggers push cluster flies deeper into insulation, wall voids, or ceiling fixtures, making them harder to reach later
- Dead flies rot and attract carpet beetles, mites, and other pests
- Heavy chemical residue can linger and affect indoor air quality for weeks
- Many attics have exposed wiring; foggers can create fire hazards if they settle on electrical components
The goal is not to kill every fly in the attic (impossible anyway); it is to safely guide the active ones outside and seal entry points so next winter’s batch cannot get in.
The Light-Trap Strategy: Use Their Attraction to Light Against Them
Cluster flies are strongly phototactic (they fly toward light). That is exactly why they end up on your windows. We can turn that instinct into the main removal tool.
Step-by-Step Light-Trap Method
- Darken the rest of the house; Close blinds/curtains on all windows except one or two in the affected room.
- Open that window fully (or as much as possible); remove the screen if safe to do so.
- Place a bright light outside the window; a shop light, portable work light, or even a powerful flashlight works. Position it so it shines toward the open window.
- Turn off indoor lights; let the flies follow the brightest light source (outside).
- Wait 30–60 minutes; most will fly out toward the light. Repeat daily on sunny days.
- Vacuum stragglers; once they are outside, vacuum any remaining flies on the sill or walls (use a hose with a stocking over the end to trap them without smell).
This method is silent, chemical-free, and surprisingly effective, especially on sunny days when they are already active.
Long-Term Prevention: Stop Next Winter’s Cluster Fly Invasion
Once you have cleared the current swarm, focus on keeping new ones out next fall:
- Seal Entry Points: Caulk gaps around windows, doors, vents, soffits, and siding (late summer/early fall is best).
- Install Tight-Fitting Vent Screens: 1/8-inch mesh over attic vents.
- Trim Tree Branches: Keep them 10+ feet from the roof and walls.
- Reduce Outdoor Lighting: Bright lights at night attract them.
- Apply Residual Insecticide: In fall, spray eaves/soffits with a labeled product (permethrin or deltamethrin); do this professionally if unsure.
Prevention is the only way to dramatically reduce numbers year after year.
Frequently Asked Questions
Do Cluster Flies Bite or Sting?
No, they do not bite, sting, or spread disease to humans or pets. They are just a nuisance.
Why Do Cluster Flies Come Inside in Winter?
They overwinter in attics and wall voids. On warm, sunny days, the attic heats up, they wake up, and they fly towards light, ending up inside your living space.
Will Bug Spray Kill Cluster Flies?
Yes, but spraying indoors spreads them deeper into walls and creates a mess. The light-trap method is safer, cleaner, and more effective for active flies.
How Do I Keep Cluster Flies Out Next Winter?
Seal gaps around eaves, vents, and siding in late summer/early fall. Use 1/8-inch mesh screens on vents and reduce outdoor night lighting.
Conclusion
Cluster flies do not bite, sting, or spread disease; they are just annoying winter squatters that wake up on sunny days and head toward light.
The light-trap method (dark house + bright window + open escape route) safely guides them outside without chemicals or mess, and sealing gaps in fall keeps next year’s swarm much smaller.
Next sunny day, darken the house, open one window, shine a light outside, and watch them leave. It is simple, effective, and gives you back control of your home.
Have you tried the light-trap trick yet? How many did you get out? Drop a quick comment; your story might help someone else win the battle this week!
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