Why do I have flies in my house in the middle of winter?
They're cluster flies that came in through gaps in the fall to overwinter in the attic or walls. A warm day wakes them and they head for the light at a window. They aren't breeding inside.
Cluster Fly (Pollenia species)
Reviewed by William "Willy" Vizcarrondo, NYS Certified Pesticide Applicator (C1825832). Updated September 23, 2026.
Quick facts
Cluster flies are the large, sluggish flies that appear at windows and in upstairs rooms of Long Island homes on warm days from fall through early spring. They aren't a sanitation problem; their larvae live in earthworms in the lawn, and the adults come inside in fall only to overwinter in the attic and wall voids.
| Size | Slightly larger than a house fly, about 3/8 inch |
|---|---|
| Color | Dull gray with black markings and short golden-yellow hairs on the thorax; wings overlap at rest |
| Look-alikes | House fly (smaller, faster, drawn to food), blow flies (metallic green or blue), fruit flies |
| Where you'll find it | Attics, wall voids, around window frames, in upstairs rooms on the sunny side; outside on south-facing walls in September and October |
| Active on Long Island | Enter September through October; appear indoors on warm winter days; leave in spring |
| Danger level | Nuisance only; no disease, no damage |
Long Island season
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
They enter in fall alongside stink bugs and lady beetles, and every warm winter day wakes a few dozen up and sends them to the nearest window.
How to identify it
Cluster flies are a bit bigger than house flies, dull gray rather than shiny, with golden hairs on the back if you look closely, and they're slow. Penn State notes they can be easily swatted or captured, which is not true of a house fly.
They gather in clusters, hence the name, and they show up in bedrooms and attics rather than the kitchen. If the flies are fast, small, and around food or the trash, that's a house fly and a different problem.
Signs you have them
- Slow flies buzzing at upstairs windows on a warm winter day
- Clusters of dead flies on window sills and in light fixtures
- Flies in the attic or coming out of ceiling fixtures
- Flies on the sunny side of the house outside in September
- Small dark spots of excrement on walls and sills
Why it matters
Mostly annoyance and volume. Cluster flies don't damage the house and don't spread disease, and Penn State is clear that they're not a sign of poor sanitation. The larvae develop as parasites inside earthworms, so a house with a big lawn full of worms and a sunny, gapped south wall gets them every year.
The one real cost is that a heavy attic population can leave stains and attract carpet beetles, which feed on dead insects and then move on to wool and stored fabric.
Can I handle it myself?
Largely. Seal the same gaps that let stink bugs in: window and door frames, siding joints, utility penetrations, soffits, and attic vents, with quality caulk and fine mesh. Indoors, vacuum them; a vacuum with a hose is the best cluster fly tool there is.
Exterior treatment works only in a short window in early fall before they congregate, and it fades quickly in sunlight (Penn State). Penn State specifically advises against spraying insecticide into wall voids, which kills flies where you can't remove them and feeds carpet beetles.
How Bobby Bugs treats it
A perimeter treatment of the sunny walls and entry points in late September or early October, and sealing what we can reach. General insect treatment covers cluster flies.
Frequently asked questions
Are cluster flies a sign my house is dirty?
No. Their larvae live in earthworms in the lawn, not in garbage. A clean house with a sunny wall and a few gaps gets them just the same.
Should I spray inside the walls for cluster flies?
No. Dead flies in a wall void you can't reach attract carpet beetles, which then go after wool and stored fabric. Seal the gaps and vacuum the flies you can see.
When do cluster flies leave?
In spring, once it's reliably warm, they work their way back outside to lay eggs in the soil. Sealing over the summer stops the next fall's group.
Sources
Think you have cluster flies? Call (516) 509-6001 for a free written quote.