Cluster fly
Pollenia species
- Identification
- Slightly larger and slower than a house fly, dark with a faint golden sheen on the thorax. Gather in clusters on warm walls.
- Lifecycle & behavior
- Larvae are parasites of earthworms in soil; adults seek structures to overwinter, entering wall voids and attics in fall.
- When it's active in Central NY
- The defining fall invader of the rural Finger Lakes. Masses on sunny walls in September–October, then emerges indoors on warm winter and spring days.
- Risk & damage
- Not a health hazard — they don't breed indoors or spread disease — but invade by the hundreds and leave spotting.
- Why DIY falls short
- Once inside the walls, they can't be sprayed out; interior spraying only addresses the few that emerge.
- How we treat it
- A timed late-summer exterior barrier plus sealing of entry points — stopping them before they get inside.
Cluster fly or house fly?
Cluster flies are slightly larger than house flies, darker, and noticeably sluggish — they crawl on windows rather than darting around. The reliable marker is golden or yellowish hairs on the thorax, visible up close, and the way the wings overlap over the back at rest. House flies are quicker, have four dark stripes on the thorax, and hold their wings more spread.
Context is the strongest tell. Large numbers of slow flies at sunny windows in fall or on the first warm days of late winter is a cluster fly pattern. House flies around food and garbage in summer is not.
Cluster flies in Central New York
Cluster flies are arguably the defining seasonal pest of this region, and they come from the ground rather than the building — the larvae are parasites of earthworms, developing in soil under pasture and lawn. That is why the pressure is worst next to open field and hayland, and why brand-new houses in former farmland get them just as badly as century-old farmhouses. Homeowners in the new Lansing subdivisions are routinely surprised by this.
In late summer, adults move to the warm sunny elevations of buildings and work their way into wall voids and attics to overwinter. They are effectively unreachable once inside, and the ones appearing at windows in February have been in the walls since fall. The whole control window is a timed exterior application to the sunny south and west walls in late summer, before they gather.
Dealing with cluster fly?
Our cluster fly control handles it — guaranteed, family-owned, and local.
Cluster fly — common questions
Why do I get flies indoors in February when it is freezing outside?
They did not come in from outside. Cluster flies entered the wall voids and attic last fall to overwinter, and the first warm spell wakes them and draws them toward light and windows. Indoor treatment in February has limited value — the effective work is exterior, the previous August.
My house is brand new. Why do I have thousands of them?
Because cluster flies develop in soil, not buildings. Their larvae parasitize earthworms in pasture and lawn, so a new house built on former farmland sits in prime habitat. A tight modern house has fewer entry points, which helps, but it does not change the surrounding ground.