Cluster Flies
Fall invaders that overwinter by the thousands
About Cluster Flies
Cluster flies are one of the most frustrating pest problems unique to the Northwoods. In late summer and fall, they gather on the sunny sides of buildings by the hundreds — sometimes thousands — and squeeze through tiny gaps to overwinter in wall voids and attics. On warm winter days they emerge into living spaces, sluggish and disoriented. Come spring, they pour out in massive numbers. Unlike house flies, cluster flies don't breed indoors — but their sheer numbers and the difficulty of eliminating them make professional treatment essential.
How to Identify Them
- Slightly larger than a house fly, about ⅜ inch long, with a dark gray checkered abdomen
- Golden-yellow hairs on the thorax (back) — visible up close
- Slow, sluggish movement compared to house flies, especially in cool weather
- Cluster on south- and west-facing walls in fall to absorb heat before entering
- Emit a faint, sweetish odor when crushed or disturbed in large numbers
Why They're a Problem
Cluster flies don't bite, sting, or spread disease — but they're a massive nuisance. A single wall void can harbor thousands of flies. When they emerge on warm winter days, they cover windows and light fixtures. In spring, the emergence can be overwhelming. Dead flies in wall voids attract other pests like carpet beetles. And because they return to the same structure year after year, the problem gets worse each season without treatment.
Signs of a Cluster Fly Infestation
- Large numbers of flies on the sunny side of the building in September and October
- Flies appearing inside on warm winter days, especially near windows and light fixtures
- Massive fly emergence in spring as temperatures rise
- Dead flies accumulating on window sills and in light fixtures
- Faint sweetish odor in rooms adjacent to infested wall voids
- Flies returning to the same building year after year
How K2 Treats Cluster Flies
The most effective cluster fly treatment is preventive — applied in late summer before flies begin entering. K2 treats exterior surfaces and potential entry points with residual insecticides that kill flies on contact. For existing infestations, void treatments and interior applications reduce active populations. Exclusion work is also recommended to seal entry points for long-term control.
- 1Exterior inspection to identify entry points — gaps around windows, soffits, fascia, and utility penetrations
- 2Residual exterior treatment to all surfaces where flies congregate and enter
- 3Void injection treatments for existing infestations in wall voids and attics
- 4Interior treatment as needed for active fly populations
- 5Exclusion recommendations to seal gaps and prevent future entry
Prevention Tips
- Schedule treatment in August or early September — before flies begin clustering
- Seal gaps around windows, doors, soffits, fascia boards, and utility penetrations
- Caulk around window and door frames, especially on south and west-facing walls
- Install fine mesh screens over attic vents
- Annual preventive treatments are the most cost-effective long-term solution
Dealing with Cluster Flies?
Don't wait — call K2 for a free estimate and fast service anywhere in the Northwoods.
(715) 891-2501Free estimates · Same-week service