Cluster Fly Treatment Across the East Midlands

What are cluster flies?

Cluster flies (Pollenia species) look like a slightly larger, slower house fly, about 8–10mm, dark grey with golden hairs on the thorax that give them a dull, sometimes yellowish sheen. When resting, their wings overlap over the body. They're sluggish, especially on cold days, and often buzz in circles at windows.

Their larvae live in the soil and develop as parasites of earthworms, so cluster flies are most common in homes near fields, pasture and large gardens, which describes much of rural Nottinghamshire, Lincolnshire, Leicestershire and Derbyshire.

Why cluster flies come back to the same house every year

From late summer into October, adult flies look for somewhere sheltered to spend the winter. They're drawn to the warm, sunlit south and west walls of buildings, crawl in through gaps around tiles, soffits and window frames, and gather in large clusters in lofts, roof voids and the corners of unused rooms. On warm days in winter and early spring they wake up and drift down into the house, often appearing at upstairs windows and light fittings.

The flies leave a scent that attracts more cluster flies, so the same buildings tend to be affected year after year. They don't breed indoors and they're not a sign of poor hygiene.

Are cluster flies harmful?

They don't bite and, because they don't breed in rubbish or food, aren't associated with the same hygiene risks as house flies. The problem is the sheer number: hundreds or thousands in a loft, dead flies on windowsills, staining, and a sickly smell when large numbers die in a roof space.

How we treat cluster flies

For an existing problem, we apply a residual treatment in the loft and roof void where the flies are clustering, and treat window frames and other resting points as needed. Large numbers of dead flies can be vacuumed up afterwards.

For homes that are affected every year, a preventative residual treatment of the loft and the outside resting areas in late August or early September catches the flies as they arrive, before they settle in for the winter. Sealing gaps helps, but with cluster flies it's rarely possible to close every entry point in a roof.