Every autumn, thousands of cluster flies descend on Scottish homes to overwinter in loft spaces, wall cavities, and roof voids. They are not dangerous, they do not breed indoors, and they do not damage property. But large numbers of slow-moving flies appearing on warm winter days, clustering at windows and in corners, are deeply unpleasant and difficult to remove once established.

What are cluster flies?

Cluster flies (Pollenia rudis) are slightly larger than house flies, with a dark grey body and golden-yellow hairs on the thorax. They move slowly and sluggishly, particularly in cold weather. In summer, they live outdoors and their larvae parasitise earthworms in the soil. In autumn, they seek warm, sheltered places to overwinter, and buildings are ideal.

They are attracted to the same sites year after year. If your loft has had cluster flies before, it will almost certainly have them again unless the entry points are sealed. Properties surrounded by agricultural land or grassland tend to have the worst problems, as these environments support large earthworm populations and therefore large cluster fly populations.

How they get in

Cluster flies enter buildings through the smallest of gaps: around roof tiles, under eaves, through gaps in fascia boards, around window frames, and through any crack or crevice in the building fabric. Once inside, they move deeper into the structure to find a cool, dark place to overwinter. On warm winter days, they become active and move towards light, which is when they appear at windows and in living spaces.

Prevention: sealing entry points

The most effective long-term solution is to seal the entry points before the flies arrive in autumn. This means checking the roofline, eaves, and fascia boards for gaps, ensuring roof tiles are in good condition, sealing around window frames, and checking for any cracks in the building fabric. This work is best carried out in late summer, before cluster fly season begins.

Complete exclusion is difficult to achieve on older properties, but even partial proofing can significantly reduce the number of flies entering.

Treatment options

Where proofing is not sufficient or practical, insecticide treatment of the loft space in autumn, before the flies settle in for winter, can significantly reduce numbers. A residual insecticide applied to the loft space kills flies as they enter and settle. This treatment is most effective when carried out in September or October.

If you already have a large number of cluster flies in your loft or living spaces, a professional treatment can clear the current population. However, without proofing, the same site will be recolonised the following autumn. Pest King provides cluster fly treatment and loft proofing advice across Aberdeen, Dundee, Angus, Perth and Stirling.