Cluster Flies

Cluster flies are found throughout the UK, but predominantly in rural areas. Their common name refers to their habit of forming clusters when hibernating, often in large numbers in the upper rooms and roof spaces of buildings during the autumn period.
Whilst there is a specific species of common cluster fly (Polleniarudis), there are four other species which vary in size and colouration, so it is possible to have mixed populations of flies causing the problem inside suitable buildings. Any of these species of fly presence is not a sign of poor hygiene. There is no need, and in any case it is not practicable to try to control the flies outdoors or to control their breeding areas.

Life Cycle

During the summer months these flies live and breed in fields and are not a problem. They are parasitic on earthworms and their larvae live in the soil, emerging as adults at the end of summer. In the autumn under natural conditions they would hibernate in dry sheltered areas such as under loose bark or in hollow trees. The survivors of the winter would then emerge the following spring to return to the grassy fields to continue their life cycle. However, with houses close to open spaces and fields it is quite common in the autumn for these flies to congregate in large numbers on the outside of buildings, especially on sunny, south-east facing, light-coloured walls. As the afternoon temperatures begin to fall they tend to crawl into crevices for shelter, sometimes through window frames, but mainly under the eaves/fascia boards into the roof space.

The following days they may emerge and sun themselves but as autumn moves on and temperatures drop they finally remain inside until the process is reversed the following spring. The big problem is that they tend to invade in large numbers. Fitting fly-screens is of little use against this specie of fly as it prefers to squeeze into small cracks around tiles, window frames etc., hundreds of flies in a loft are a big nuisance. The flies tend to return to the same property year after year. Even if there are similar buildings nearby, they may not be affected. It is not known why one building is preferred, it may be that the flies are attracted to lighter coloured buildings, sunnier spots, or use pheromones to locate old roosts, it is certainly no suggestion that it is due to poor hygiene.

What can be done?

There is very little that you can do to perminantly stop the flies getting in. Making sure that gaps around window frames are re-pointed and sealing obvious entry points can offer limited help, but it is not really possible to stop a fly getting into a loft and if one fly can get in, so can the hundreds of others. The good news is that although you may be unfortunate and have invasions of cluster flies every year, we can treat the affected areas using a licensed pesticide that will eradicate them almost immediately.