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 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, control is fairly easy.Using an ordinary aerosol fly spray in the loft will kill all the cluster flies it hits, but will not penetrate into the cracks where flies may be hibernating.

For small infestations, if you do not want to use insecticides, a vacuum cleaner will give limited effect effective in sucking up flies. Like a fly spray, this will only deal with the flies you see - it will not control any that are hidden and hibernating. Old fashioned fly-papers again give a limited control effect and are not really suited to large scale fly control.

Calling in professional help

A pest control technician has access to more effective insecticides and equipment. Normally they will carry out an inspection of the property and suggest the best method of control. The methods that may be used are

Using a powered ultra-low volume generator. This can clear large buildings of flies very quickly and will penetrate into hiding spaces. This equipment allows us to do the job without having to use expensive scaffolding to reach roof spaces in big buildings like churches.

Where it is easy to reach all parts of the building treatmentusing a low pressure residual spray may be used. This will give a quick knock down, and will continue to kill any emerging flies for several months.

Caution

In old houses or large buildings, cluster flies may be in a loft where there are also bats. It is illegal to use insecticide if there is any possibility that it might come into contact with bats. You as the occupier and your chosen contractor both have a legal duty ensure that the bats come to no harm.

You must ensure smoke alarms are covered (only whilst treatment is carried out) to prevent false detection alarms with mists.