Cluster flies are particularly noteable in some properties, usually in rural areas, in the spring and autumn. Whilst there is a specific species of common cluster fly (Pollenia rudis), there are four other species which vary in size and colour, so it is possible to have mixed populations of flies in one building. By way of reassurance, whilst it is not pleasant having large numbers of flies in a property, they are not an indicator of poor hygiene.
Cluster flies can be found in large numbers particularly in upstairs rooms or loft spaces in the spring and autumn. In the autumn they will be coming in to hibernate over winter and in the spring they will be leaving hibernation to head outside. They are often seen clustering around windows at these times. Cluster flies are not considered a public health pest but more a nuisance due to their ability to accumulate in numbers of up to several thousand.
Cluster Fly Lifecycle
During the summer months cluster flies live and breed in fields and do not cause a particular 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 cluster flies hibernate in dry sheltered areas such as in log piles, under loose bark or in hollow trees. In the spring, they emerge and return to the grassy fields to continue their life cycle. 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, light-coloured walls. As temperatures begin to fall in the evenings they crawl into crevices for shelter, sometimes through window frames, but mainly under the eaves/fascia boards into the roof space. Once the cold weather sets in they will hibernate inside.
The flies tend to return to the same property year after year, and it is believed this is in part due to pheromones attracting them back to their favoured winter residence.
What Can Be Done to Prevent Cluster Flies in Your Home or Workplace?
It is almost impossible to totally proof a property against cluster flies as they are experts at crawling through confined spaces. It is therefore more effective to try and control numbers present rather than aim for total prevention.
In living areas, cluster flies can often be easily removed with a vacuum cleaner and aerosol fly killers might help deal with smaller numbers. Devices can also be fitted to windows to discretely capture flies in affected rooms area.
Use of insecticidal fogs, smoke generators or ULV treatment can be used to help reduce heavy infestations particularly in lofts. Other controls can include installing ultra-violet electronic fly killers in the loft, sticky fly boards or tape, or use of a residual insecticide spray. Each treatment method poses different risks so needs careful consideration. Extreme care must also be taken to check for the presence of bats before carrying out any treatment as it is illegal to disturb a bat roost.