As the weather begins to cool in autumn, you may notice more flies gathering around windows, walls or roof spaces. These may not be ordinary house flies at all, but cluster flies looking for somewhere sheltered to spend the winter.
Cluster flies are a common seasonal nuisance in the UK. They usually spend the warmer months outdoors before moving towards buildings in autumn, where they look for protected spaces to overwinter.
Although they are not generally considered harmful, they can gather in surprisingly large numbers once they find their way into a property. Understanding why they appear and where they are likely to enter can make it easier to work out how to keep cluster flies out before they become a problem.
What Are Cluster Flies?
Cluster flies are a type of fly belonging to the Pollenia genus. They are usually greyish in colour, often with small golden or yellowish hairs across the body, and tend to move more slowly than the house flies you may see during summer.
Their name comes from their habit of gathering, or clustering, together in large numbers.
Unlike house flies, cluster flies do not usually breed inside buildings. They spend much of their life outdoors and only become a particular nuisance when they begin looking for somewhere protected to spend the colder months.
Why Do Cluster Flies Come Indoors in Autumn?
Cluster flies are most noticeable during autumn because falling temperatures encourage them to search for warm, dry and sheltered places.
Buildings provide plenty of suitable spaces, particularly lofts, roof voids, wall cavities and upper rooms. They may also gather around sunny walls and windows before finding their way indoors.
Once inside, cluster flies can remain relatively inactive throughout winter. On warmer or sunnier days they may become active again, which is why they can suddenly appear around windows even in the middle of the colder months.
When temperatures rise properly in spring, they generally begin making their way back outside.
Check for Gaps Around Windows and Doors
One of the simplest ways to reduce the chances of cluster flies entering is to check around your windows and doors.
Small cracks around frames, damaged seals and other gaps can provide an easy route indoors. Where appropriate, sealing these openings before autumn can make it more difficult for flies and other insects to gain access.
It is particularly worth checking older windows and doors where small gaps may have developed over time.
Look at Eaves, Rooflines and Other Openings
Windows and doors are not the only places cluster flies can get into a property.
Because they frequently overwinter in lofts and roof spaces, it is worth checking accessible areas around eaves, fascias, rooflines and external walls for obvious gaps or cracks.
Completely sealing a building against cluster flies is unlikely to be practical, and some openings are essential for ventilation. However, repairing unnecessary gaps can help reduce the number of potential entry points.
Use Flyscreens on Frequently Opened Windows
Autumn weather in the UK can still be surprisingly mild, which means windows may remain open long after summer has ended.
A properly fitted flyscreen creates a physical barrier across the opening, allowing fresh air into the room while helping prevent cluster flies and other flying insects from coming inside.
Flyscreens can therefore be particularly useful in rooms where windows are regularly opened for ventilation.
They will not prevent cluster flies entering through other parts of the building, so they should be considered one part of a wider approach rather than a complete solution.
Protect Suitable Vents and Openings With Insect Mesh
Some vents and other external openings may also provide access for insects.
Where appropriate, suitable insect mesh can be used to create a barrier while still allowing airflow through the opening.
It is important, however, that any mesh used is suitable for the application and does not restrict essential ventilation. Air bricks, mechanical ventilation systems and other specialist openings may have specific airflow requirements that need to be maintained.
Be Careful With Unscreened Windows and Doors
If cluster flies are gathering around your property, leaving windows and doors wide open can give them a straightforward route indoors.
Where an opening does not have a flyscreen, keeping it closed while large numbers of flies are active around the building may help reduce the number entering.
This is particularly useful during warm autumn days, when cluster flies can become more active and may gather on sunny walls and around windows.
Remove Cluster Flies That Are Already Inside
Prevention is useful, but sometimes the first sign of a problem is discovering cluster flies already inside the house.
If you are wondering how to get rid of cluster flies that have appeared around a window or room, a vacuum cleaner is often one of the simplest options for dealing with small numbers.
Avoid crushing them where possible, as this can leave marks on surfaces.
It is also worth remembering that removing the visible flies does not necessarily mean there are no others elsewhere in the building. If they are repeatedly appearing in the same area, there may be a larger group overwintering nearby.
Are Cluster Flies Harmful?
Finding large numbers of flies indoors is unpleasant, but cluster flies are generally regarded as a nuisance rather than a serious health risk.
Their presence is also not usually a sign that your home is dirty or unhygienic. Unlike some common household flies, they are not coming indoors because they are breeding in waste or food.
The main problem is their ability to gather in large numbers and return to suitable overwintering sites.
Why Are Cluster Flies Often Found Around Windows?
There is no single way to guarantee that cluster flies will never enter a building. They can make use of very small gaps and may reach areas that are difficult to inspect or protect.
However, checking for unnecessary gaps, repairing damaged seals, protecting suitable openings and using flyscreens across regularly opened windows and doors can all help reduce opportunities for them to get inside.
Taking action before temperatures begin to fall is particularly worthwhile if you have experienced cluster flies in previous years.
If you need help choosing a flyscreen or insect mesh for a particular window, door or opening, please get in touch. We will be happy to discuss your requirements and help you find a suitable solution for your home, workplace or commercial premises.
How to Keep Cluster Flies Out
A handful of cluster flies is very different from finding hundreds of them in a loft or roof space.
If large numbers appear every year, or you suspect there is a significant group overwintering somewhere within the property, professional pest control may be the most appropriate option.
A pest controller can identify where the flies are gathering and advise on suitable treatment as well as possible entry points that could be addressed before the following autumn.
Extra care may also be needed before treating lofts and roof spaces, particularly where protected wildlife such as bats could be present.
Get Professional Help With Large or Recurring Infestations
If you are dealing with large numbers of cluster flies, or they return to your property year after year, it may be worth contacting a professional pest controller.
For customers in our local area, we recommend Venables Pest Control, run by Chris Venables, who has extensive experience dealing with a wide range of pest problems.
If you are based elsewhere in the UK, you can use the British Pest Control Association’s Find a Pest Controller service to search for a qualified pest control company in your area.
Flyscreens and insect mesh can help reduce some of the ways cluster flies enter a property, particularly through open windows, doors and suitable ventilation openings. However, if flies are already established within a loft, roof space or other part of the building, professional treatment may be required.
If you would like advice on choosing a flyscreen or insect mesh to help protect an opening, please get in touch. We will be happy to discuss your requirements and help you find a suitable solution.
