If cluster flies seem to appear in your home or business at roughly the same time every autumn, you’re probably wondering the same thing, why do they keep coming back?
Unlike many pest problems, recurring cluster fly activity isn’t necessarily connected to food, waste or poor hygiene. Instead, cluster flies are looking for somewhere warm, dry and sheltered where they can spend the colder months. Unfortunately, once a building provides the right conditions, it can experience the same problem year after year.
So why are some properties particularly attractive to cluster flies, where are they hiding, and what can you do to reduce the chances of another autumn infestation?
What Are Cluster Flies?
Cluster flies are slightly larger than the common housefly and are generally slower moving. Their name comes from their tendency to gather, or ‘cluster’, together in large numbers when looking for somewhere to overwinter.
During spring and summer, cluster flies spend much of their time outdoors. As temperatures begin to fall in late summer and autumn, however, their behaviour changes. They start looking for protected places where they can remain relatively undisturbed through the colder months and our buildings can provide exactly what they need.
Why Do Cluster Flies Come Inside Every Autumn?
For cluster flies, your building can effectively become their winter accommodation. Warm, dry and sheltered spaces such as lofts, roof voids and wall cavities provide protection from colder conditions outside.
Before entering, you may notice flies gathering on warm, sunny external walls. From here, they can exploit small gaps and openings around a building to reach protected areas inside. Potential access points can include:
- Gaps around windows and frames
- Rooflines and eaves
- Gaps beneath roof tiles
- Vents and air bricks
- Cracks and gaps within the building fabric
- Openings around pipework and other services
Once inside, they can congregate in surprisingly large numbers. This is why a cluster fly problem can seem to appear very suddenly. You may only see a handful initially, while considerably more are hidden away elsewhere within the property.
Why Do Cluster Flies Keep Coming Back to the Same Building?
This is one of the most frustrating aspects of cluster flies. If your building has provided suitable overwintering conditions previously, it may continue to provide those same conditions the following year.
Certain properties can be particularly susceptible because of their location, construction, orientation and the number of suitable sheltered spaces available. The problem can therefore become seasonal.
September and autumn arrive and so do the cluster flies.
Simply removing the flies you can see doesn’t necessarily address the characteristics of the building that made it attractive to them in the first place.
If you’ve experienced significant numbers for several consecutive years, it’s worth considering the problem as a recurring seasonal pest issue rather than a series of unrelated infestations.
Where Do Cluster Flies Hide?
One reason cluster flies can be difficult to manage is that they don’t necessarily remain in easily accessible areas. Common places to find them include:
- Lofts and roof spaces
- Wall cavities
- Around window frames
- Eaves and rooflines
- Upper floors
- Quiet storage areas
- Other dark and undisturbed spaces
During colder periods they may remain largely inactive, which can make it seem as though the problem has disappeared. Then the heating comes on, or an unusually warm and sunny winter day raises the temperature. Suddenly, the flies become active again.
This can lead to the slightly confusing experience of finding flies around windows in the middle of winter when you thought the problem had ended months earlier.
Are Cluster Flies a Sign of Poor Hygiene?
No. This is an important distinction, particularly for commercial premises. Cluster flies aren’t entering because your property is dirty. They’re primarily looking for somewhere suitable to overwinter. However, that doesn’t mean their presence isn’t a problem.
Large numbers of flies around windows, offices, accommodation or customer-facing areas can be unpleasant and disruptive. Customers, employees and visitors are also unlikely to know why the flies are there.
For hotels, offices, schools and other commercial environments, recurring cluster fly activity can therefore become an issue that needs professional management even though hygiene isn’t the underlying cause.
How Can You Prevent Cluster Flies Coming Back?
Reducing access to a building can help. Where practical, gaps and defects around windows, rooflines and other potential entry points should be identified and appropriately sealed, while ensuring that necessary building ventilation isn’t compromised.
However, cluster fly proofing isn’t always straightforward. Their ability to exploit small openings, combined with the complexity of roofs, lofts and older buildings, can make identifying every potential route difficult. This is particularly true for properties that have suffered substantial recurring infestations.
That’s where understanding where the flies are gathering, how they may be gaining access and when the seasonal activity begins becomes important. If you already know your property suffers from cluster flies every autumn, don’t wait until large numbers are appearing around the windows again before thinking about the problem.
What Should You Do if You Have Cluster Flies?
A few individual flies can often be removed with a vacuum cleaner. However, if you’re discovering large numbers or experiencing the same problem every year, professional pest control can help establish the extent of the infestation and determine an appropriate treatment.
Our technicians can assess cluster fly activity, identify likely harbourage areas and advise on treatment and preventative measures appropriate to your property. For businesses, we can also consider cluster flies as part of a wider pest-management programme, helping you prepare for known seasonal pest pressures rather than simply reacting when they appear.
Get Help With Recurring Cluster Flies
If autumn means cluster flies in your building every year, it probably isn’t a coincidence. Your property may simply be providing the warm, dry and sheltered conditions they are looking for.
Understanding why they’re there and where they’re getting in, is the first step towards managing the problem. We provide professional cluster fly control for homes and businesses across Wiltshire and the South West. If you’ve started noticing cluster flies gathering around your property, or you already know from previous years that they’re likely to return, contact ASW Pest Control for professional advice and treatment.
