Cluster flies in your house or loft: why they appear and what to do

A bedroom window suddenly covered in flies can be unsettling, especially when you have kept the windows closed and cannot find anything attracting them. If it happens in autumn or on a mild winter day, cluster flies may be the explanation.

Cluster flies enter buildings to shelter through winter. They often gather in lofts, roof spaces and window frames, then appear indoors when warmer conditions make them active.

If you think you are suffering from a cluster fly infestation, this guide explains what is happening, what you can do now and what to expect if you arrange treatment.

The key things to know

• They are seeking winter shelter. Common cluster flies and yellow swarming flies do not breed inside your home, and they do not bite or sting.
• A few flies may be manageable without treatment. Removing accessible flies with a vacuum or domestic fly spray can be enough if they are causing little disruption.
• Large numbers? We can help now. We offer ad hoc treatments to reduce heavy activity and make affected rooms more tolerable.
• For recurring problems, we usually recommend a later seasonal treatment. This is generally after the first frost, often in November, depending on the weather and whether flies are still arriving.
• The aim is to reduce the problem. Further flies may emerge or enter after treatment, and affected buildings may need ongoing seasonal management.
• Complete proofing is rarely practical. Cluster flies can get through small gaps in the building, such as under tiles, under facias and soffits and around windows.

If you are unsure whether the flies are cluster flies, we can offer an inspection to confirm identification.

What are cluster flies?

Cluster flies take their name from their habit of gathering together in sheltered places over winter. The familiar common cluster fly is a dark fly with fine golden hairs on its body. It can look like a house fly at first glance, the overwhelming numbers are the time of year are generally the give away.

“Cluster fly” is also used more broadly in pest control for several flies that behave this way. They do not all look identical or share the same life cycle.

Two types we commonly encounter are the common cluster fly and the yellow swarming fly.

Common Cluster Fly – Pollenia rudis

The common cluster fly belongs to a group of similar-looking Pollenia species. Adults are roughly 8 mm long, with a dark bodies with yellowish or golden hairs. Identifying the exact species may require closer examination.

Their early life takes place outdoors: eggs are laid in soil and the larvae parasitise earthworms. The adult flies later seek somewhere sheltered to spend winter. Your loft provides accommodation, rather than a breeding site. BPCA cluster fly guide

Yellow Swarming Fly – Thaumatomyia notata

The yellow swarming fly, Thaumatomyia notata, is sometimes called a yellow swarming cluster fly. At around 3–4 mm long, it is considerably smaller, with yellow colouring and dark markings.

It belongs to the grass fly family. Its larvae feed mainly on root aphids in the soil, rather than earthworms. Adults can congregate on buildings in very large numbers before slipping into gaps to overwinter.

There are other cluster fly species, and other kinds of fly can also overwinter in lofts. Some bluebottles, for example, use buildings for winter shelter. A fly in the loft is therefore not automatically a common cluster fly: identification matters, particularly if its appearance or the pattern of activity is unusual.

Why are there suddenly so many flies in my house?

There are two usual explanations: flies are moving into the building in autumn, or flies already sheltering inside have become active.

In autumn, adults gather on sunny walls and around windows before finding their way into crevices. Later, warmth from sunshine or the house can disturb resting flies. They emerge from concealed spaces and may end up in bedrooms and other occupied rooms.

That sudden appearance can give the impression that a batch has just hatched indoors. With cluster flies, it is generally overwintering adults becoming visible.

In most cluster fly cases we attend, the house is near grassed areas: fields, pasture, playing fields or substantial gardens.

When is cluster fly season in the UK?

The main period for flies entering buildings is late summer and autumn, with activity often particularly noticeable during September and October. The precise timing depends on the weather and location.

They may then remain tucked away for weeks, appearing again during mild winter weather or as spring approaches.

We notice another peak in spring, when emerging flies looking to leave the property become apparent.

Do cluster flies breed indoors or mean something has died?

Common cluster flies and yellow swarming flies do not reproduce inside homes or other buildings. The adults are using the building as winter shelter, while their immature stages develop outdoors.

An identified cluster fly problem is not evidence of a dead animal in the loft or poor housekeeping. However, other flies have different breeding habits. If you find maggots, a strong decay smell or large numbers of a different-looking fly, have the cause checked rather than assuming it is the usual seasonal invasion.

Are cluster flies harmful?

Common cluster flies and yellow swarming flies do not bite or sting. They are primarily nuisance pests, although hundreds of flies in a bedroom can be a substantial nuisance.

Typical frustrations include repeated cleaning, flies gathering at windows and dead insects accumulating in out-of-the-way spaces. Remove accessible dead flies: accumulations can provide food for scavenging insects such as carpet beetles. Cluster flies do not eat your roof timbers or cause direct damage themselves.

How do you get rid of cluster flies?

The right response depends on how many there are, where they are sheltering and how much disruption they cause. There is no single number of flies that means every property needs treatment.

For small numbers, remove the visible flies

A vacuum cleaner is often the most practical starting point. Use it to collect accessible flies around windows and other affected areas, and empty it promptly into a sealed bag for disposal outdoors. Avoid squashing flies against curtains or painted surfaces. Domestic fly can also help reduce low numbers.

This deals with the insects you can reach. It will not remove a hidden population elsewhere in the building. Only enter a loft if access and footing are safe.

For heavy activity, consider professional treatment

If flies are repeatedly covering windowsills or making rooms difficult to use, we offer ad hoc cluster fly treatments to bring the numbers down and make the problem more tolerable.

You do not have to struggle through heavy activity while waiting for the ideal seasonal treatment date. An earlier visit can provide useful relief, provided expectations are clear: more flies may still arrive, and inaccessible resting flies may emerge afterwards.

Professional treatment starts with identifying the flies and assessing the affected spaces. The method depends on the building, access and where the insects are sheltering. Treatment of exposed flies cannot be assumed to reach every concealed gap.

Before any insecticide treatment in a loft, tell the technician about known or suspected bats. The space needs to be assessed for suitability, and preparation, ventilation and re-entry instructions must be followed.

When is the best time to treat cluster flies?

For properties with a recurring problem, we generally suggest treating affected spaces after the first frost, usually in November, adjusting the visit to local weather and fly activity.

The aim is to tackle the flies once the main autumn movement into the building has slowed. Treating while substantial numbers are still arriving may provide short-term relief, but the space can soon contain more flies.

We think of these treatments as a way to “clear the decks”: reduce the accumulated flies and make the winter period more manageable.

Where activity is particularly heavy, the approach may therefore involve an ad hoc treatment for relief, followed by a later treatment once the seasonal influx has settled.

Will one treatment stop cluster flies coming back?

Usually, a recurring cluster fly problem needs managing over time. Treatment can reduce the flies in the building, but it does not remove the outdoor population or every possible entry point.

Clients who follow our annual treatment routine often report numbers reduce year to year.

For a house that regularly attracts overwintering flies, an annual treatment when needed is a realistic way to manage the problem. If activity remains high after treatment, let our office know so we can review it with you.

What should I expect before and after treatment?

Before the visit

Tell our office which rooms are affected.

We would provide preparation instructions for your particular treatment. These should clarify which areas need to be accessible, whether people and pets need to leave any part of the property, and when the treated space can be used again. Mention any known or suspected bats before treatment is arranged.

After the visit

The purpose of treatment is to reduce fly activity in the affected spaces. It does not seal the building or necessarily reach every fly in a concealed void. You may find dead flies afterwards, and some live flies can still appear.

Can you proof a house against cluster flies?

Complete proofing is rarely practical in the properties we see. These flies can enter through very small holes and gaps, and a typical roof or older window assembly contains numerous potential routes. Sash windows are a common culplit.

That does not mean repairs are pointless. Closing obvious defects can help reduce the numbers gaining access.

However, attempting to find and seal every route through roof details, eaves, frames and concealed voids can become extensive work with no guarantee of complete exclusion. Necessary ventilation must also be maintained. Proofing should be proportionate to the building and the problem.

More questions about cluster flies

Why do they return to my house but not my neighbour’s?

Buildings differ in their exposure to sunlight, available gaps and sheltered spaces. One property can attract substantial numbers while another nearby has little trouble. It is not a verdict on how clean either house is.

Will cluster flies go away by themselves?

Surviving overwintering flies generally leave as conditions warm in spring. Some die indoors. Waiting may be reasonable for a few occasional flies, but it can mean living with repeated activity for months if the numbers are high.

Why am I finding dead flies on the windowsill?

Flies that become active indoors may reach the window but fail to get outside. Dead flies there can be part of an overwintering problem, although identification is still needed if the cause is unclear.

Why am I still seeing flies after treatment?

Flies may emerge from concealed spaces that treatment did not reach, and further flies may enter during autumn.

Should I treat the lawn to stop cluster flies?

We do not recommend treating lawns or trying to remove earthworms as a solution. Flies can arrive from beyond your garden, and outdoor breeding-area treatment is not a practical way to manage a household infestation. Focus on the affected building.

Need help with cluster flies?

If your property has a cluster fly problem, please contact The Pest Company to discuss possible solutions.

To help us advise you, please include:

• Your postcode and the type of property.
• Which rooms or windows are affected, and whether loft access is available.
• When the flies appeared and roughly how many you are seeing, for example a few each day or windows repeatedly covered.
• Whether this has happened in previous years and any recent treatment.
• A clear close-up photograph of a fly, if you can take one safely.

You can also read about our cluster fly treatment service.