Why Cluster Flies Invade Lofts Every Autumn (and How to Stop Them Before They Hibernate)

As the crisp chill of autumn settles across London and Essex, a quieter invasion begins above our heads. Homeowners from Romford to Ilford, Barking to Dagenham start noticing slow-moving flies gathering around windows or buzzing faintly in the loft.

These aren’t houseflies — they’re cluster flies, and every year they seek out warm, dark roof spaces to hibernate through the winter.

In this guide, our BPCA-certified pest control team at Bugwise explains why cluster flies choose lofts, how long they stay, and how to get rid of them before they turn your attic into a winter retreat.

What Are Cluster Flies?

Cluster flies (Pollenia rudis) are larger and darker than houseflies, with golden hairs on their thorax that shimmer slightly in sunlight. They’re sometimes called attic flies or loft flies because of their tendency to gather in large numbers in roof voids and upper walls during colder months.

Unlike the common housefly, cluster flies don’t breed in rubbish or food waste. Instead, their larvae develop in soil — feeding on earthworms before emerging as adults in late summer.

When temperatures drop, they look for warmth and shelter — and our homes provide exactly that.

👉 For a detailed guide on identifying them, visit our post:
What Are Cluster Flies and How Do You Get Rid of a Cluster Fly Infestation?

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Why Your Loft Is the Perfect Hibernation Spot

Cluster flies are driven by instinct to find quiet, warm, and undisturbed environments for overwintering. Unfortunately, lofts tick every box:

• Warmth – Heat rising from the home below keeps loft spaces cosy through winter.
• Shelter – Lofts are dark, dry, and rarely disturbed.
• Entry points – Tiny gaps around soffits, vents, tiles, and window frames let them in easily.
• Pheromone trails – Once cluster flies have chosen a property, they release chemical signals attracting others year after year.

That’s why some homes experience repeat infestations every autumn — even after cleaning or DIY sprays.

Did you know?
Cluster flies are not a hygiene issue. They enter clean and tidy homes just as readily as older properties.

When Do Cluster Flies Appear in the UK?

From late August through October, cluster flies begin gathering on south-facing walls that warm in the afternoon sun. As evenings cool, they crawl through gaps and roof vents, settling in the loft to hibernate.

They remain dormant all winter, only becoming active on mild, sunny days — or when loft insulation and central heating create artificial warmth.

By early spring, they start emerging again, often gathering at loft windows or skylights, drawn to the light.

Are Cluster Flies Dangerous?

Not at all — cluster flies don’t bite, sting, or spread diseases. But their presence is far from pleasant:

• Large numbers can cause unsightly staining on walls or windows.
• The odour of dead flies in wall cavities can attract other pests like beetles.
• For landlords or commercial properties, they can lead to complaints and hygiene concerns.

So while they aren’t hazardous, they’re still best dealt with promptly and professionally.

How to Get Rid of Cluster Flies in Lofts

Removing cluster flies effectively requires more than just a quick spray. Here’s how to tackle them step by step:

1. Foggers or Smoke Bombs

Ideal for lofts or inaccessible spaces. The fog fills the area, killing visible and hidden flies instantly.

2. Residual Surface Sprays

After fogging, apply a residual insecticide spray to rafters, joists, and surrounding surfaces. This prevents re-infestation for up to two months.

3. Vacuuming

Regular vacuuming of visible flies and dead insects helps prevent odour build-up and secondary pests like larder beetles.

4. Seal Entry Points

Inspect soffits, vents, window frames, and tile gaps. Use silicone sealant or fine mesh to block entry routes.

5. Professional Pest Control

For recurring problems, BPCA-certified technicians can perform safe, targeted ULV fogging and exterior barrier treatments — ensuring thorough eradication.

Cluster fly infestations can be persistent, so a follow-up visit may be needed if new flies emerge over the following weeks.

👉 Learn more about specific removal methods in our in-depth post:
Can You Get Rid of a Cluster Fly Infestation?

Why Do Cluster Flies Keep Coming Back?

Even after treatment, cluster flies often return the following year.
That’s because they leave pheromone markers that attract future generations to the same building.

The best way to break this cycle is with preventive treatment in early autumn — applying residual pesticides to external walls before flies enter to hibernate.

Bugwise recommends scheduling this service between late August and mid-September, especially if you’ve had cluster flies before.

How to Prevent Cluster Fly Infestations

You can’t stop cluster flies existing outdoors — but you can stop them choosing your loft:

• Seal soffits, vents, and window gaps using fine mesh or sealant.
• Treat exterior walls and eaves with a residual insecticide before autumn.
• Improve loft ventilation; flies prefer stagnant, warm air.
• Remove any organic debris or dead insects.
• Keep roof voids well-lit when possible; they prefer darkness.

These steps, combined with professional fogging, will drastically reduce the chance of overwintering infestations.

Professional Cluster Fly Removal in London & Essex

At Bugwise Pest Control, we use advanced fogging equipment and professional-grade insecticides to eliminate cluster flies safely.

Our technicians are BPCA-registered, fully insured, and trained to handle loft treatments discreetly — whether it’s a domestic home, rental property, or commercial building.

Areas We Cover:
Romford, Barking, Ilford, Dagenham, Chigwell, Loughton, Woodford, Hornchurch, Upminster, and across East London & Essex.

We provide:

• One-off cluster fly removal
• Preventive exterior treatments
• Follow-up inspections
• 30-day guarantee