Cluster Flies:

What You Need to Know

Understanding Cluster Flies

Cluster flies are a recurring nuisance in many commercial buildings, particularly in roof voids, service areas, and other low-traffic spaces. While they do not pose a health risk or indicate poor hygiene, their presence in large numbers can disrupt operations, create an unpleasant environment for staff or visitors, and impact the professional appearance of a site.

What Are Cluster Flies?

The most common species of cluster fly in the UK is Pollenia rudis. Unlike typical household flies, cluster flies have a unique life cycle. Their larvae develop inside earthworms, feeding internally until they mature and emerge as adult flies. Adults measure approximately 7 mm in length and can be identified by their dark colouring and distinctive yellow-gold hairs on the thorax.

In the UK climate, cluster flies typically have two generations per year, although warmer weather conditions can increase this number to six.

Why Are Cluster Flies a Problem for Commercial Premises?

Cluster flies pose a seasonal challenge, particularly in late summer and autumn. During this time, they are commonly seen congregating on the sunniest sides of buildings, typically south or south-west facing walls. As temperatures drop, the flies seek warmth and shelter to overwinter, often infiltrating roof voids, attics, service risers, and cavity walls.

Once inside, they form large, sluggish clusters in hidden areas, often returning to the same location year after year. While they do not transmit disease or cause structural damage, their presence can result in:

• Unpleasant odours and stains on paintwork or ceilings
• Disruption to staff or occupants in office and accommodation spaces
• Negative visual impact for customers or visitors
• Repeated seasonal infestations without professional intervention

In sensitive industries such as food manufacturing, hospitality, and healthcare, even a seemingly harmless pest can breach audit standards and damage reputational trust.

Can You Prevent Cluster Flies?

Unfortunately, completely preventing cluster fly entry is extremely difficult, as they exploit tiny cracks and gaps in building structures. However, several measures can help reduce the risk:

• Sealing obvious entry points around rooflines, vents, soffits and window frames
• Installing fine mesh screening where appropriate
• Conducting regular inspections of attics, lofts, and unused void spaces

Despite best efforts, large infestations often require professional treatment. Because cluster flies are sluggish during hibernation, they can sometimes be removed using a vacuum cleaner, although this is only a short-term fix and not suitable for large populations.

Professional Treatment Options

For high-traffic or heavily affected commercial areas, Ultra-Low Volume (ULV) fogging treatments can offer a fast and effective knockdown. These treatments are best applied in loft voids and roof spaces before cluster flies settle for the winter or as a reactive solution during infestations.

Important: Before any loft or attic treatment is carried out, a qualified pest control technician must assess for the presence of protected species such as bats, as well as open water tanks that may be exposed to chemical vapours.

Contact the Experts

If your business is experiencing issues with cluster flies or recurring seasonal infestations, contact the OCS pest control team. Our trained technicians provide safe, compliant, and discreet pest control solutions tailored to commercial environments – helping you maintain operational continuity and brand integrity all year round.