Cluster Flies in the UK: How to Identify, Prevent, and Control Them

Preventing and Managing Cluster Fly Infestations in UK Homes

Cluster flies (Pollenia rudis) are a common seasonal pest in the UK, particularly during the autumn and winter months. Unlike typical house flies, cluster flies are not usually linked to unhygienic environments. These flies primarily live outdoors but seek indoor shelter to overwinter, often entering through small gaps in roofs, walls, and windows.

Once inside, cluster flies gather in large numbers—typically in lofts, attics, and unused rooms. Though they pose no direct health threat, their presence can be disruptive, especially when they become active during unseasonably warm winter days or early spring.

Key Identification Features:

• Sluggish movement

• Dark grey, non-metallic body with golden hairs

• Grouping behaviour in warm, undisturbed spaces

  
  

Causes of Indoor Infestation:

As temperatures drop in autumn, cluster flies look for warm, sheltered environments to hibernate. Buildings in rural areas, especially near fields or woodland, are more susceptible due to the natural habitat of the flies. Entry points include gaps in tiles, eaves, fascia boards, and around window frames.

Signs of Infestation:

• Clusters of slow-moving flies around windows, especially on upper floors

• Audible buzzing from loft spaces or cavity walls

• Accumulations of dead flies near windowsills or in attic corners

  
  

Prevention Strategies:

• Seal access points: Inspect exterior walls, windows, and roofs for entry holes and seal them with caulk or appropriate materials.

• Install mesh screens: Fly screens on vents and windows can help block entry.

• Reduce attraction: Clean and regularly inspect loft spaces, which often provide ideal overwintering conditions.

  
  

Management Methods:

• Vacuuming is effective for small groups.

• Residual insecticides may be used in roof voids, but should be handled carefully.

• ULV fogging and misting treatments can treat larger spaces effectively.

• Traps and electric fly killers may reduce visible numbers indoors but won’t resolve the source.

  
  

For severe or repeated infestations, professional assessment and treatment is recommended to manage hidden hibernation sites and provide long-term exclusion strategies.

Reference: Bowles, M. (2024). Preventing Cluster Fly Infestations in Homes. MB Pest Controllers Blog. https://www.mbpestcontrollers.co.uk/blog/cluster-fly-prevention