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There are more than 110,000 species of flies with the largest being around 6cms long and the smallest only around 0.15mm long. Some are more bothersome than others – depending on the season – and some carry more germs than others.

In late summer if staff in top-floor offices are complaining about flies – or your loft space is overrun with insects – it’s probably cluster flies. 

Cluster flies such as Common cluster fly Pollenia rudis are ‘field’ flies – they lay eggs in the soil and the larvae burrow into earthworms where they pupate. As the weather becomes cooler, they seek out shelter in nooks and crannies in buildings to hibernate in large numbers. Their favourite places are roof spaces and lofts and people living in houses in the country are often plagued with swarms of cluster flies in their lofts. 

Normally, several thousand flies will cluster together – hence the name cluster flies. They will usually infest the highest warm point available – and this is often the top floor of offices or loft spaces in people’s homes.

Cleankill Pest Control’s Managing Director Paul Bates explains: “As cluster flies don’t breed indoors, controlling them outside is impractical. It’s often impossible to keep flies from entering premises, no matter how well sealed or modern the property is. The flies enter through air vents, breathing points in mortar, roof tiles and even through gaps in modern double glazing.”

Insecticidal spraying around window and door frames and other entry points can help. Once flies are inside, control can be achieved with a range of insecticides and electronic ultra-violet fly machines available from Cleankill.

While there is a specific species of Common cluster fly, there are other species of swarming flies which overwinter.

The life cycle of the cluster fly is very much dependent on the prevailing weather conditions and, in this country, two generations a year are usual but, in hot summers, up to four generations per year are possible.

Curiously, a single house or one building in a row of similar buildings is often chosen year after year for this clustering phenomenon.

With offices across the South East in Kent, Sussex, Surrey and in Bristol and Buckinghamshire, Cleankill Pest Control is an award-winning pest control company and was last year named Company of the Year in the British Pest Management Awards. Cleankill is the only pest control company in England to hold the Investors in People Gold accreditation.

To book a free survey or a no obligation appointment to compare Cleankill’s costs with your current provider go to or call 0800 056 5477.

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