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Cluster Fly Entry Points

Cluster Fly Entry Points

Protecting your home from Cluster Flies

Protecting your home from flies may sound like a mammoth task but using the correct products will dramatically increase your chances and start you on the right route to eradication. When temperatures start to deteriorate, cluster flies look for harbourage from adverse weather conditions and will make their way into residential and commercial properties. They tend to seek refuge in lofts/roof spaces and farm buildings and will successfully hibernate until temperatures increases, unless they are disturbed. It widely is thought that cluster flies leave pheromones in properties to encourage other flies to return year on year, however, this is still under debate.


Due to the size of cluster flies (10-15mm) they can enter buildings through tiny gaps, which can make preventing access rather problematic. Open windows are easy access points for flies, they will travel from the garden where they reside and enter the building through the open window. There is a simple fix, closing the window and keeping it closed though autumn and winter will remove the entry point. Trickle vents are put on windows to allow natural ventilation to the property but also double as an entry point for cluster flies and other flying insects.  If they are not in place to allow ventilation then closing the vent will seal the entry point, alternatively, if they are needed to allow ventilation opening them for short bursts will allow ventilation but keep the majority of flies out. As Pest Expert Formula C spray leave residual protection for 12 weeks, spraying the trickle vent will leave protection. If a fly tried to enter through a trickle vent which had been sprayed they would come into contact with the Cypermethrin and would be killed before it entering the building. Another entry point to buildings for cluster flies is through cracks in roof tiles and felt. The majority of cluster fly infestations originate in loft spaces, removing entry points in the roof can tremendously help to prevent access for cluster flies. Replacing any torn or loose felt and cracked tiles will prevent flies entering the loft through the roof. Any DIY need to be done to an adequate standard or other problems with the property may occur. Weep holes are gaps in the mortar and they are in place to allow water to drain from the brickwork and to allow ventilation to the building. However, they are also an entry point for flies and insects. Entering weep inserts or brick vents will remove the entry point whilst still allowing sufficient ventilation and drainage from the property. If for any reason these entry points cannot be sealed and removed then are insecticidal treatments can be used which leave residual protection behind. The Pest Expert Formula C spray contains the powerful active ingredient Cypermethrin. This can be sprayed on window frames and sills, exposed beams, walls and any alighting surfaces. Formula C will kill flies on contact but once the solution is dry it will leave a clear film of Cypermethrin behind. The film is harmless to people and pets but lethal to insects such as cluster flies. Once they come into contact with the residual film they will die. In areas where spraying a water-based solution is not suitable, e.g. around electrical sockets, the Pest Expert Formula P powder can be used. This contains the synthetic pyrethroid Permethrin which, like the Formula C spray, will leave behind residual protection. If cluster flies entered the property and came into contact with the powder they would perish. The use of fumigators will kill a whole population of flies in the space of three hours and are great tools to ‘clean the slate’ before applying residual insecticides. At Pest Expert we have two fumigators, our Formula P Fogger and the Formula P smoke bomb. They work in the same way in that they both fill the room with an insecticide which kills cluster flies. However, the fogger releases an aerosol whereas the smoke bomb emits a smoke. For knockdown of flies in loft spaces, foggers may be more suited as a naked flame is not needed and smoke alarms will not be set off when using them.

All of the entry points outlined can be sealed with appropriate products which will help to prevent access to the building for cluster flies. As stated due to the size of cluster flies they can enter through the smallest of gaps, inevitably some flies will make their way into the building. Using both a reactive and residual treatment process will put you in a strong position to be able to eradicate any sign of infestation and help to prevent failure and re-infestation occurring.