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Cluster Fly

Cluster flies will start to enter buildings from August through to late November and it is normally a drop in temperature that starts them entering the house.  You may often encounter Green Cluster flies (Dasyphora cyanella) along with the common Cluster flies (Pollenia rudis). 

You are advised to treat the attic area with a smoke generator placed on a heatproof surface.  Make sure that all water tanks are sealed or covered and that no bats are in residence, isolate all smoke alarms and in high-density residential areas advise the local fire brigade. 

If any flies have been found in sash boxes or around window frames a treatment with a residual insecticide will also help.  In the spring they start to find their way down from the loft into the house, proofing the loft with draft excluder can help to keep them out.

External treatments are a waste of time, but window frames and other obvious entry points can be sealed with a caulking gun, and attention should be paid to any gaps at the eaves and fascia boards and with poor fitting tiles or slates this will not stop the flies but may reduce the problem. 

Finally, a follow up visit in the spring would be adviseable, as this is when any survivors start to leave harbourages warmed by the sun during March to May.