Found throughtout the UK, but most commonly in rural locations, cluster flies live and breed in fields during the summer months only becoming a household pest in the autumn and winter when they move indoors to hibernate, most often in roof spaces or wall voids. They congregate in large numbers and group together for warmth giving rise to their name.
In the spring the cluster breaks up giving rise to alarmingly large swarms of flies and on warmer winter days, cluster flies may also try to move towards the light being found in the home near windows.
This treatment in a Norfolk farmhouse used a fogging agent to kill the flies and provide a residual protection.