Cluster Flies are not easy to kill without insecticides because of the difficulty of accessing all their hibernating places. If you can get access to the flies' hibernating voids, then insecticide Smoke Bombs are vey effective. Make sure there are no bats in you loft as they will be killed and that is illegal.
Prevent Auto Dispensers are a means of dispensing natural pyrethrins into the atmosphere and give long term control. Dusting with insecticide powder using a bellows duster in any cracks and crevices where they may congregate.
The Cluster Fly overwinters in lofts, cellars and voids within the structure of buildings
slightly larger than the house fly
non-metallic, dark grey in colour
dark grey abdomen with lighter patches
characteristic golden hairs on body
wings overlap at the tip when at rest
adult lays her eggs singly in damp,moist soil or leaf litter
emerging maggot penetrates and enters a living earthworm
feeds on worm for 13 - 22 days
emerges from worm at pupal stage and pupates after 11-14 days
up to four generations over the summer months
hibernates as an adult fly in lofts etc
crowds on the internal side of windows trying to get outside with the first warm days of spring
Cluster flies tend to overwinter in lofts, cavity walls, cellars and other enclosed dark voids. There they can accumulate in very large numbers and subsequently appear with the first warm sunny days of spring.
They are usually first noticed at the windows or in conservatories as quite large, sluggish, dark coloured flies. Most simply exit the building the way they came in and go unnoticed by us.
In some cases many hundreds can find their way into rooms through ill-fitting window frames, especially so in the case of sash windows.