Cluster Flies

Biology

Cluster flies are parasites of earthworms and breed outdoors in gardens and fields during the spring and summer periods. Eggs are laid in damp soil and rotting vegetation and hatch in approximately 7 days. The larvae seek out and parasitise earthworms which are eventually killed. The larvae then pupate in the soil for about two weeks before emerging as flies. Adult cluster flies live out of doors in the summer but seek refuge (usually loft spaces) in the autumn to hibernate, sometimes in vast numbers to emerge in the spring.

Cluster flies are extremely difficult to control, and often cannot be fully controlled. It is common for the problem to arise year after year, sometimes twice a year. 

There are numerous types of control which include knockdown treatment (removing visible flies from your property with an aerosol) and more long term - trying to reduce the number of flies entering the property.  For the most efficient control it is recommended that you use a combination of methods.

Using fly sprays, swats, sticky papers, UV insectocuters (they use an ultraviolet light to attract flies, then electrocute them, take care not to install over food preparation areas and do not install in lofts as they may be a fire hazard, recent models incorporate a sticky board instead of electrocution) and other such methods can be effective at reducing the visible population.

Flies often gain access to lofts via air bricks and gaps in the structure, so placing fly papers in lofts can prove successful, as well as vacuuming any visible cracks or crevices to remove flies/debris.

The best advice would be to contact a qualified pest control contractor.