If you are suffering from a large quantity of flies in your house during the build-up to winter or even during winter months, then these are more than likely Cluster Flies.
A Cluster Fly looks very similar to a house fly, except Cluster Flies have a small yellow colouration on their thorax.
Cluster Flies live outside in fields and hedgerows during the spring and summer months. However, when the temperature starts to drop and we head towards autumn and winter, the flies tend to gather in large numbers and start to enter houses in order to find warm places in which they can hibernate. They enter buildings through small gaps in the property, such as cracks and crevices around window or door frames and gaps in the roof tiles.
Once inside, they cluster together in isolated places such as attics or false ceilings and begin to hibernate. Cluster Flies are attracted to light, so you may find them near windows and lamps at night. Cluster Flies are not harmful to you or your property; they do not bite or feed from foods carrying harmful bacteria, so they do not spread diseases or harmful pathogens. Cluster Flies are strictly parasitic on earthworms.
To treat Cluster Flies, the best technique is to ‘fog’ the loft with an insecticide. This is very effective in killing the flies within the loft space. However, a check needs to be carried out to make sure that there is no bat activity present.
To prevent future activity, you could try to seal as many holes around the exterior of the property as possible. However, given the size of flies, they may find another entry point into the loft or roof space.
