Cluster flies are not a health risk, they do not spread disease and they do not lay eggs in your house. But they are a nuisance because they come in large swarms. You will notice them as autumn turns to winter, when they come inside to hibernate, and also in early spring when they leave.
Conventional fly sprays can be effective against visible flies, but will not touch the ones hidden in cracks and crevices. A timed insecticidal spray from a dispenser such as the Prevent Auto will kill them on an ongoing basis. Cluster flies are regularly found in loft spaces, but can migrate to rooms below. A Cluster Buster fly trap is a chemical free way to eradicate them from windows where they like to ‘cluster’.