The cluster fly (Pollenia rudis) is common throughout the UK especially in the south.
It is slightly larger than the common house fly at approximately 7mm long with a 10mm wingspan. It is non-metallic with both light and dark grey areas on the abdomen. There are no distinct lines or stripes behind the head. The thorax is covered in yellowish hairs. It has large reddish compound eyes and holds its wings folded along its back. The female lays eggs loosely on damp soil and in leaf litter near the burrows of earthworms upon which it is parasitic. The tiny larvae hatch out after a week and seek out earthworms to feed upon. When full grown, the parasitic larvae leave the body of the host and enter the soil to pupate. The flies emerge and live outdoors. Depending on the weather conditions two generations are normal but up to four are possible each year.
Adult flies of the last generation of the year become numerous during the latter days of September to mid-October. As cold weather progresses, the adults seek protected places to spend the winter. Cluster flies emit a clustering pheromone that encourages them to hibernate together and for subsequent generations to follow to the same property. The flies will invade many building types and will enter roof spaces and voids via small gaps and crevices in the building fabric. They are commonly found in roof spaces, behind facia boards, around and within window frames, above suspended ceilings and under roof tiles, and particularly favour the warmer south and south west facing elevations of a building. Isolated properties in the country are especially prone to invasion, since they generally offer the only warm shelter for miles around.
It should be noted that although there is only one specific fly known as the cluster fly, i.e. Pollenia rudis, a number of other fly species exhibit the same behaviour of harbouring in buildings during the winter months although their life cycle is different. Although they can be extremely difficult to totally exclude from properties, a number of control measures are available which can help to lessen the burden of their presence. For small infestations around windows and behind curtains they can be removed with the use of a vacuum cleaner. For large infestations in loft spaces, treatment is usually carried out with an insecticidal space spray or smoke generator. Electronic fly killers adapted for the purpose can also be a beneficial means of control.