Cluster fly infestations mostly occur in rural or semi-rural areas and are often found in lofts, roof spaces and sash window boxes. Once a cluster of flies has found a suitable place to settle they emit a pheromone which attracts others of its species. If has been known that if the 'clusters' are large enough to detect the pheromone, they will return year on year in even greater numbers. The presence of cluster flies is usually first detected on a warm, sunny day when the flies become active and are often then seen in large numbers at windows or in little-used rooms.