Cluster flies are so named due to their habit of forming ‘clusters’ when hibernating. They are also known as ‘attic flies’ as they are usually found in roof spaces and lofts. They are frequently confused with ‘autumn flies’, which also have a tendency to gather in large numbers in the autumn/winter months. They live outdoors during summer and lay their eggs in soil, compost and leaf litter, but when the temperature drops during the autumn months (from late August to November), they migrate indoors seeking a protected place to overwinter. They favour lofts and roof spaces and can often be found in vast numbers in roof voids. They enter by crawling through small openings in walls or roof tiles, which makes them very difficult to guard against.
The most common species, Pollenia rudis, is slightly larger than a house fly at around 6-10mm in length. Unlike a house fly, they are rather sluggish in flight. Cluster flies can be recognised by their distinctive stripes behind the head, short golden hairs on their thorax and a mottled light/dark grey abdomen. Other species include Dasyphora cyanella, which is similar to a greenbottle, although less shiny. Autumn flies (Musca autumnalis) are also commonly found in large numbers, sometimes with cluster fly colonies – the males of the species have an orange abdomen with a dark central stripe; females are similar to a common house fly. Although annoying, cluster flies pose no risk to humans as they do not lay eggs in human food. They are classed as a pest because of the numbers that 'cluster' together, which can be very large.
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.
Trying to prevent cluster flies from entering your home by sealing window frames, doors and any other potential entry points will help but can be costly and is rarely effective.
Cluster fly control is best achieved by finding and treating resting
sites and harbourages by applying a residual cypermethrin-based insecticide to alighting surfaces and by activating foggers or smoke bombs to fumigate the area and flush them out. If initial cluster fly treatment is carried out in the autumn months, a follow-up treatment in the spring is often advisable.
