The Cluster Fly is from the blowfly family Calliphoridae. Their name derives from their habit of clustering together – where there is one there is likely to be many, many more. The good news is that they do not spread diseases that affect humans and that they do not lay their eggs on human food as other species of blowfly do.
Cluster flies are dark or olive in colour but have dark and light grey over their abdomen. They have stripes or lines behind their head making them quite distinctive. They tend to be slow moving and around 7mm in length.
When Cluster Flies emerge from hibernation in late spring they can be seen gathering in sunny areas such as roofs, walls and fences to warm up in the sun.
They will then lay their eggs in earthy areas such as compost heaps or leaf litter. They survive on a diet solely of earthworms as larvae. Adult flies will also feed on nectar.
Cluster flies are not really a nuisance until autumn when they start to come indoors to find a place to hibernate. They generally ‘cluster’ overwinter in inaccessible areas such as wall cavities and lofts or in unused rooms. They emit a sickly sweet smelling pheromone to attract more to their group – they also often return to the same site year after year – so if you have them now, it is best to deal with them promptly by professionals as they are difficult to get rid of without the proper methods.