Cluster Flies
These flies are commonly found throughout Europe and the UK the name refers to their habit of clustering together in large numbers and hibernating in buildings. There are several species found in the UK but they are often found in mixed swarms. The sizes vary between species from 3 to 8 mm, but the commonest species are 3 to 4mm long.
Life Cycle
Eggs are laid on damp soil or beneath dead and rotting leaves. The larvae of one species seek earthworms and bore through the body wall and are therefore more common in the country. When the earthworm dies the larvae bore out again and pupate in the soil. The adult flies feed on the nectar of garden and wild flowers.
As a free-living insect the length of the life cycle is weather dependent. In Britain two generations are common, but in hot summers up to four are possible. During the summer and early autumn cluster flies are of no importance. As the season cools they seek shelter in nooks and crannies in houses and other buildings.
When the temperature drops further they will seek greater protection and often form very large clusters in lofts or roof spaces. Often nuisance is caused by the presence of these large clusters outside buildings while they try to find a way in to hibernate. These masses may consist of several thousand flies clustered together. These large clusters produce a sickly smell. If the flies are warmed up during their hibernation they may emerge rather lazily and be a source of nuisance within the building.
It has been observed that a single building in a row of apparently identical buildings will be selected for clustering year after year. This is no indication of poor hygiene standards and no explanation for the phenomenon has been found.