The Cluster fly is often mistaken for the common house fly but they are more sluggish and often appear to fly very slowly. They are about 8mm in length with a dark grey-olive body with dense matted yellow hair.

Habitat The flies are common throughout the UK and, as the name implies, they are often found in large numbers hibernating in buildings. During the summer and early autumn these flies are of little consequence. As the weather cools they seek shelter in small nooks and crannies (e g window frames) They may also search further protection and frequently form vast clusters of several thousand flies in roof spaces and lofts.

It is not uncommon that a single house will be chosen year after year for this clustering phenomenon

Some large collections of flies produce a sickly smell and if warmed up during hibernation may emerge rather lazily and slow flying.

Life Cycle

The adult female lays eggs on and around damp soil, rotting leaves etc. about one week later the eggs hatch and the larvae actively seek earthworms to which they cling and subsequently burrow through the body wall. The maggots then develop inside the worm and eventually bore their way out to pupate in the soil.

Obviously the timing of the above is dependant on weather conditions with anything between two and four generation a year being possible.

These flies are of no particular harm but can be alarming because of their great numbers. The Council do not undertake treatment for these flies but the following might be of some use.

Control of the flies outside is almost impossible however once the flies are inside control is somewhat simpler. Any fly spray will have an instant knock down effect and a nozzle type vacuum cleaner can be used to collect the dead flies.