Cluster Fly

Cluster flies are found commonly throughout Europe and the UK. The common name refers to its habit of clustering and hibernating in numbers in buildings.

The adult female lays eggs on and around damp soil beneath dead and rotting leaves etc.

The life cycle of a fly is very dependent on weather conditions. In Britain it seems that two generations per year are common, but in hot summers up to four generations per year are possible.

During the summer and early autumn these flies are of no consequence. As temperatures drop they search for shelter and frequently form vast clustering masses in roof spaces and lofts, with several thousand flies clustered together. Often a single house or building in a row of similar buildings will be chosen year after year.

Green Cluster Fly

A species very similar in appearance to the quite common greenbottle.

Yellow Swarming Fly

yellow swarming fly.jpg

This is a much smaller species of hibernating fly, seldom being longer than 3mm with a typical fly appearance - fairly large rounded wings in relation with its yellowish body, it may be confused with the fruit fly.

Autumn Fly

A fly very similar to the house fly in appearance, distinguishable by its clearly defined grey and yellow abdomen.

Window Fly

The adult fly is about 3-4mm in length reminiscent of a small crane fly, with noticeable long spindly legs. The wings with a fairly complex venation may sometimes be seen to have cloudy grey-brown patterns. It is one of the more common species to enter houses and other buildings.


Control methods for cluster and swarming flies are often ineffective and at best incomplete.

It is often not possible to keep flies from entering premises and it is recognised that while they are a great nuisance on occasion, are unlikely to represent a health risk. They do not breed indoors and are not indicative to poor hygiene.

Control of flies outdoors in their breeding areas is considered impractical.

Once flies are inside, for example a loft or attic, control methods are relatively simple with physical methods and a range of insecticides and formulations.

Occasionally, a vacuum cleaner (nozzle type) can be used as a sole control method and the collected flies can be disposed of in a sealed bag i.e., a vacuum bag inside a polythene bag. Dichlorvos strips may have some value in killing flies in well sealed loft spaces.

CAUTION:- There is a possibility of bats living in areas in buildings used by Cluster flies. Care must be taken to look for signs of bat presence, and in cases of doubt contact the Bat Conservation Trust Helpline on 0171 627 8822.

Date of last review: 01 September 2015