Common pests

Cluster flies are common throughout Europe and Britain. The adults live harmlessly out of doors in summer but then may enter buildings, usually roof spaces and lofts, in autumn to hibernate, sometimes in vast numbers. They get their name from this habit of clustering in buildings.

Facts about cluster flies

The adult female lays eggs loosely on damp soil and beneath dead and rotting leaves. After about a week the larvae hatch from the eggs and seek earthworms, which they bore into. The fly maggots develop inside the earthworm and at or near the death of the earthworm the larvae bore their way out again and pupate in the soil. The adult flies are 6mm long, blackish, with fine golden hairs on the thorax. The life cycle of the cluster fly is very dependent on weather conditions but in Britain it seems that there are commonly two generations per year and in hot conditions, up to four generations per year are possible.

How do they affect me?

Cluster flies are of no consequence during summer and early autumn and only become a problem when the temperature drops and they seek protection in roof spaces and lofts. At this time of year they can infest homes, crawling through eaves, tiles and small holes in walls. It has been found that cluster flies often choose the same house or building year after year.

How do you control them?

Control methods for cluster flies are often ineffective. It is normally not possible to prevent flies from entering buildings. Whilst the flies can be a great nuisance they do not normally represent any health risk, although they may produce a rather sickly smell when clustered in large numbers and can be most upsetting or unnerving if disturbed during hibernation. Control of the flies inside a loft or attic is relatively simple, using a range of insecticidal sprays or smoke formulations. Sometimes a vacuum cleaner is all that is needed to collect the flies. Where the same building is used year after year a thorough cleansing of the affected area may help deter the flies from returning the following year