Cluster Flies 
 

What do they look like

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. There are however a number of species which can give problems in urban areas.

The habit of laying eggs in damp material can on occasions result in foodstuffs in domestic pantries becoming infested.

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.

Control Methods

Control of these insects is at best difficult. Ideally one would wish to prevent them entering the building, but this is rarely possible. Fly-proofing a building is not 100% effective. However, caulking around windows, and sealing obvious entry points can help. When the insects are inside control can be achieved by a combination of physical and chemical methods. If your premises have been used for hibernation before, hanging insecticidal strips in the loft or attic may be helpful in reducing the numbers of flies.

If the cluster is accessible a vacuum cleaner can be used to collect them, and the vacuum bag can then be disposed of sealed inside a plastic bag. Aerosol fly killers will deal effectively with small clusters, but larger ones may need to be treated by a pest control contractor. However, it must be borne in mind that even treatment by a pest control contractor may not be 100% effective. The insecticide used may not kill those flies which are already hibernating at the time of the treatment, and as the area warms up they will emerge.

Local Authority Service

Cardiff County Council Pest Control Department does Not offer treatment for cluster flies. 
 

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