The cluster fly is approximately 8mm long and recognised by its abdominal patterns and dark grey/olive thorax with dense yellow matted hairs. The adults are frequently mistaken for the housefly, although they are somewhat darker and larger and fold one wing over the other when at rest. They are also much more sluggish in their movements and fly much slower.
Found mainly throughout Europe and the U.K. The common name refers to its habit of clustering and hibernating in numbers inside buildings.
During the summer and early autumn, these flies are of no consequence. As the season cools, they seek shelter in nooks and crannies in buildings. As temperatures drop they search for more protection and frequently form vast clustering masses in roof spaces and lofts, with several thousand flies clustered together. It has often been observed that a single house or building in a row of similar buildings will be chosen year after year for this clustering phenomenon.
Some large aggregations of flies do produce a rather sickly smell and, if warmed up accidentally or artificially during their hibernation, may emerge rather lazily and create some concern amongst the people using the building. This has occurred commonly in places such as church halls and domestic bedrooms. They are of no particular harm but are a nuisance because of their great numbers.
The adult female lays eggs loosely on and around damp soil, beneath dead and rotting leaves, etc. After about a week the larvae hatch from the eggs and actively seek earthworms to which they cling and then bore through the body wall. The conventionally shaped fly maggots develop inside the earthworm.
Mature larval stages have been observed to push their hind ends back through the earthworm's body wall to allow their breathing spiracles to gain access to the free air. At or near to the death of the earthworm, the larva bores its way out again and pupates in the soil. Since this a free-living 'field' insect, the life cycle is very dependent of weather conditions. In Britain it seems that two generations per year are common, but in hot summers up to four generations per year might be possible.
There has not been a lot of research on the control methods for cluster and swarming flies and they are often ineffective or at best, incomplete. It is often not possible to keep flies from entering premises and control of the flies outdoors in their breeding areas is considerable and impractical.
It is recommended that a combination of physical and chemical control methods are used and it should be stressed that using silicone or other suitable exterior sealing compound around window frames and blocking other obvious entry points can contribute greatly to control. It is difficult to use insecticide in and around unsealable entry points to some buildings, (e.g. thatched roofs, or under tiles or slates) but in some cases where recurrent problems have been experienced, prophylactic dusting of access points can be partly effective.
Once the flies are inside, e.g. in the loft or attic space, control is relatively simple both with physical methods and a range of insecticides and formulations. Occasionally a vacuum cleaner (nozzle type) can be used as the sole control method where aggregations exist within reach and the collected flies can be disposed of in a sealed bag. Alternatively, or in addition, most pyrethrum and pyrethoid based space sprays will quickly kill exposed flies and in some situations, smoke formulations based on permethrin can be very effective.
Proofing of Buildings
- Fill in any holes around pipes and cables where services, (gas, water, electricity) enter the house, using cement/mortar or other suitable external filling compound.
- Fill in any holes around overflows, sink waste pipes, soil pipes and chimneys, etc., with cement/mortar or other suitable external filling compound.
- Fill in gaps/holes where roof tiles meet the soffit boards. Use crumpled wire mesh then fill in with cement or other suitable external filling compound.
- Re-point loose/missing mortar in brickwork.
- Seal outside windows and doorframes with mastic. Seal inside windows and doorframes with general purpose household sealer.
- Cover air vents or airbricks with fine zinc gauze, fixed securely.