Cluster flies are parasites on our friendly earthworms. The eggs are deposited singly into cracks in the soil of lawns or open areas in late summer or early fall.

The eggs hatch in approximately three to seven days and the larvae then enter the body cavities of earthworms to overwinter until spring when feeding activity resumes. The larvae feed for up to 19 days; they then moult and begin the final larval stage. The larvae finish feeding on what is left of the earthworm hosts and then enter the soil where they pupate for approximately 10 weeks. In mid-summer, adults emerge from the soil. These adults and the cluster flies that hibernated in households will mate and lay eggs in late summer or fall, repeating the cycle. Adult cluster flies can often be seen sunning themselves on stumps of trees and sides of houses in autumn before they hibernate.

Cluster flies, particularly Pollenia rudis are very common occurring throughout Great Britain. The adult flies after hibernation during the winter months lay thier eggs in the earth or animal dung. Permenant areas of grassland are favorite sites eg. permenant grazed pasture or parkland as solid cultivation such as ploughing disrupts the flies breeding cycle. The eggs remain in the earth and hatch when soil temperature increases in the spring. The larval stage is parasitic, entering earthworms and feeding upon them until emerging as an adult fly.

Cluster flies migrate from outdoors into the lofts of houses and farm buildings during the winter months. Obvious signs of an infestation is a roof space containing a large quantity of lethargic flies and a quantity of dead flies. Unless the loft is used for storage purposes or is accessed regularly the flies will hibernate successfully leaving in early spring dependant on the weather.

Reasons for control

Cluster flies are not damaging to property when they enter buildings in the autumn purely to hibernate over the cold winter months. As mentioned above they feed on decaying vegetation and earthworms. Cluster flies are not to be associated with dead animal bodies, faeces and the resulting maggots. Fibre glass insulation material in lofts can look unsightly if covered with the bodies of flies that have not hibernated successfully although its insulation properties will not be altered and no damage to the roof structure will occur. If the loft is home to a hibernating colony of flies, first floor lighting diffusers or sunken spot light lenses can become full of dead or dying flies. This often occurs in houses where the loft area has been converted into a living room. In most buildings built after 1950, the external roof covering has an under felt layer. Cluster flies can successfully hibernate undetected between these two layers. Often cluster flies can be seen basking on sunny grassland or resting on south facing walls of buildings. Cluster flies, like birds, return to roof spaces previously occupied so re-infestation is almost inevitable.

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