The term ‘cluster’ relates to their habit of gathering in large numbers in lofts and other suitable hibernating locations. They can also be seen basking in sunshine on sheltered walls. When a householder mentions the flies, often dead, on upper floor windowsills, invariably an inspection of the loft will identify the root of the infestation. Not all lofts suffer from cluster flies while other lofts have a recurring infestation year upon year. This is due to the flies leaving pheromone markers during their hibernation which next year’s flies can detect and use the same loft for their hibernation.
Hibernation of course happens over the winter, with the flies reemerging in the spring to lay their eggs in soil or rotting vegetation through the summer. When the night-time temperatures drop during autumn, flies begin to seek out suitable hibernation locations. Cluster flies in ‘the wild’ are also known to hibernate in burrows and tunnels made by a range of animals from rabbits to timber boring beetles. The first frosts lead to the greatest numbers of flies entering hibernation, and those crisp sunny days associated with frosty nights are perfect conditions for cluster fly hibernation and perfect conditions for cluster flies to annoy householders! With our centrally heated homes, the cue for the flies to re-emerge from hibernation in spring is less marked than the drop in outside temperatures in the autumn which instigated hibernation, so the nuisance to the householder of their re-emergence is often less noticeable.
Once eggs have been laid in the soil, the first instar (the phase between two periods of moulting) begin searching to feed in earth worms. The larvae feed inside the earth worm until they are ready to pupate. At this time, they bore their way out of the earth worm and make their way to the surface of the soil to pupate. In most cases the adults are herbivorous, feeding on vegetation, plant sap, etc. and cause very little nuisance through the summer. Because of this metamorphosis, cluster flies are more usually a rural pest than an urban pest.
Cluster flies congregate on windowsills and inside window frames in houses, usually on the top floor and at south facing aspects where the sun provides some warmth. They are characterised by the way they can be nudged with a finger before they reluctantly move and are generally encountered in the autumn when there are warm sunny days and cold nights. This species is likely to be Pollenia rudis, though there are over twenty different species within the Pollenia genus in the UK, most of which cannot be distinguished without a microscope.
Control of cluster flies in residential properties comes down to breaking the part of the cycle which takes place within the property. Proofing against and excluding flies is not practical. You only need to see how they can congregate inside window frames to see what small spaces they can fit through! As mentioned above, successive generations of cluster flies leave pheromone markers in hibernation locations for the next generation to detect. One insecticidal treatment in the loft, timed to coincide with the first frosts when the greatest numbers of flies are present at any one time, will significantly reduce the fly numbers. Another treatment a year later, again timed to coincide with the first frosts, usually breaks the cycle of cluster flies returning to the same loft for hibernation due to the lack of pheromone left in the loft as the result of the first treatment. At this point, the recurring seasonal cluster fly issue is usually resolved.