The term ‘cluster fly’ relates to a number of species of flies and refers to their habit of gathering in large numbers in lofts and other suitable hibernating locations. The most commonly encountered by householders is Pollenia rudis. The focus of this blog will be relating to this species, though treatment will be the same regardless of species. They resemble the common house fly but tend to be lethargic. When at rest, the wings lie along the length of the body. They can often be seen basking in sunshine on sheltered walls. When a householder mentions flies, an inspection of the loft will often 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. The next year’s flies will use the same loft for their hibernation.
Hibernation of course happens over the winter. 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. These are made by a range of animals from rabbits to timber boring beetles. The first frosts lead to the greatest numbers of flies entering hibernation. Crisp sunny days associated with frosty nights are perfect conditions for cluster fly hibernation. With our centrally heated homes, the cue for the flies to re-emerge from hibernation in spring is less marked. The drop in outside temperatures are more marked in the autumn compared to the increase in temperature in spring. This is why cluster flies are more noticeable in Autumn.
Eggs are laid in soil or rotting vegetation during the summer. 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. This means they feed on vegetation, plant sap, etc. and cause very little nuisance through the summer. Cluster flies are more usually a rural pest than an urban pest due to this life cycle.
Cluster flies congregate on windowsills and inside window frames in houses. This is 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. They 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 cluster fly species cannot be distinguished without a microscope.
Control of cluster flies in residential properties comes down to breaking the part of the life cycle which takes place within the property. Cluster flies leave pheromone markers in hibernation locations for the next generation to detect. One insecticidal treatment in the loft will kill the flies present at the time. 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. A loft which has had historical cluster fly issues might need another treatment treatment the following spring or autumn to break the pheromone trail.
Conventional fly spray in a can will not work. This is because the droplet size is too large to carry in the air. A special piece of equipment needs to be used which generates fine droplets of insecticide as a mist. These fine droplets will penetrate any cracks crevices the flies are congregating in.
An electric fly killing unit hung in a loft can be a useful monitoring device. However it is unlikely that these devises will be effective in removing an infestation. Moreover, this approach would not be cost effective.