Cluster flies, Pollenia spp., (Diptera: Polleniidae) are common throughout the UK especially in the south. Eight species are currently recorded in Britain (2021). The common name referring to their habit of ‘clustering’ and hibernating in vast numbers in buildings.
Adult Pollenia live out of doors during the summer months where their larvae are parasitic on earthworms and are of no consequence. However, as the temperature drops in the autumn, adult flies of the last generation of the year become numerous and they often enter buildings to hibernate. This is typically in roof spaces and lofts but also in soffits and fascias, cavity walls, ventilation bricks, around window and door frames, behind steel cladding and in other suitable nooks and crannies. They particularly favour the warmer south and south west facing elevations of a building. Isolated properties in the country are especially prone to invasion since they generally offer the only warm shelter for miles around. If warmed up, either accidentally or artificially during their hibernation, they may emerge in large numbers and create some consternation among people using the building.
One extremely important aspect to understood is that Pollenia spp. are believed to release a pheromone that attracts others to the area – indicating that previous generations have successfully overwintered. Successive generations further enriching the pheromone. This attractant will linger even after all the flies have been destroyed or have naturally left their overwintering site and will continue to attract other cluster flies to that area. Therefore, the likelihood of recurring seasonal problems is highly likely with this group of flies.
Although they can be extremely difficult to totally exclude from properties, a number of control measures are available which can help to lessen the burden of their presence. For small infestations around windows and behind curtains they can be removed with the use of a vacuum cleaner. For large infestations in loft spaces, treatment is usually carried out with an insecticidal space spray or smoke generator. Electronic fly killers adapted for the purpose can also be a beneficial means of control.
It should be noted that a number of other fly species exhibit a similar behaviour of harbouring in buildings during the winter months although their life cycle is different to Pollenia spp.