Cluster Flies in the Autumn
Cluster Flies in the Autumn:
Soon pest control companies will be getting calls about Cluster Flies. These are flies that hibernate in cavities, before humans made buildings they would use caves or hollow trees to spend the Winter. These flies are not the same as ‘normal’ flies such as House Flies or Blue Bottles and there are several species. The most common type is parasitic on Earthworms. The adults lay eggs in the soil which hatch into grubs which then burrow until they find an earthworm. Once found the grub burrows into the earthworm and feeds on the worm (probably eventually killing the earthworm) until the grub is ready to turn into a pupae and then the adult fly. Most of the eggs that are laid and hatch will not survive. In years when there is not much rain (or as in last Winter too much) and there is a lack of earthworms near the surface then Cluster Fly numbers are small. The fly numbers may be effected by the flooding last Winter which killed a lot of earthworms. We shall have to wait and see. The second most common type of Cluster Fly is one that feeds on cow dung. Properties near farms with uncovered dung heaps are susceptible to this type of Cluster Flies infesting lofts and cavity walls. The flies are nor considered a health hazard as are House Flies, Green Bottles, Blue Bottles etc nor do they bite, sting or damage foodstuff. They are more a nuisance. The numbers of dead flies can build up over time in lofts. I have seen layers several inches thick of dead flies. The amazing thing about these flies is that although the flies that emerge from hibernation will never see another Winter (they will die from old age in the next few weeks after they have mated and layer eggs) their descendants will return to the same hibernation site. This is known as Genetic Memory and is bad news if you have a Cluster Fly problem.
Treatment will only reduce the problem, not eliminate it as in a lot of cases you will only be dealing will part of a larger infestation and it is not always possible to get insecticide deep into cavity walls.