With the days getting shorter and the nights longer & colder the Natural World begins to shut down for the Winter. Rats and mice, perhaps displaced by harvesting activities begin coming into warmer buildings to survive the winter, their activity often going un noticed until droppings or gnawing have caused damage or spoiled foodstuffs.
The pest of the moment however is the Cluster Fly. After several nice sunny days of late the phone has been red hot with people troubled with plagues of flies.
During the summer months these flies are of no consequence and the adults live harmlessly out of doors, feeding on pollen and nectar, however when the weather cools they seek shelter in nooks and crannies in houses and other buildings and as the temperature drops they will look for more protection and frequently form huge clustering masses in lofts and roofspaces.
These large masses of flies are not considered a health risk, as they aren't associated with manure or rotting flesh like bluebottles or house flies would be, as they do not breed indoors they aren't indicative of poor hygiene. Large accumulations can though, produce a sickly smell.
Control of Cluster Flies can be very difficult, it's not possible to control the larval stage as they are parasitic on earthworms, sealing entry points around an entire building is usually impossible, or at best very difficult. Vacuum cleaners can be the best method of getting rid of adult flies around windows and sealing around window frames can help to prevent access.
Once the flies are inside an attic or loft accessable clusters can be vacuumed up or an insecticidal space spray or "smoke bomb" used to quickly kill any exposed flies, however these treatments are only effective for those flies present on the day and care must be taken to ensure there is no risk of fire, no danger of triggering smoke alarms and no bats present.
Unfortunately, for anyone who is bothered by cluster flies they do seem to favour the same buildings year after year. Houses that face south and are light coloured outside (painted white, magnolia or yellow, or locally, Bath Stone), seem to be particularly to their liking,
however, as I mentioned before they aren't a health risk, just a (sometimes severe) nuisance and no matter how horrible looking the infestation is, they are unlikely to pose a threat to your health or well being.