About Cluster Flies
Some insect colonies, such as wasps, die off each winter and then start building again the following Spring. Others such as Cluster Flies, have developed ways of surviving the winter. Cluster Flies get their name from the fact that they cluster together in large numbers in order to survive. If you see flies in winter, often in large numbers, then they will most probably be Cluster Flies (Pollenia rudis). They are similar in size to, and often mistaken for, Common House Flies, although they are noticeably slower in flight. They breed outside and then seek refuge when the weather turns cold. As long as the weather is cold, they remain inactive, but they get fooled by central heating and sunlight and often start to appear around your windows.
How do you know if you have Cluster Flies?
Typically Cluster Flies will find warm places to over-winter. It is not unusual for people to go to their attic in search of Christmas decorations, only to be confronted by a swarm of flies covering an entire wall (Cluster Flies are sometimes referred to as Attic Flies). They are attracted to light and can often be seen congregating around windows and door frames. They can be seen any time from late autumn to early spring often appearing on the warm (South-facing) side of the house.
Treatment
Treatment of Cluster Flies will depend on the location. In attics, a very fine spray of insecticide is used to fill the void they occupy. This is very effective at combatting Cluster Fly Infestations. There are also products, new on the market, that interfere with the pheromones (chemicals used by insects to communicate) and break the yearly cycle of Cluster Flies.