Homeowners are often bothered by the appearance of sluggish flies in their homes in late winter and early spring. These flies are collectively called “clustering flies” but may actually be any of the three most common types of flies.
Cluster flies are approximately 8 to 10 millimetres long. They are dark grey with black and silver (non-metallic) checkered abdomens, with many golden hairs on their thorax. These may or may not be present on older flies. Unlike house flies, their wings overlap when they are at rest.
At first glance they resemble house flies; however, they are larger, darker and slower in their movements. These flies tend to be more abundant in houses surrounded by large lawns or those backing onto open parks. The name “cluster fly” is derived from their habit of gathering in clusters after entering a house in the fall and these may emit a sickly, sweetish odour if disturbed.
Face flies are another fly that closely resembles the common house fly. Face flies are pests of cattle and may hibernate in homes or invade them during the summer. Hibernating face flies have very similar habits to those of cluster flies.
Face flies are most likely to invade farm homes or homes located near pastures or where cattle are kept since the larvae develop in fresh cattle manure. During the summer, the adult female flies feed on the mucous secretions from the eyes and noses of cattle and horses.
Blow flies , also referred to as blue or green bottle flies, are robust flies with shiny metallic bodies that can often be found in homes during winter and early spring. These “buzzing” insects develop in manure or dead animal carcasses. They are strong fliers and are attracted to lamps or lights. Blow flies are usually the first insect to come in contact with a dead animal. The green bottle fly is also attracted to dog feces and garbage.
Cluster flies hibernate in secluded areas of houses, such as in wall voids, attics, closets and empty rooms. They are a particular nuisance as they leave stains on the walls and curtains.
Blow flies or face flies may also hibernate in homes that are located near livestock farms or places where animal wastes or garbage accumulation are not well managed. If the flies die in the wall voids, they may attract larder beetles which will feed on the dead flies and then migrate to other areas of the house. In the spring, the sluggish clustering flies will migrate from their hibernating areas to living spaces and gather on windows as they are attracted to light.