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These flies are commonly referred to as cluster flies and can be abundant in houses with large lawns or those backing onto open parks. The name "cluster fly" is derived from their habit of entering a house in the fall and gathering in clusters that may emit a sickly, sweetish odour if disturbed.
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. 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 cluster flies will migrate from their hibernating areas to living spaces and gather on windows as they are attracted to light.
Cluster flies (Pollenia rudis) belong to the same order as fruit flies, houseflies and mosquitoes (Diptera). Adult cluster flies are approximately eight 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). Their wings overlap when they are at rest.
Cluster flies are parasites on our friendly earthworms. The eggs are deposited singly into cracks in the soil of lawns or open areas in late summer or early fall. The eggs hatch in approximately three to seven days and the larvae then enter the body cavities of earthworms to overwinter until spring when feeding activity resumes.
The larvae feed for up to 19 days; they then moult and begin the final larval stage. The larvae finish feeding on what is left of the earthworm hosts and then enter the soil where they pupate for approximately 10 weeks.
In mid-summer, adults emerge from the soil. These adults and the cluster flies that hibernated in households will mate and lay eggs in late summer or fall, repeating the cycle. Adult cluster flies can often be seen sunning themselves on stumps of trees and sides of houses in autumn before they hibernate in Fall in your home.