Cluster Flies are a common pest in Ontario. Many people living in the country or cities alike see cluster flies gather in windows, buzz around your lights and sometimes spinning to the floor.
The common name reflects this species habit of forming compact clusters of hibernating individuals, typically in wall voids or attics. Cluster flies are widely distributed throughout Ontario. Specifically, cluster flies occur wherever their host earthworm, occurs, which is usually in a well drained silt or loam soil with grass cover.
Adults about 8 mm long, dark gray color, non metallic; thorax lacking distinct stripes but with numerous short golden hairs; abdomen with irregular lighter areas. Wings are bent forward near their tips.
Adults over winter in sheltered places emerging in the spring to mate. Eggs are laid in soil cracks and hatch in about 3 days. The larvae are parasitoid upon the earthworm host, entering at almost any point along the body wall. Developmental time varies from 27 to 39 days. There are usually 5 generations per year.
As days shorten and the weather cools, cluster flies often enter structures to over winter, sometimes traveling more than a mile to do so. They usually occupy attics or wall voids which receive the most sunlight. Typically they use the same structure year after year. They do not multiply within structures. They can be a problem at any time of the year; autumn when they enter to hibernate; on warm, sunny winter days; and again in the spring when they attempt to leave the structure. Usually on bright sunny day cluster flies will become active and seek light. This is why they usually come out around wall switches, outlets, ceiling fixtures, window and door frames. Cluster flies can usually be found crawling on window frames or around lights.
Cluster flies can be found in fields throughout the summer, and movement towards shelter appears to be initiated by a sudden drop in temperature. In the daytime they will settle on your exterior walls and roof, to sun themselves. As the sun sets, they crawl into any crevice on the exterior of buildings, but usually near the roof. For a few days, they come out during the day and return to the warmth of the building at night. Eventually they continue to move into the interior of the building and remain there to spend the winter. As warm spells occur throughout the winter, flies may break their dormancy and begin to move about inside the infested building.
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