Bad Bugs: Big Black Hairy Slow Flies

Cluster flies and flies found during the fall and winter often are found in upper stories or attics. These larger flies use homes and commercial buildings for shelter from the cold but do not reproduce inside the structure. The best control includes caulking entry points and using a fly swatter. If nothing is done, these flies will die on their own. Have you ever found a big black hairy fly on your windowsill in the winter?

Biology and Habits:
The best way to prevent fly problems in a structure is to exclude them. The Cluster fly has a unique biology. The larvae lives as a parasite in earthworms in the soil. After pupating it emerges on the sunny side of a structure in the fall. It climbs the wall to a crack in the roofline and crawls into your house or attic only to be found in your house, office building, or cabin. The cluster fly is a big nuisance pest in Canada.

Some food plants have found that besides exclusion, sterilizing the perimeter soil with an approved soil sterilant or cyfluthrin can reduce a Cluster fly problem.

Cluster flies are one of the most annoying flies found during cool seasons. They also can be pests in office buildings, especially upper stories. Cluster flies are larger than houseflies and during their indoor period they are semi-dormant, flying sluggishly. Cluster flies do not feed on garbage or animal manure like most flies. In late summer, cluster flies seek over wintering shelter and fly to buildings in the afternoons and rest on sun-exposed areas. As the sun sets, the flies seek out cracks and other openings into the building and move to upper stories. When temperatures are cool, cluster flies remain dormant, but on sunny warm days in the fall and winter, these flies become active. It may seem that they are invading the house from outside, but outside temperatures are too cold.

Cluster Fly Control:
Cluster flies (earthworm parasites) often are difficult to control by breeding area  management. The best management tactic is to caulk or fill cracks and crevices around the homes or other structures, especially an earlier infestation has occurred. Before these flies move in for over wintering, treatments can be applied to upper stories of building exteriors for Cluster fly control. Tempo SC® is an effective insecticide to use on the exterior for controlling Cluster flies.

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