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Overwintering adults of the cluster fly are an annual problem in many houses in North America. Cluster flies are slightly larger and darker than house flies, and have a coating of short golden hair on their thorax. These flies normally live outdoors where they occur on flowers and fruits. In late summer and fall, when the weather turns cool, cluster flies may congregate on sunny walls facing South or West.

They enter the home to overwinter through any available access. In the home, the flies hide, often in a cluster in attics and wall voids, in dark corners, cracks, under clothing in closets, beneath curtains, behind pictures and furniture etc. On warm, sunny days in the winter and spring, the flies often emerge from their hiding place and swarm on windows causing great annoyance by their buzzing and by their presence.

Dead flies provide a food source for larder beetle larvae and can contribute to the presence of other nuisance insects in the house.

In April cluster flies lay their eggs singly in cracks in the soil. During the summer, the eggs hatch and the emerging larvae (maggots) penetrate earthworms and develop as a parasite in its host. There are several generations during the summer. The adults stop egg laying in late August and September, and seek hiding places in which to overwinter. Most of the flies spend the winter outside in and on fenceposts, under stones, and in other protected places. Cluster flies are usually a greater problem in rural and suburban areas due to the large grassy areas that favour high earthworm populations.

Cluster flies are not related to livestock production. They do not develop in manure or manure pits. They are not interested in food, do not develop in garbage, and are not a sign of unsanitary conditions.


To prevent the entry of the flies in the fall, screen attics and roof vents and fill all cracks, crevices and other points of entry with caulking or other crack filler. Tighten baseboards and quarter round. Flies found on the windows can be killed with a fly swatter and removed with a vacuum cleaner.

Planting tall vegetation on the south and west sides of the house will reduce the attractiveness of these walls to flies seeking warmth and shelter in fall.


We do a thorough inspection and determine the areas of infestations and points of vulnerability in the house. Then we apply registered pesticide spray on the surfaces where these pests are likely to come into contact with the insecticide. We not only only eliminate existing infestation but also deter any future such occurrences.

Cluster flies congregating in a window.
This image depicts a country home.

Please, click for full resolution image.

Usually Cluster flies are noticed when they are attracted towards the light in windows. Many of them would die nearby.

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