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Cluster flies

Cluster flies belong to the Diptera order (one pair of wings and mouth pieces). They look like houseflies only they are bigger, 8 mm to 10 mm long, and darker. They can be distinguished by their lethargic and erratic behaviour and by their wings that cross each other when at rest. These flies are parasites of earthworms (night crawlers). They are mostly visible during humid periods. When cold weather approaches, they seek a place to spend the winter, and they gather on outer walls that are exposed to the sun and in attics.

Development and Behaviours

These flies normally live outside where they feed on flowers, fruits, and, sometimes, on tree sap. When they begin seeking a place to spend the winter, they may fly for a distance of over 2 km. Then, they gather in considerable numbers on walls that are most exposed to the sun. They get inside houses and other buildings through cracks in structures and through other available openings. Once inside, the flies take cover in groups within attics, walls, and other empty places they can find in buildings. The situation becomes a problem when the heat is turned on in the infested building and they start to follow the heat. Since these flies are often stuck inside the building, they are stopped by windows and cover window frames and other surface with cadavers and droppings, being a nuisance and causing insalubrity. It is essential to prevent an infestation as soon as possible, because they secrete an aggregation pheromone (attractive substance), attracting an ever-increasing population of these flies to enter the building year after year.


  • Seal windows and doors.
  • Keep screens in good condition.
  • Install screens over vents and other openings that allow air to circulate in attics.
  • Clean up the dead insects if possible, because their cadavers become food for larder beetles, which become a problem themselves.

Contact us to meet with a pest management and extermination technician and to establish a structured and safe action plan for quick and long-lasting results.