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A source for information on flies

Cluster Fly      Pollenia rudis

Cluster flies, also known as attic flies, are household pests. Adults measure between 8 and 10 millimeters in length and have light and dark grey-checkered abdomens. The thorax of an adult cluster fly is covered in short, golden hairs and the wings overlap when at rest. Cluster flies are slightly larger and darker than the common housefly and move more sluggishly.

They appear on the sunny side of the structure in heaviest concentrations in late fall and early winter, as they seek warm locations in which to live during cold months. Although cluster flies are observed buzzing and congregating at windows, screens prove ineffective in preventing their entrance. Cluster flies are capable of crawling through small openings in the walls of a structure. They hibernate in secluded parts of houses like attics and wall voids. On sunny winter days, the wall voids become warm and the cluster flies try to move toward light. Very often they find themselves in the inhabited parts of the house and the move to the windows. They cluster around the windows and they leave stains on walls and curtains. They eventually move to other areas of the house.

The cluster fly life cycle begins when a female lays her eggs inside the soil in late summer or early fall. These eggs hatch within a few days, after which larvae seek to enter the body cavities of earthworms. Cluster fly larvae feed on earth worm hosts for up to 19 days, at which time they molt and begin the last of their larval stages, consuming what remains of the host and returning to the soil. Cluster flies then pupate for approximately 10 weeks, emerging as adults by mid-summer.

If cluster flies gather around your home, contact your local pest control professional for a consultation.


Medium-sized flies from 1/4-inch to 3/8-inch in length.




A number of insects have discovered that heated buildings are ideal for surviving the cold of winter. Such pests include box elder bugs, cluster flies, and ladybugs. As the weather cools in late summer and early fall, the sun warms the southern and western walls of buildings. The warmth attracts these insects to buildings where they crawl inside cracks and stay there for the winter. This would be fine, but during warm winter days, some insects "wake up" and end up on the inside of the building.


Cluster flies are actually parasites of earthworms. The adult flies deposit their eggs in the soil and the larvae seek out and live off of earthworms.


The best way to control cluster flies is by prevention as described below. If it's too late and they are already inside, it takes a professional to find and treat the right areas to minimize the numbers of pests seen inside. If your home has experienced a problem with overwintering pests, take the following steps next summer to prevent a recurrence:

  • Seal as many cracks and holes on the outside of the home as possible, especially on the south and west walls where the sun heats the home during the late summer and fall.
  • Be sure that all foundation and attic vents have tight-fitting screens.
  • Check the soffit vents and any gable vents or turbine vents on the roof.
  • Have your pest professional treat the outside west and south walls of the home near the eaves. This treatment should be completed in mid- to late August.
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-- Duggan Cooley, former CEO- RCS

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