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Cluster Fly

Danger/Damage

Cluster flies are nuisance pests in the home. They usually appear in the late fall, early winter, and early spring. Cluster flies do not damage your home but can occasionally leave stains on walls and windows. If flies die within the wall voids of a home or structure, they may attract larder beetles, warehouse beetles and even meal worms, which will feed on the dead flies and then potentially migrate to other areas in the home or structure.


Did you know?

Cluster flies are dark grey with black and silver (non-metallic) irregularly coloured abdomens, ranging from 8 mm to 10 mm long, slightly larger than a house fly. They have many golden hairs on their thorax (these may or may not be present on older flies), and their wings overlap when they are at rest. A telltale sign of cluster flies is their sluggish behavior when indoors.


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Cluster flies are dark grey with black and silver (non-metallic) irregularly coloured abdomens, ranging from 8 mm to 10 mm long, slightly larger than a house fly. They have many golden hairs on their thorax (these may or may not be present on older flies), and their wings overlap when they are at rest. A telltale sign of cluster flies is their sluggish behavior when indoors.


Buildings provide cluster flies numerous crevices and cavities, such as wall voids, attics, closets, and empty rooms that make good overwintering sites.

In the spring, they will sluggishly migrate from their hibernation areas, to living spaces and windows in search of light. If disturbed during hibernation, a gathering of cluster flies may emit a sickly, sweetish odour.

Cluster flies can be seen resting on sun-warmed windows and buildings beginning as early as mid-August. They can enter a building through cracks around windows or soffits and ventilation openings.

During the cooler months, cluster flies remain in a dormant state, moving little if at all, usually clustered together. During this time, cluster flies stop reproduction, slow down their metabolism, and are in a dormant stage that allows them to survive through to spring. Despite this, cluster flies can easily be stimulated by warmth, be it from a furnace or bright sunlight. This sensitivity allows them to resume activity almost anytime.


Female cluster flies’ mate between late summer and early fall and lay the eggs in cracks in the soil. The eggs hatch within three to seven days. Shortly after hatching, the larvae look for an earthworm and bore into. The cluster fly larva will use the earthworm for food until its development is complete.

The development period for an adult is about 27 - 39 days. During mid-summer, adult cluster flies will emerge from the soil, repeating the cycle in late summer or early fall.

The name “cluster fly” is derived from their habit of gathering in clusters before and during hibernation.


  1. Cluster flies found in buildings are in a dormant state and cannot reproduce indoors.
  2. Cluster flies do not develop in manure, do not like food, and are not a sign of unsanitary conditions.
  3. Cluster flies feed on the nectar from flowers.

Are Cluster Flies hazardous to humans?

Cluster flies are a nuisance as they leave stains on walls and curtains. If the flies die within the wall voids, they may attract larder beetles, warehouse beetles and even meal worms, which will feed on the dead flies and then migrate to other areas in the house.

Can Cluster Flies cause damage to my property?

Cluster flies may occasionally leave small dark-colored spots of excrement on windows and walls. They can also leave greasy stains squashed on windowsills, walls, curtains, or carpets.

How do I remove a Cluster Fly infestation?

As cluster flies are sluggish and congregate in large volumes around windows and lights. Use a vacuum to remove large numbers of cluster flies in hibernation. Electric light traps also prove effective in attracting and capturing flies inside structures.

How can I prevent a Cluster Fly infestation?

The best defense against a cluster fly infestation is to prevent their entry into the property. Seal the roof, screen attics, and roof vents and fill all cracks, crevices, and other points of entry around homes and buildings.

However, once you have a cluster fly infestation, you will most likely experience an annual pestering of them. Contact Abell Pest Control, we have expertise in cluster fly preventative and control, and we can help you solve your infestation problem.


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