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Controlling the Exasperating Cluster Fly

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Controlling the Exasperating Cluster Fly
Whitney Cranshaw, Colorado State University, Bugwood.org

Homeowners generally expect to have to swat a fly or two in the house during the summer months. With family members constantly filing in and out; doors being propped while groceries are carried in; windows opened to screens that need mending, it is generally more likely that a fly will get in than that all will be kept out.

But what is an expected annoyance in the summer can be exasperating in the winter when doors and windows are sealed tight, and one wouldn't think that any flies are existing in the cold outdoors to come inside. So where are they coming from?

In all likelihood that large fly is not a house fly, but is rather a cluster fly that sought to overwinter in the protected areas within your walls, attic or basement. Overwintering insects generally stay in harborage until the warming and lengthening days of spring pull them from harborage. But the overwintering cluster fly will be drawn to the warmth of the home's interior and find its way out the inside the same way it found its way into the outside wall: through cracks and openings.

Cluster Fly Identification

The cluster fly can be distinguished from the house fly by its:

  • body characteristics. It is a bit larger than the house fly with a black/silvery-black checkered body. Young, newly emerged flies have short light-brown/yellowish hairs on its lower body.


  • sluggish movement. It will fly around the home, but at a less frantic pace than that of the house fly.


  • overlapped wings. When at rest, the cluster fly will overlap its wings; the house fly's will remain separate.


  • clustering at windows (thus its name). If a large population exists, it will tend to cluster at windows or attics, especially in little-used areas on warmer days.

The flies normally live outdoors and are most likely to appear in the spring and fall. But overwintering cluster flies can emerge into homes and building throughout the winter.

Cluster flies are not known to transmit disease or damage structures, and they do not reproduce after emerging into the home. However, they can leave dots of excrement in their wake, and large numbers can be a significant nuisance within a home or other building.

Cluster Fly Control

Once cluster flies enter a building - to harbor within the walls or buzz around the rooms, control methods are limited. Although insecticides can kill the flies harboring within walls, large numbers of dead or dying flies can attract secondary pests such as beetles and rodents.

Because the flies are sluggish, it is generally fairly easy to swat or vacuum those that find their way indoors, but more are likely to emerge, causing an ongoing process of swatting, vacuuming, and exasperation.

Thus, the keys to cluster fly control are exclusion and prevention. Maintain the home in good condition and implement pest-proofing procedures. Caulk or fill all cracks and crevices in the home's structure, including:

  • near doors and windows.
  • under and in eaves and siding.
  • around electrical outlets.
  • at pipe, wire, and cable openings.
  • around vents.
  • along the roof.

Also:

  • ensure all doors and windows are tightly fitted.
  • check for and repair any holes in screens.
  • use yellow sodium vapor or halogen lighting to reduce insect attraction.

Pesticides have limited effectiveness in control of these flying insects, however some control can be attained by a professional application of an insecticide to the south sun-warmed side of the structure prior to the onset of cluster fly harborage in early fall, and application of dust in wall voids to kill the flies prior to breeding.

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