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Cluster Fly biology, identification, image, control tips and products for eliminating cluster flies.
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Cluster Fly Biology and Identification

The Cluster Fly is listed in our category of Biting & Nuisance flies, but is the only one of the group which does not have a metallic coloration; they are dark gray in color.  These medium-sized insects are 1/4 inch to 3/8 inch in length.  Cluster flies are distinguished from other flies by the presence of golden yellow hairs on the front, top of the thorax.  Another noticeable feature of the Cluster Fly is the way its wings are held flat over its back while resting; the wing tips overlap when held in this position.  This fly is often mistaken for a house fly or a face  fly.

Cluster fly females lay their eggs singly in cracks in the soil; the larvae emerge in three days and begin to seek out their food source: earthworms.  After burrowing into the earthworm, the larvae feed for about three weeks before pupating.  The adult Cluster Fly emerges from its pupa after 12 to 14 days.  The adult flies ( of which there 3 to 4 generations per year) feed on flowers.

Homeowners are often bothered by the appearance of sluggish flies in their homes in late winter and early spring.  These flies are not breeding in the home but will become quite a nuisance if not controlled.  These flying pests are parasites of earthworms. The more abundant earthworms are around a home, the more likely it is that cluster flies will become an indoor pest. Earthworms are most abundant around places where manure has been piled or stored and are common in grassy areas where the soil has good moisture.

In the fall, adults search for a protected overwintering site: an attic, loft, loose bark, holes in trees, or other crevices.  Siding without cracks, crevices or other protected areas are usually fly free, but white loose-fitting vinyl or aluminum siding will attract nuisance flies.

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Cluster Fly Image

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Cluster Fly

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Eliminating Cluster Flies

Cluster flies, face flies and blow flies are difficult to control once they have gained access to homes because they hibernate in wall voids and other inaccessible places.

  1. Exclusion: Close and seal as many openings as possible through which the flies can enter.  This procedure is time consuming and may require a dedicated long term effort.  Window screens offer no protection from Cluster flies because they crawl into the home through small openings in the walls of the building.  These same overwintering flies get into rooms during the winter and spring months entering through window pulley holes, around the baseboards and through other small openings in walls.   A thorough inspection for locating and eliminating all possible entry points is a must, but will not guarantee success.  For best control, use chemical sprays with your exclusion methods.
  2. Surface Spray: Use a  compressed air sprayer (Chapin Sprayer) to apply a surface spray to surfaces in attics, basements, closets, store rooms and other areas where the flies congregate.  These sprays also are effective if applied to the outside of the house in the fall when the flies collect in these areas before entering the home. Pay special attention to the areas beneath the eaves and around the windows and other areas where flies are likely to seek entry.
  3. Crack and Crevice Treatment: Using a pyrethrin aerosol for treating cracks and crevices will compliment your surface spray.   This method is only necessary if there are areas to be treated that you cannot properly treat with a surface spray.

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Summary of Cluster Fly Elimination

Cynoff WP is the best surface spray for nuisance flies.

  1. The first step in Cluster Fly elimination is to seal or eliminate all possible entry points.
  2. Use a good surface spray on all areas where cluster flies congregate, hibernate or enter a structure.  Cyper WP is the best surface spray for nuisance flies.  When used according to label instructions, this product is safe around children and pets, and also gives a very long residual protection against a wide variety of pests -- with no odor.  For ordering information and product description, go to our Fly Control Products page.
  3. If necessary, use a crack and crevice aerosol for hard to reach areas.  Delta Dust is a good crack and crevice tool.


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