How to Identifiy the Cluster Fly

The Cluster Fly is considered an overwintering fly and they cannot breed nor survive inside your home or structure.  They typically enter the home in the fall months, on at a time.  The cluster fly will congregate together similar to maybe a swarm of bees.

They are a little bigger than a house fly and has a nonmetallic, dark gray color.  The cluster fly averages between 1/4 to 3/8 inch long. Their thorax has golden hairs and this is characteristic to them. When crushed, they give off an odor like buckwheat honey. Cluster flies closely resemble house flies, but they are usually larger and have a yellowish sheen on the thorax.

BIOLOGY AND HABITS OF THE CLUSTER FLY

They live outdoors and frequent flowers and fruits.  But with the increase in cooler temperatures they enter the building to overwinter.    They will typically hide under furniture, nooks and dark corners.    The cluster fly is a parasite of earthworms and breeds outdoors in lawns and fields during the spring and summer. You can find cluster flies almost everywhere in the United States and Canada, except for the Southern states bordering the Gulf of Mexico.

Female Cluster Flies lay their eggs in cracks in the soil, which hatch in three days. The larvae use earthworms as a food source. The larvae feed for about 22 days. After that, they go into the pupae stage, which lasts 11-14 days before emerging as adults. Adult flies feed on flowers. There are about four generations hatched per summer.

When fall approaches, the cluster flies begin to enter structures in large numbers. Problems with cluster flies begin in late August as they move to winter quarters to over-winter. The cluster fly is seeking warm sites with protective cracks for shelter, crawling back as far as they can get. It is important to consider treatment before this happens.

Cluster flies are known to enter from very tight spaces and even facilities that have sealed up very tight. As the number of cluster flies attracted to the building increases, large clusters of flies huddle inside wall voids, attics, and false ceilings. Most infestations occur in the upper regions of buildings, such as the attics of homes. In multi-story buildings, the cluster flies can be found in the upper two or three floors, and almost always of the south and west sides of the buildings.

They will fly very sluggishly and can be strongly attracted to light and warmth, so they are typically located on the southern side of structures and can be found in large numbers at the windows.

Control of Cluster Flies or other overwintering flies 

You will want to check thoroughly around windows for any live or dead cluster flies. If you happen to find the entry point in a wall void then this can be tretaed with a dust or aerosol labeled for this type of application.   
When you are trying to locate the flies entry point you will want to start on the warmer side of the structure, southern side.  Check the attics, crawlspaces and false ceilings.

Steps to Controlling and Eliminating the Cluster fly

You can to some extent prevent some of the problem from occurring, but this should be done before the fall.  Using a good quality vacuum to suck the flies up often is very effective for the adults.  You can also spray a residual aerosol like CB-80 which is a pyrethrum based spray to kill the adult Cluster Flies.  You will want to spray light and repeat if necessary.

You may have some success with fly lights as attractants with a good sticky glue board, but for home use this may be excessive.

Before the fall months, the use of a good residual spray on the exterior of the structure can help eliminate the cluster flies before they enter the structure.   Spray the outside walls of the building where cluster flies are likely to land and rest before entering the building, southern and western sides.

You will want to apply these residual insecticides outdoors to window frames, door frames, soffits and eaves.  You will also want to apply to any other areas that are accessible for them to entry

You can also apply a barrier treatment of residual insecticide to the surrounding soil may also help minimize Cluster Fly populations and later entry.   This will also help eliminate other unwanted insects from entering the structure.  Spray in a band about 3 feet on the ground and 3 feet up the side.

Recommendations for controlling cluster flies

Cb-80 Aerosol

Cyzmic cs Residual Insecticide

PT 565 Aerosol is a good aerosol for doing a fogging type treatment

Monica Bird (922 Posts)

Monica’s compassion for her customer's struggles with pest control issues and passion for pest control stems from over 10 years in the industry. With a master’s degree in entomology, she uses her knowledge and experience in chemistry, insects and pests to educate her customers.


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