Look what the dog let in: Cluster flies


My wife called the other day, notably upset. The call was: “Craig you must take care of all these flies in our house now. I have fly swatted and have lost count. The cat just plays with them.”

Sure, enough when I arrived home, there were flies by our kitchen window and near the ceiling of our living room.

Upon closer examination I discovered these flies are commonly called cluster flies, or botanically (Pollenia rudis). My wife was not amused or impressed with my identification skills.

Cluster flies resemble the common house fly, except they are slightly larger, darker in color and clumsy fliers. The cluster fly has an interesting life cycle. The fly is an internal parasite of earthworms. The adult fly overwinters in the adult stage and in late summer and early fall and begin seeking protected overwintering sites.

Like other fall invading insects such as the boxelder and ladybugs, these insects do not know your home from other outdoor over wintering sites. They are attracted to the solar heat radiating from southern or western facing roofs and outside walls of our homes as well as the warmth radiating from within. They enter homes and other structures by squeezing through cracks around windows and doors, attic and soffit vents, louvers and other similar entry points. In our case they came in every time we let the dog in and out.

Over-wintering outdoor cluster flies are among the first to be active in the spring. They can be observed buzzing around yards just above the ground. They lay their eggs in cracks in the soil and the eggs hatch in about three days. Newly hatched maggots grab onto earthworms as they slither by and burrow into the worm to feed. Four to five weeks are required to complete a life cycle. The worm does not usually survive the experience. This cycle is repeated several times till we reach late summer, and the last generation of cluster flies adults begin seeking overwintering sites again.

The good news is cluster flies besides being a tremendous annoyance, are harmless; they do not feed or lay eggs during the winter months. They do not harm woolens, fur or feathers, nor do they infest foods in kitchen cupboards. They do not bite, and they are not known to transmit diseases. The bad news is there is no easy remedy for cluster fly problems. It is practically impossible to kill enough of them to make a difference once they have found their way into wall voids, ceilings voids and attics. Once they are in, they are in and there is not much we can do about it. They may also remain active during the winter months and may continually emerge from their hiding spots and find their way into living areas throughout the winter. I am sure their presence will not be appreciated except possibly by the cat.

Control measures include sealing exterior cracks and holes on the outside of the house with caulk, especially south and west facing walls. Sealing cracks around electrical outlet boxes, switches, and light fixtures, and around window and baseboard molding on the inside walls will help keep the flies trapped within the walls. Indoor aerosol insecticides are effective in killing exposed flies during the winter and spring months. A vacuum cleaner is an effective method of removing the sluggish, slow-moving flies from the house as well.

Treating yards with insecticides to kill earthworms has not been shown to be effective in reducing the number of flies entering homes. Spraying the outside walls of homes with a synthetic pyrethroid insecticide, especially the south and west facing walls, in late summer and early fall can also help reduce the number of flies entering homes. Make sure to choose a pesticide product that is safe to use on outdoor structures, and to always read and follow all labeled directions on the pesticide product.

Now if I can just train the dog to keep the flies away and the cat to be a better fly catcher.

Howard Russell, entomologist diagnostic services with Michigan State University, was a source for this article. More information can be found at https://www.canr.msu.edu/resources/cluster-fly.

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