Cluster Flies


  • Latin Name: Pollenia rudis
  • Size: 3/8” to ½”
  • Color: Cluster flies are gray and black with yellow hairs.
  • Shape: Oval, wings overlap when at rest
  • Found in: All over the United States
  • Overview: Cluster flies are also known as attic flies because large groups to spend the winter months.  They look similar to a house fly but are larger and more lethargic.

Origin: Cluster flies originated in Europe and were transported to the United States in ships containing soil and earthworms.
Climate: Cluster flies will stay outside as long as the temperature is over 50 degrees Fahrenheit. The ideal temperature to reproduce is 80 degrees Fahrenheit. They are found all over the United States.
Favorite Foods: Cluster flies help control the earthworm population. Earthworms are the only food cluster flies’ larvae eat to develop.
Family: Female cluster flies lay eggs burrowed in the soil near the presence of earthworms. The larvae will enter earthworms to develop before pupating. The larvae are parasites and eat the earthworm from the inside out. It takes one month for an egg to turn into an adult.
Unique Facts: Cluster flies are attracted to light. They cluster on the sunny side of building and look for cracks in the windowsill or siding to enter and stay warm. Once inside, they try to stay away from humans. They live inside the wall, in the attic or under the baseboards.
Signs of a problem: If you live near a golf course or pasture where earthworms are present, you may have a problem with cluster flies. Cluster flies appear in the fall when they are searching for their winter home.  They will not reproduce inside a home. When the temperature is above 54 degrees Fahrenheit on a warm winter day, cluster flies will wake up and head towards the windows looking for a way out. Homeowners will find the quantity of cluster flies annoying, but not damaging like many other pests.
Do they attack humans? No, cluster flies do not bite. Cluster flies are not dangerous to humans or pets and do not carry diseases like the black flies. They are sluggish and easy to swat, although they may leave a small black mark on your wall when squashed and emit a sweet sugary smell.
Stopping an infestation: Make sure all cracks and crevices are sealed with caulk prior to the fall season. Repair any holes in screens, however a cluster fly may still be able to fit through the screen. A pest control professional may recommend spraying the perimeter of your home with a chemical to deter cluster flies. It is not recommended to spray inside your home once cluster flies are found as dead flies may attract other unwanted insects like carpet beetles. Instead, follow the slow moving flies around until you can determine where they are entering the house and make repairs as necessary. Ask a pest control professional about treating earthworms in your yard. Removing the earthworms could cause damage to the ground.

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