Cluster Fly


Actual Size: 8 mm

Characteristics: Dark gray, not shiny.

Legs: 6

Wings: Yes

Antennae: Yes

Habitat: Drawn to ripe fruit and flowers. Will overwinter inside houses and buildings.


  • Not attracted to human foods, unlike blow flies. 
  • Are slightly larger in size than houseflies. 
  • Known for hibernating and overwintering inside of houses.

Cluster Flies in Anaheim

Cluster flies, which belong to the blowfly family, can be found all over the United States, except for the states bordering Mexico. During the fall, these insects seek shelter inside houses to hibernate. They often cluster in quiet and sometimes hard-to-reach spots like wall voids and false ceilings, hence their name. When spring arrives, cluster flies become active again and while trying to leave their winter hideouts, they often end up in the living areas of homes and buildings, becoming quite a nuisance. In the early spring, they move slowly and don’t buzz around loudly indoors like some other invading flies.

Cluster Fly Habitat

Cluster flies are typically outdoor insects that enjoy visiting flowers and ripe fruits. As the weather turns colder, they seek refuge inside homes and buildings for the winter. Inside, they hide in various places like corners, closets among clothes, behind curtains, within wall gaps, and behind furniture and pictures. When spring brings warmer days, they become active again, sometimes emerging indoors rather than outside. Inside, these flies move slowly along walls and may end up in food on tables.

Cluster Fly Behaviors, Threats, or Dangers

Unlike common blowflies, cluster flies are generally not considered a health risk because they don’t show interest in human food. Nevertheless, their presence can be quite bothersome as they often form large swarms comprising thousands of flies. In the fall, as these adult flies seek warmth and shelter for the winter, they tend to cluster noisily near doors and windows. When you attempt to swat them, they may leave behind greasy marks. While they hibernate throughout winter, their accumulated waste can produce a noticeable odor and may also stain fabrics and walls.

If you’re dealing with a cluster fly issue on your property, it’s advisable to contact your local pest control experts for assistance.