University of Maryland Extension

Cluster Flies

cluster fly on siding
Photo:Whitney Cranshaw, Colorado State University,

Key Points

  • Cluster flies resemble large house flies and may suddenly appear indoors around windows or lamps in the fall, spring or winter.
  • They often become active on warm, sunny, winter days or when heat in the home arouses them.
  • Cluster flies are 3/8" long, dark gray, and non-metallic.
  • They are sluggish fliers.
  • These flies are not associated with garbage or manure. The larvae are actually parasites of earthworms.
  • Adult flies usually enter homes in late August to overwinter and occupy attics and/or wall voids that are warmed by winter sun exposure (most often southern-facing). 


  • Prevention is the best control. In summer, seal up access openings. Place tight screens over vents. Caulk or seal cracks and holes.

  • Inside the home, swat or vacuum any flies that appear.

Rev. 2020

Maintained by the IET Department of the College of Agriculture and Natural Resources. © 2020. Web Accessibility

University programs, activities, and facilities are available to all without regard to race, color, sex, gender identity or expression, sexual orientation, marital status, age, national origin, political affiliation, physical or mental disability, religion, protected veteran status, genetic information, personal appearance, or any other legally protected class. If you need a reasonable accommodation to participate in any event or activity, please contact your local University of Maryland Extension Office.