cluster fly

Cluster fly (Pollenia pediculata). Photo: Whitney Cranshaw, Colorado State University,

Updated: March 1, 2023

About cluster flies

  • 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