University of Maryland Extension

Cluster Flies

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

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). 

Management

  • 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

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