The Cluster Flies

Common Name:
Cluster Flies
Scientific Name:
Pollenia rudis (Fabricius)


This species gets its common name from its habit of forming compact clusters of hibernating individuals, commonly in wall voids or attics. It is widely distributed in Europe, Canada, and throughout the United States except for those states bordering the Gulf of Mexico. Specifically, cluster flies occur wherever their host earthworm, Allotbophora rosea (Savigny), occurs, which is usually in a well-drained sill-loam soil with grass cover.


Adults about 3/8" (8 mm) long, robust. Color dark gray, non-metallic; thorax lacking distinct stripes but with numerous short golden hairs (may be lost in older specimens); abdomen with irregular lighter areas. Wings with 4th long vein sharply bent forward near tip towards and almost meeting 3rd vein at wing margin; wing tips overlap at rest. Sluggish movements. With buckwheat honey odor when crushed.


  1. House Fly (Musca domestics) and other muscid flies (Muscidae) have thorax dark with 4 distinct stripes and lacking golden hairs, sides of abdomen usually pale.

  2. Blow/Bottle flies (Calliphoridae) with abdomen or entire body metallic colored, shining or if body or abdomen non-metallic, then thorax with 3-4 distinct stripes.

  3. Other flies either have stripes on thorax, lack golden hairs on thorax, and/or lack 4th vein of wing strongly bent forward and almost meeting 3rd vein at wing margin.


Adults overwinter in sheltered places, emerging in the spring to mate. Eggs are laid in soil cracks and hatch in about 3 days. The larvae are parasitoid upon the earthworm host, entering at almost any point along the body wall. Developmental time (egg to adult) varies from 27-39 days. There are usually 4 generations per year.


As days shorten and the weather cools, cluster flies often enter structures to overwinter, sometimes traveling more than a mile to do so. They usually occupy attics and/or the between-wall voids of walls which receive the most sunlight, usually the south and possibly the east or west walls. Typically they use the same structure year after year. They do not multiply within structures.

They can be a problem or nuisance in the autumn, winter, and/or spring; autumn when they enter to hibernate; on warm, sunny winter days; and again in the spring when they attempt to leave the structure. Actually, they can be stimulated by warmth to resume activity almost anytime. Sometimes it takes no more than the furnace to be turned on and thoroughly warm the inside of the structure to start activity, but it usually additionally requires a bright sunny day to warm the walls from the outside. Once stimulated, cluster flies seek light. This is why they usually come out around loose- fitting wall switches and outlets, ceiling fixtures, window and door frames, window pulleys, etc.

Cluster flies can usually be found at windows crawling on the panes or frames, or around lamps or lights. They are sluggish in their movements in comparison to house flies. They give off a buckwheat honey odor and leave a greasy spot when crushed.