INTRODUCTION.
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.
RECOGNITION.
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.
SIMILAR GROUPS.
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.
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.
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.
BIOLOGY.
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.
HABITS.
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.
