Adults are about 3/8 inches long. Their coloration is dark gray, non-metallic, thorax lacking distinct stripes but with numerous short golden hairs, and the abdomen with irregular lighter areas. They are slow moving and when crushed smell of buckwheat honey.
Cluster flies occupy attics and/or between-wall voids of walls which receive the most sunlight when they are ready to hibernate. They usually try to use the same structure year after year. While they are there they will not multiply.
They are a nuisance when the do fly into structures to hibernate. They can be stimulated by warmth to resume activity almost anytime. Once they are stimulated, cluster flies will seek out light.
Cluster flies can be found on windows, lamps, or lights crawling around. 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.
Adults hibernate in sheltered places, emerging in 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 varies from 27-39 days. There are usually 4 generations per year.
Pollenia rudis Fabricius