Clusterfly

The common name reflects this species' habit of forming compact clusters of hibernating individuals, typically in wall voids or attics. It is widely distributed in Europe, Canada, and throughout the United States except for those stated bordering the Gulf of Mexico. Specifically, cluster flies occur wherever their host earthworm occurs, which is usually in well-drained silt-loam soils with grass cover.

Recognition

Adults are about 3/8 inch long, robust. Color dark gray, non-metallic; thorax lacking distinct stripes but with numerous short golden hairs; abdomen with irregular lighter areas. Wings with fourth vein sharply bent forward near tip towards and almost meeting third vein at wing margin; wing tips overlap at rest. Sluggish movement. Emits a buckwheat honey odor when crushed.

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 at almost any time. 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 also 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.