Cluster Fly

The cluster fly seeks to overwinter inside homes from fall onward. It accumulates in large numbers in attics, walls and window frames, and reappears on mild winter days and in spring.

Identification sheet

Appearance

Fly slightly larger and slower than a house fly, with a thorax covered in golden hairs. Moves sluggishly and clusters near sunny windows.

Size

8 to 10 mm

Color

Grey-black with golden sheen

Habitat

Attics, roof voids, walls, south- and west-facing window frames. Larvae parasitize earthworms in the soil.

Lifespan

Several weeks (overwintering adults)

Danger level

Very low

Health risks

The cluster fly doesn't bite, doesn't lay eggs in food and transmits no disease. It's mainly a major nuisance due to its numbers, especially in fall and spring when it invades rooms.

Signs of infestation

Clusters of slow flies near sunny windows
Mass appearance in fall and during winter thaws
Dead flies accumulated on window sills
Dark stains in attics and lofts
Buzzing in walls and window frames

Prevention tips

Seal cracks, soffits and window surrounds before fall
Install screens and vent grilles
Apply an exterior preventive treatment in late summer
Seal attic entry points
Have recurring accumulation zones inspected

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