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Cluster Flies

picture of a cluster fly

General Facts

Found throughout Canada as well as the United States and Europe, cluster flies belong to the Calliphoridae family of blowflies and are members of the genus Pollenia. The insects are named for their tendency of clustering together in groups while overwintering indoors. Unlike other Canadian fly species, cluster flies neither pose a significant health threat nor breed in the homes they invade. The sluggish insects enter indoor structures simply to hibernate during the winter and wait for warmer weather to return.

Appearance / Identification

Cluster flies appear similar to the common house fly and blow/bottle flies. Adult cluster flies are dark grey in colour, 8 to 10 mm long, and have numerous golden or yellowish hairs on thorax. The abdomen of a typical cluster fly features a black and silver chequered pattern. Despite bearing a strong resemblance to house flies, cluster flies are larger in size and darker in colour, they lack the dark stripes found on thorax of house flies and the metallic coloured shining bodies of bottle flies. They are sluggish in movement compared to house flies and bottle flies. The wings of cluster flies overlap across the abdomen when the insect stands at rest.

image of a cluster fly

Habitat

Cluster flies begin to appear in homes and buildings during the late summer months and the early fall. As the days become shorter and temperature begins to fall, the flies begin searching for overwintering sites. At this time of year, west and south facing sides of buildings are warmer as they are exposed to more sunlight. The warmth from the west and south exterior walls attract the insects, where they eventually enter through openings in the wall such as cracks and crevices near window and door frames, open and unscreened windows, and unscreened vents that provide access to the into the building. Once inside, cluster flies gather together in isolated and protected locations, such as attics, and wall voids, to hibernate for the winter. Indoor warmth and unseasonably warm weather may interrupt overwintering, as cluster flies become active again when outdoor temperatures breach 12 degrees Celsius.

Homeowners frequently observe the insects emerging from hibernation and flying toward windows and other sources of light on warmer winter days. Cluster flies tend to enter and hibernate in homes with large, open lawns rather than ones shaded by trees.

Life Cycle

Like other flies, cluster flies develop by undergoing complete metamorphosis from egg to adult. They emerge to mate after the winter and the females lay eggs in the soil during the spring. Larvae hatch from the eggs in approximately 3 days. Shortly after hatching, the larvae look for an earthworm and bore into it. The cluster fly larva will use the earthworm for food until it completes development, which can last for 2 to 3 weeks before it pupates. Cluster fly pupae mature into fully developed adults in 11 to 14 days. The entire cluster fly life cycle generally takes 25 to 39 days to complete. Despite invading and overwintering in homes and structures, cluster flies do not breed  indoors.

Larvae

Rarely seen and interesting behaviour of cluster flies is that the larvae search for earthworms living in the ground. Once located, the earthworm is parasitized by the  larva. Cream coloured and shaped like an elongated wedge, cluster fly larvae feed on live earthworms for about 13 to 22 days before advancing to the pupal stage, which also takes place in the soil.

Problems Caused by Cluster Flies

Although cluster flies are not considered a health hazard like other fly species, the insects can still cause problems when they overwinter in homes. Widely regarded as a nuisance, cluster flies hibernate in large groups and collectively emerge in the spring, producing swarms that gather around windows in unsightly masses. Furthermore, the excrement of hibernating cluster flies may stain the curtains and walls of indoor overwintering sites. Cluster flies that die during hibernation may also attract larder beetles, which use the fly carcass as a food source and then remain in the home.

Prevention Tips

Use a vacuum to catch these pests. Insect light traps attract and capture pests in enclosed spaces. Seal cracks and crevices in exterior walls. Pay special attention to the south and west sides of your home, which provide the most warmth.