Cluster flies

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Identification

  • Colour Dark grey
  • Size From 8 mm to 10 mm long
  • Description Have golden or yellowish hairs on their thorax, while their abdomen features a black and silver checked pattern. Their wings overlap across the abdomen when at rest.
  • Notes Cluster flies strongly resemble house flies, but are larger, darker, and slower.
Cluster fly on leaf

Appearance

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.

Lifecycle

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.

Why do I have cluster flies

Cluster flies enter homes in the late summer months and the early fall, looking for somewhere to hibernate as the temperatures drop and winter approaches. West and south facing buildings, with large, open lawns, exposed to more sunlight, are especially attractive to cluster flies.

The pests find their way in through openings in the wall, like cracks and crevices near window and door frames, open and unscreened windows, and unscreened vents that provide access into the building.

Once inside, cluster flies then gather together in isolated and protected locations, such as attics and wall voids, to hibernate for the winter.

How worried should I be about cluster flies

Cluster flies do not pose a danger to humans and do not breed in the homes they invade, but they are widely regarded as a nuisance, especially as they usually hibernate en masse. When they collectively emerge in the spring, it produces swarms that gather around windows.

As well, indoor warmth and unseasonably warm weather can interrupt hibernation, prompting the mass of cluster flies hidden away to suddenly become active again. Homeowners often see the emerging insects flying toward windows and other sources of light on warmer winter days.

As well, the excrement of hibernating cluster flies can stain curtains and walls. Cluster flies that die during hibernation can attract larder beetles, which use the fly carcass as a food source and then remain in the home. To stop an infestation, get help from a professional pest control service.

How can I prevent cluster flies invading

Vacuum up any cluster flies you see, Use insect light traps to capture them, Seal cracks and crevices in outer walls

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