Cluster flies

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  • 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


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 the 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 the 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.


Like other flies, cluster flies develop by undergoing a 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 three 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.


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.

What are cluster flies?

Cluster flies are a unique type of pest compared to other flies. They are larger and darker in colour and prefer to live close to the ground, where they can easily feed on plant sap, flower nectar, and fruits. Cluster flies prefer to hibernate throughout the winter and will often enter homes through a door or window crack. They are attracted to sunlight for its warmth, which is why you will often find them clustered together in windows.

Where do cluster flies come from?

Cluster flies can be found on many continents worldwide, including Europe, Australia, and North America. They are very active in the summer months, which is when they reproduce. Cluster flies spend their first few weeks of life underground before emerging from the soil as full-grown adults. Adult female flies can lay up to four generations of eggs a year. When the weather turns colder, they seek the shelter of an interior heated space where they can hibernate during the winter. It is around this time they can usually be found indoors.

Where do cluster flies lay eggs?

Adult female cluster flies lay their eggs in cracks in soil, near earthworm burrows. After hatching, the larvae move beneath the soil and enter the bodies of the worms. Here, they feed on the worms for approximately two to three weeks until they are ready to pupate in the ground, which takes another 11 to 14 days. It is estimated that a female cluster fly will lay eggs four times per year, and approximately 130 eggs each time.

How long do cluster flies live?

Cluster flies can live up to two years or more if conditions are ideal. This is a significant jump compared to most other flies, which only live a few months. Hibernation plays a large role in the lifecycle of cluster flies, and invading indoor spaces before winter arrives is vital to their survival.

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 the 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 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 and other dermestid 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 from invading

Vacuum up any cluster flies you see. Use insect light traps to capture them, and exclusion -seal voids, cracks and crevices in outer walls. Application of exterior perimeter treatment will prevent them from entering indoors.

How to get rid of cluster flies?

Getting rid of cluster flies can be a difficult endeavour due to the time of year they invade your home. Preventative measures are exceedingly helpful in dealing with this pest and can be done in the months leading up to the fall to avoid an infestation. Here are some solutions to consider:

  • A professional pest management company can use an exterior perimeter targeted treatments on your home or business to create a barrier and stop cluster flies from entering your property.
  • Examine possible entry points, including walls, windows, and doors. Seal any cracks or crevices and ensure screens are in good condition.
  • If you find these pests within your home or business, a vacuum or a fly swatter can often get rid of them easily.
  • Do not apply insecticide treatment to control them when in hiding spots. This will kill them and the carcass attract other insects, which will result in secondary infestation.

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