Cluster Flies

Learn about Utah Cluster Flies

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Cluster flies belong to the family of flies known as Calliphoridae. Although these flies are not native to Utah, they are found in many parts of the state. They are a nuisance pest because they inhabit homes in large numbers during the fall and winter, especially on sunny days.
The adults are slightly larger than house flies measuring about 1/4-inch long. Their body color is dark grey, with black hairs on their upper surface. Larvae develop inside earthworms as parasites. They pupate underground, and adults emerge in late summer or early fall.


Cluster flies are most common in rural areas and may also be found in suburban and urban areas. They are commonly found in homes, outbuildings, churches, schools, and other buildings. Cluster flies typically invade buildings during fall seeking remote overwintering sites.
Cluster flies often enter attics through roofs and soffit vents or louvers. They first become active on warm days in late winter or early spring, constantly emerging from wall voids at windows where they attempt to fly outdoors. The flies continue to emerge as temperatures rise into the 70s, especially on sunny days.

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Cluster flies are considered secondary pests as they do not cause any damage to plants, structures, or stored products. The diet of the cluster fly is not well known, but adults are thought to feed on various flowers and plant exudates. Larvae probably feed on earthworms. When the weather gets cooler, and the food supply is reduced, cluster flies will seek shelter in warm places to overwinter.

Life Cycle & Reproduction

The life cycle of the cluster fly is similar to that of the common housefly, although it takes between eight and ten weeks to complete. This means that development occurs more slowly than in the common housefly. They do not reproduce until spring when they emerge from their winter hiding places and lay eggs near earthworms on grassy lawns.
Adult females lay their eggs in cracks and crevices in the soil. The eggs hatch into larvae which immediately burrow into the ground. They feed on earthworms, where they grow and develop for two to three weeks. The larvae then pupate within earthen cells for about ten days, after which adult cluster flies emerge.
An adult cluster fly may look similar to a common housefly. Still, it is slightly larger (8-10mm in length) and has a golden sheen on its thorax when viewed from above. The critical difference between the two species is that cluster flies are a nuisance pest only indoors (they do not breed or feed indoors), whereas the common housefly breeds and feeds indoors and outdoors. The female cluster fly can lay several hundred eggs during her lifetime, lasting up to six weeks.


Cluster flies are slow-moving and often fly into windows, doors, and gaps around window frames. They do not bite or sting and do not carry diseases. They will, however, contaminate food. Cluster fly infestations can be a nuisance because of the large number of flies that enter structures at one time. They are especially prevalent in buildings near wooded areas where there is an abundance of earthworms to serve food for the developing larvae.

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