Cluster Flies


Cluster flies look very much like large house flies, are about 3/8 inches long; and are colored dark gray. In addition, they have goldish-colored hairs on their body.


Cluster flies become a nuisance inside structures due to their behavior of seeking warmer locations to overwinter and then once again becoming active in the spring as they try to move outdoors. While this behavior is troublesome, it is important to know that cluster flies are not a medically important species of fly, and they do not reproduce inside structures since they will either die or leave before reproducing.

These flies are hard to manage since they seek out places to overwinter that are hidden and hard to reach like attics, wall voids, under piles of clothing or behind window blinds and furniture. If cluster flies do not move inside, they will overwinter under tree bark or in other protected outdoor locations. In general, cluster flies seem to prefer structures that are located on the higher elevations within their environment and tend to gravitate more to the sunny, exposed southern and western sides of a home or other building.


Cluster flies don’t develop on garbage as do filth flies, but instead are parasites of earthworms in the larval stage. As adults, they are primarily consumers of sugars and carbohydrates from plant sap, fruit and flowers.


As adults, their preferred habitat is similar to many other flies, with the exception of not being filth feeders and not colonizing dead animals or fecal matter. As larvae, they live in the soil where they parasitize earthworms.


Cluster flies go through complete metamorphosis – egg, larva, pupa and adult stages – and their life cycle is completed in about two months or less under favorable conditions. Female flies mate in the spring and lay their eggs in the soil near an earthworm. After about three days, the eggs hatch and the larvae burrow into the earthworm and feed on the inside of the earthworm. In time, the larvae become inactive pupae and then become adults.


Cluster flies are usually noticed by the buzzing sound they make as they fly; their appearance in or near light fixtures and sightings around windows; and on the sunny sides of buildings in the fall and spring months.


The most effective preventive technique for cluster flies is to keep them outside of your home. Therefore, preventing their access by screening entry points in attics and around doors and windows is the best thing to do.


If cluster flies get inside, using a vacuum to remove them or a fly swatter to kill them are the best approaches. One may also use an aerosol insecticide to directly spray adults and apply an exterior insecticide to locations where flies may congregate or in openings into the structure they are using as entry points to overwinter. Specialized light traps placed in the attic or other locations where flies are found can also work well. Controlling earthworms is not an effective or practical way to help reduce cluster fly populations.