Cluster Fly Identification
Adult cluster flies are approximately 8 to 10 mm long. They are dark grey with black and silver (non-metallic) checkered abdomens, with many golden hairs on their thorax (these may or may not be present on older flies). Their wings overlap when they are at rest. The name “cluster fly” is derived from their habit of gathering in clusters before and during hibernation.
Cluster Fly Habitat
Cluster flies hibernate in hidden areas of homes, such as in wall voids, attics, closets and empty rooms. They are a particular nuisance as they leave stains on the walls and curtains. If the flies die in the wall voids they may attract larder beetles, which will feed on the dead flies and then migrate to other areas of the house. In the spring, the sluggish cluster flies will migrate from their hibernating areas to living spaces and gather on windows as they are attracted to light. If disturbed during hibernation, a gathering of cluster flies may emit a sickly, sweetish odour.
Cluster Fly Life Cycle
Female cluster flies mate in spring and lay their eggs in the soil. The eggs hatch about 3 - 7 days at which time the larvae tunnel into the body cavities of earthworms to overwinter until the following spring when feeding on the earthworm (host) resumes.
The growth stage of cluster fly larvae to adult requires 27 - 39 days. In midsummer, adults emerge from the soil. These adults and the cluster flies that hibernated in households will mate and lay eggs in late summer or fall, repeating the cycle.
Cluster Fly Prevention
Since the larvae of a cluster fly is parasitic on earthworms (they develop within earthworms), there is no effective control against the larval stage. The best defense against a cluster fly infestation is to deter their entry into the building so they do not accumulate in large numbers. Seal the roof, windows, doors, and wall cracks. Use a vacuum to remove large numbers of cluster flies that are in hibernation. Electrical light traps also prove effective in attracting flies inside structures.
Preparation Steps for Cluster Fly Treatment
1. Treatment performed by Abell Pest Control will vary depending on the type of fly. An inspection and report detailing breeding sites will be performed.
Recommended sanitation must be performed prior to any treatment. Fly traps may be used, in this case any trap should be left where the technician places them - Please do not disturb. Outside a treatment around window sills and eaves may take place.
2. Screen or seal all openings to the outside.
3. Vacuum up dead cluster flies and remove the vacuum bag.
4. It may be necessary to vacate the premises during service and not re-enter until at least 4 hours after treatment. Persons with underlying health conditions should consult their family physician where they have concerns regarding re-entry. Open windows for 20 to 30 minutes after re-entry is possible.
Download the Prep Sheet for Cluster Fly (PDF)