Cluster fly is a collective name given to several species that spend winter as adults in sheltered places, including inside homes and structures. They tend to accumulate in wall voids, attics, and rooms with southern or western exposures. One or more species in this group may be present. While cluster flies are an annoyance, they do not breed indoors, and they do not cause any damage. The flies will become active on warm days, crawling and flying to windows in an attempt to escape outside.
Cluster fly species
The face fly is the most common cluster fly species in structures near pastured cattle. The larvae develop only in fresh cattle manure. During summer, adult flies feed on mucous secretions from the eyes and faces of cattle and horses. They leave animals in the fall to move to wintering sites, often in white or light colored buildings. Face flies return to cattle in late spring and remain near them during summer.
Blue or green bottle flies have shiny dark blue to green metallic bodies and make a loud “buzzing” sound as they fly. They will come to shelters in the fall but can be around most any time of year if there is accessible garbage, pet waste, or an animal carcass in which they can breed. They live in urban and rural areas. Infestations during summer usually disappear after a few days.
Cluster fly is the recognized common name for a sluggish gray fly with short golden “hairs” on its body. The larvae of this insect develop as parasites in earthworms. They occur in urban and rural areas.
Managing Cluster Flies
Unfortunately, it is not practical to eliminate all breeding sites used by the cluster fly group.
That leaves exclusion as the first line of defense. Many obvious entry ways can be plugged; however, it is impossible to eliminate or seal every possible opening that they may use. Some flies may find their way inside after even the most conscientious pest-proofing efforts.
Often cluster flies will congregate in large numbers on outside walls before they enter structures. A number of products are available to homeowners for application to outside walls for control of accidental invaders such as cluster flies, boxelder bugs, etc. Common active ingredients in these products include: cyfluthrin, cypermethrin, lambda-cyhalothrin, deltamethrin, or permethrin. Results are best if flies are contacted directly by the spray. There will be some lasting effect against flies landing on treated surfaces, but the residue will be degraded by sunlight and weathering. Several applications may be needed because the flies move to shelter over a period of time rather than all at once. These applications may leave visible residues on treated areas.
Once indoors, the best tactic is to swat individual cluster flies or suck them up using a shop vac. Cluster flies do not move about much so fly paper, fly strips, and bug electrocuters generally have limited effect on fly numbers. Indoor use of insecticides is rarely very effective. In fact flies killed in attics and wall voids by indoor treatments can be attacked by scavengers such as carpet beetles.
By Lee Townsend, Extension Entomologist