Background on the Cluster Fly
Cluster Flies closely resemble House Flies, but they are usually larger (3/8"-1/2")
and have a yellowish sheen on the thorax. With a mostly black body, their thoracic
hairs are short, curly, and yellow to golden. They have tufts of crinkly yellow-orange
hairs where their wings attach to the thorax. Older specimens may have some of the
yellow thoracic hairs rubbed off. The wings often overlap at their tips when the
fly is resting.
Cluster Fly larvae are parasites of earthworms and breed outdoors in lawns and fields
during the spring and summer. Earthworms are usually most abundant around places
where manure has been piled or stored and are common in grassy areas where the soil
is moist.
The large numbers of adults that emerge in late summer and early fall seek protected
places to spend the winter. Often, as with Face Flies, this site is a structure.
Inside a structure, the flies tend to “cluster” in large numbers at windows or on
ceilings or other high places.They often occur within the walls, attics and basements
of homes. Window screens alone offer no protection from these flies because they
crawl into the home through small openings in the walls of the building. Both Face
Flies and Cluster Flies may be attracted to the same structure year after year.
Cluster Flies are also bothersome in the spring when they try to escape from the
home. Vacuuming is the most effective means of removal from inside a home.
While no scientific studies have been completed to determine the effectiveness of
Fly Predators for Cluster Flies, anecdotal reports from Fly Predator users indicate
that one application of Fly Predators around the perimeter in spring provides control
for the following year.
Spalding Labs would like to thank the following people who contributed to or reviewed
this guide. Dr. Bill ClymerFort Dodge Animal Health, Dr. Roy EllisPrairie Pest Management,
Dr. Kevin FloateAgriculture and Agri-Food Canada, Dr. Robert M. Miller, DVM, Dr.
William QuarlesBio-Integral Resource Center. All illustrations 2006 Dr. Roy Ellis.