Cluster flies are close relatives of blow flies and are similar in size
to house flies (3/8-inch) but are more robust in body structure. They
are nonmetallic gray, lack stripes on the thorax (segments with the
wings and legs attached), and have yellow or golden hairs on the back,
behind the head, and around the base of the wings. Cluster flies appear
narrow when at rest because their wings completely overlap over their
backs. The larvae are typical spindle-shaped maggots but are never seen
because they develop as parasites in earthworms.
- Biology: The female
flies mate in the spring and lay their eggs in soil crevices. The
eggs hatch in three days and the larvae burrow into the bodies of
earthworms where they develop. Development (egg to adult) requires
27 to 39 days. There are usually four generations per year.
- Habits: Cluster
flies are annoying because they over winter as adults in the attics
and wall voids of structures, especially older frame buildings. The
common name of this species reflects its behavior of gathering in
clusters before hibernation. They enter structures in early fall to
seek shelter from cooling temperatures. Soon, a "cluster"
of adult flies accumulates in wall voids and dark corners, under
shelving, beneath curtains, and in other protected areas.
On warm days in winter and spring, they annoy building occupants
when they become active and crawl sluggishly over walls or windows.
When the weather warms, the cluster flies emerge from their
hiding places and either exit the building or enter interior
areas. They are stimulated by warmth and are often found on the
south and west sides of buildings. Once stimulated, cluster flies
are attracted to light.
- Control: There is no
effective control of the larval stage of these flies because they
develop in earthworms. Control tactics for cluster flies should be
initiated before they enter buildings in large numbers. The most
effective long-term control in structures attractive to
overwintering adults is to seal entry points in the walls and roof
of the structure. To prevent entry into interior rooms, entry
routes, e.g., around window pulleys, electrical outlets, switch
boxes, and window and door frames should be sealed. Large
accumulations of these flies can be removed with a vacuum cleaner.
During the winter, a bare light bulb in an attic will cause the
flies to die from cold exposure and exhaustion of their food
reserves. Insect light traps can also be placed in attics but
require frequent servicing.
Cluster flies can not be controlled by disrupting the life cycle of
the larvae (maggots) because they are parasitic on earthworms which
are beneficial contributors to the environment. Residual
applications of micro-encapsulated and wettable powder formulations
should be applied in the fall to the exterior surfaces of structures
in order to control these flies prior to entry.