Barbara J. Bromley, Mercer Co. Horticulturist 2004
Homeowners may notice clusters of hibernating flies in attics or wall voids. These flies become active on mild days in winter, in spring when they attempt to leave the building, or in autumn when they come indoors to hibernate. The warmth of an indoor heating system or of the sun on exterior walls can stimulate them enough that they wake up and become active in winter. When they emerge they seek light at windows or ceiling light fixtures.
The adult fly is about 3/8 inch long and robust. They are dark gray, non-metallic, have many short golden hairs on the thorax or behind the head (visible with a 10X hand lens,) and the abdomen has irregular lighter areas. Their movements are more sluggish than houseflies. They give off a buckwheat honey odor and leave a greasy spot when crushed.
Cluster flies parasitize earthworms and, therefore, are found wherever earthworms are found. Adults overwinter in sheltered areas such as attics and wall voids and emerge in spring to mate and lay eggs in soil cracks. The eggs hatch in about 3 days; then the larvae enter and feed on earthworm hosts. Development time from egg to adult is from 4 to 6 weeks. There may be several generations per year.
Outdoor controls are generally unsuccessful since the earthworm is their host. Repellent sprays (such as a long lasting pyrethroid) on the outside of buildings are generally made about mid to late August to prevent the flies from coming inside for the winter. Screen, caulk, and seal outdoor entry points in June or July after all flies have gone outside for the summer, but before they come back inside in fall.
Sprays can be used in the attic or in individual rooms when the flies become active indoors, but no attempt should be made to kill them in wall voids. [Dead flies will attract dermestids (carpet beetles, larder beetles etc.) which would become a worse pest.] Temporary relief can be obtained indoors by vacuuming up pesky flies or trapping them on flypaper or sticky boards. A continuously burning 60 watt light bulb in the attic or an infested room will attract the flies. They will use up their available food reserves and die near the light. They must be swept or vacuumed up.
IPM CONTROL STRATEGIES: CLUSTER FLIES
Biological Fly-eating insects and animals: not reliable control Cultural Mechanical/Physical Screen attic vents and louvers in summer Caulk or use weather stripping on cracks and crevices or any outside point of entry in summer Caulk interior entry points (ceiling fan or light base, electrical sockets, baseboards) anytime Hang fly paper or glue boards (sticky traps) Pick up with vacuum cleaner Fly swatter Constantly burning 60 watt light bulb in winter Chemical – indoors Spray of pyrethrin or pyrethrin+ piperonyl butoxide Chemical – outdoors Cyfluthrin Cypermethrin (Cynoff) Pyrethrin