- Color: nonmetallic gray; lack stripes on the thorax (segments with the wings and legs attached); and yellow or golden hairs on the back, behind the head, and around the base of the wings.
- Size: 3/8-inch
- Appear narrow at rest because their wings completely overlap over their backs.
- Larvae are typically spindle-shaped maggots but are seldom seen.
Habits of Cluster Flies:
- Female flies mate in the spring and lay eggs outdoors in soil crevices.
- Eggs hatch and the larvae burrow into the bodies of earthworms where they develop. Development time (egg to adult): 27 to 39 days.
- There typically are four generations per year.
- They overwinter as adults within the attics or wall voids of structures, especially older frame buildings. They enter structures in early fall to seek shelter from cooling temperatures. A “cluster” of adult flies soon accumulates within wall voids and dark corners, under shelving, beneath curtains, and in other protected areas.
- When the weather warms, the cluster flies emerge from their hiding places and either exit the building or enter interior areas.
- Stimulated by warmth, they often are found on the south and west sides of buildings.
- Once stimulated, cluster flies are attracted to light.
Damage Caused by Flies:
- On warm days in winter or spring, cluster flies annoy building occupants when they become active and crawl sluggishly over walls or windows.
- Adult flies on well-lit or warm areas.
How to Control and Kill Cluster Flies:
- There is no effective control of the larval stage of these flies because they develop in earthworms.
- Initiate control tactics for cluster flies before they enter buildings in large numbers.
- Entry points into walls and roof of structures should be sealed. To prevent entry into interior rooms, possible 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 lit 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.