About Cluster Flies

Cluster flies (Pollenia rudis) resemble house flies but are slightly larger, with dark gray bodies and golden hairs on their thorax. Unlike house flies, they do not breed in garbage or food waste. Instead, they develop outdoors as parasites of earthworms.

Risks & Concerns

While not harmful to humans, cluster flies invade buildings in large numbers, often hiding in attics, wall voids, and window frames to overwinter. They emerge on warm days, sluggish and annoying, and leave stains on walls and windows.

Prevention & Control

Seal cracks around windows, doors, and siding in the fall. Vacuum up individual flies and avoid crushing them, as they leave oily marks. Professional treatment may be needed to prevent large overwintering populations.