Cluster Flies: Identification, Prevention, and Safe Removal

Cluster flies—also called attic flies—are larger than house flies, about 8–10 mm long, with grey‑checked abdomens and golden hairs on the thorax. They move sluggishly, seeking warmth, and often gather in large numbers on sunny walls or inside attics.

Life Cycle and Behavior

They breed in late summer to early fall. Eggs hatch in days; larvae parasitize earthworms, then pupate. Development from egg to adult takes as little as 27 days. In autumn, they cluster on warm exterior walls and may enter homes through cracks.

Why They’re a Nuisance

Cluster flies don’t damage structures, but they leave dark stains on windows, walls, and curtains. Their sheer numbers can be overwhelming, and dead flies may attract secondary pests like carpet beetles.

Prevention Is Easier Than Cure

• Seal cracks around windows, doors, and utility penetrations with silicone or silicone‑latex caulk.
• Repair or replace damaged window and door screens.
• Inspect the exterior before breeding season if you’ve had issues before.

Don’t Spray Indoors—Call a Pro

Using over‑the‑counter insecticides inside can kill flies but leave carcasses that attract other pests. A pest control specialist can safely eliminate the current infestation and implement exclusion measures to prevent recurrence.