Cluster Flies
(Pollenia rudis)
Large, sluggish flies that enter attics and wall voids in fall to hibernate in large clusters.
Overview
Cluster flies are fall-invading nuisance pests that enter Bowling Green homes in large numbers seeking warm places to overwinter. They are often confused with house flies but are slightly larger, move more sluggishly, and produce a distinctive buzzing sound. In attics and wall voids, they can cluster by the thousands — hence their name. They do not bite, spread disease, or breed indoors, but their large numbers and tendency to emerge on warm winter days make them a persistent seasonal nuisance.
Appearance
Cluster flies are about 3/8 inch long, slightly larger and more robust than common house flies. They are dark gray with distinctive golden or yellowish curly hairs on the thorax (visible with a hand lens). Their wings overlap when at rest, giving them a narrower silhouette than house flies. They fly more sluggishly than house flies.
Habitat
During warmer months, cluster flies live outdoors as parasites of earthworms. In fall, they seek warm overwintering sites, typically on the upper stories and south-facing walls of buildings. They enter through small gaps in soffits, window frames, and around attic vents. Inside, they hibernate in wall voids, attics, and behind window casings, emerging on warm, sunny days throughout winter.
Diet
Adult cluster flies do not feed indoors. Outside, they feed on flower nectar and the juice of decaying fruit. Their larvae are internal parasites of earthworms in the soil.
Lifecycle
In spring, overwintering adults emerge from structures and lay eggs in soil near earthworm burrows. Larvae hatch, burrow into earthworms, and develop as internal parasites. They pupate in the soil, and adults emerge in late summer. Two to four generations per year are typical. The last generation of the year migrates to buildings in fall.
Signs of Infestation
- !Large numbers of sluggish flies in attics, around upper-floor windows, or on warm, south-facing walls
- !Buzzing or tapping sounds in walls or ceilings on warm winter or spring days
- !Dead flies accumulating on window sills, in light fixtures, and in attic spaces
- !Flies emerging from behind baseboards or from ceiling fixtures on warm days
Prevention Tips
- ✓Seal gaps around soffits, attic vents, window frames, and roof intersections before September
- ✓Ensure all attic ventilation openings have fine-mesh screening
- ✓Apply exterior perimeter treatments to the upper stories of the home in early fall
- ✓Caulk around window and door trim on upper floors
Professional Treatment
Exterior perimeter treatments applied in August or early September — before flies begin entering — are the most effective approach. Once flies are in wall voids, they are difficult to treat. Emerging flies can be vacuumed or removed individually. Light traps in attics can capture emerging adults during winter. Thorough sealing of entry points on the upper stories provides long-term prevention.
Did You Know?
- •Cluster fly larvae are parasites of earthworms — one of the few fly species with this lifecycle.
- •They get their name from their habit of clustering in huge numbers in attics and wall voids.
- •Unlike house flies, they do not breed in garbage or filth and pose no sanitation risk.
- •A single attic can harbor thousands of overwintering flies.
About 3/8 inch
Dark gray with golden hairs on thorax
6
Stockier and slower than house flies
Attic fly, Buckwheat fly
Don't let them take over your home. Our expert technicians can eliminate the problem quickly and safely.
Or call us directly at
(270) 510-0473
Other Occasional Invaders in Our Library
Brown Marmorated Stink Bug
Shield-shaped invasive insects that enter homes in large numbers in fall and release a foul odor when disturbed.
Boxelder Bugs
Black-and-red bugs that swarm on sunny walls in fall before entering homes to overwinter.
Earwigs
Distinctive insects with rear pincers that hide in moist areas and enter homes during wet weather.
Millipedes
Slow-moving, many-legged arthropods that invade homes in large numbers after heavy rains.