Cluster Fly
Pollenia rudis
Anatomy
Identification
Distinguishing Features
- Wings overlap almost completely at rest (scissor-like pattern)
- Larger and more robust than common house flies
- Golden-yellow hairs on thorax (diagnostic when present)
- Sluggish flight and movement, especially in cold weather
- No metallic sheen unlike blow flies
Similar Species
- House flies are smaller, more agile, and hold wings at an angle (not overlapping). They breed in filth.
Biology
Lifecycle
Females lay eggs in soil near earthworm burrows. Larvae are internal parasites of earthworms, feeding until pupation. There are typically 4 generations per year, with the final generation seeking overwintering sites.
Diet
Adults feed on flower nectar and plant sap. They do not feed on garbage, feces, or decaying matter like house flies. Larvae feed exclusively on earthworms.
Nesting
Do not breed indoors. They enter structures in late summer/fall to overwinter in wall voids, attics, and behind siding. Attracted to south and west-facing walls that absorb heat. They cluster together for warmth, sometimes numbering in the thousands.
Signs of Infestation
Large numbers of sluggish flies
Clusters of slow-moving flies on warm, sunny windows in winter or early spring. They often appear disoriented and lethargic.
Aggregations on exterior walls
Masses of flies gathering on south or west-facing walls in autumn, seeking entry points to overwinter.
Flies in attic spaces
Finding hundreds or thousands of flies (dead or alive) in attic spaces, wall voids, or around window frames.
Sweet, buckwheat-like odour
Large cluster fly populations produce a distinctive sweet smell when disturbed or crushed.
Where They Nest
- Attic spaces and under roof insulation
- Wall voids, especially on south and west sides
- Around window frames and behind curtains
- In soffits and behind fascia boards
- Around light fixtures and ceiling voids
Distribution in Canada
Common throughout southern Canada, particularly in rural and suburban areas with earthworm populations. More problematic for homes near open fields and lawns.
Active Season
Adults seek overwintering sites in late August through October. They emerge sluggishly on warm winter days and in early spring before leaving to lay eggs outdoors.
Prevention
- Seal gaps around windows, doors, and siding in late summer before flies seek entry
- Repair damaged screens and install screen vents in soffits
- Caulk around utility penetrations and cable entries
- Apply exterior perimeter treatment in late August/September for prevention
- Install door sweeps and weather stripping
Professional Treatment
Prevention is key - exterior treatments in late summer create a barrier before flies seek entry. For existing infestations, treatment of attic and wall void areas combined with exclusion work provides control. Unlike house flies, sanitation is not a factor.
Get General Pest Control ServiceFrequently Asked Questions
What does a Cluster Fly eat?
Adults feed on flower nectar and plant sap. They do not feed on garbage, feces, or decaying matter like house flies. Larvae feed exclusively on earthworms.
Where does a Cluster Fly nest?
Do not breed indoors. They enter structures in late summer/fall to overwinter in wall voids, attics, and behind siding. Attracted to south and west-facing walls that absorb heat. They cluster together for warmth, sometimes numbering in the thousands.
When is a Cluster Fly most active?
Adults seek overwintering sites in late August through October. They emerge sluggishly on warm winter days and in early spring before leaving to lay eggs outdoors.
What are the signs of a Cluster Fly infestation?
Large numbers of sluggish flies: Clusters of slow-moving flies on warm, sunny windows in winter or early spring. They often appear disoriented and lethargic. Aggregations on exterior walls: Masses of flies gathering on south or west-facing walls in autumn, seeking entry points to overwinter. Flies in attic spaces: Finding hundreds or thousands of flies (dead or alive) in attic spaces, wall voids, or around window frames. Sweet, buckwheat-like odour: Large cluster fly populations produce a distinctive sweet smell when disturbed or crushed.
How can I prevent a Cluster Fly infestation?
Seal gaps around windows, doors, and siding in late summer before flies seek entry. Repair damaged screens and install screen vents in soffits. Caulk around utility penetrations and cable entries. Apply exterior perimeter treatment in late August/September for prevention. Install door sweeps and weather stripping.
How is a Cluster Fly treated?
Prevention is key - exterior treatments in late summer create a barrier before flies seek entry. For existing infestations, treatment of attic and wall void areas combined with exclusion work provides control. Unlike house flies, sanitation is not a factor.
Scientific Classification
- Kingdom
- Animalia
- Phylum
- Arthropoda
- Class
- Insecta
- Order
- Diptera
- Family
- Polleniidae
- Genus
- Pollenia
