Identify Your Pest Flying insects Cluster fly
Cluster fly
Pollenia spp. · Also called: Attic fly
Cluster flies are slow, dull grey flies that crowd onto sunny windows in fall and again in late winter and spring. They spend the winter hidden in attics and walls, but they do not breed indoors and are not known to spread disease.
Size 8 to 10 mm long
Colour Dull dark grey with a checkered silvery abdomen
Key feature Golden, crinkly hairs on the thorax and wings that overlap flat at rest
Season Enter late summer to fall; reappear on warm winter and spring days
Where Attics, wall voids and sunny south or west windows, often in upper rooms
What it looks like
Check a resting fly up close. It measures 8 to 10 mm, longer than most house flies, and its grey body has a flat, dull look. Short golden hairs cover the thorax, the part behind the head. The abdomen shows patches of silver and dark grey that shimmer as it moves. When it sits, one wing lies almost fully on top of the other.
These flies move slowly and seem clumsy, especially indoors. They often buzz against windows, crawl on warm walls or drop onto sills. They show up in groups on the sunny side of a building. You usually will not see them around food or garbage.
Often confused with
House fly
Blow fly
Signs to look for
Flies at sunny windows
Dead flies on sills
Flies in upper rooms
Flies resting on siding
Small dark spots
Risks and damage
Cluster flies are mainly a nuisance. They do not bite, breed indoors, damage fabric or feed on stored food, and Health Canada does not see them as a usual health risk. Their droppings can speckle glass and sills, and a swatted fly can leave a greasy mark. Carpet beetles and larder beetles feed on dead insects, so a big build-up of dead flies in an attic or wall can start a second pest problem.
In Alberta
Cluster flies are found in Alberta, and some homes, often those beside farmland or wide stretches of grass, see them each fall and spring. Published records from the Prairies are fewer than from eastern Canada, so how often they turn up varies by area.
Young cluster flies are parasites of earthworms. The larvae hatch in the soil, find an earthworm and feed inside it before turning into adult flies.
How to keep them out
Close gaps by mid-August
Repair screens and vents
Close gaps at doors and windows
Vacuum flies indoors
When it is time to bring in a professional
Call a professional if flies return in large numbers every year or you cannot find how they get in. The same goes if dead flies build up in an attic or wall. Technica’s licensed technicians can inspect the outside of the home and help plan sealing and treatment for the right time of year, which is usually late summer.
Frequently asked questions
Are cluster flies breeding in my house?
No. Cluster flies breed outdoors in soil, where their young feed on earthworms. The flies you see indoors came in as adults to spend the winter. They do not lay eggs inside the home.
Why do they show up on warm winter days?
Sun and heat warm the walls and attic where the flies are resting. They wake up and crawl toward the light, often ending up in living spaces by mistake. On cold days they go quiet again.
Will cluster flies come back every year?
Often, yes. The same buildings tend to be chosen each fall, especially tall homes with sunny walls near open grass. Sealing gaps before late summer is the most useful step you can take.