Quick facts
- Size
- About 3/8 inch, slightly larger than a house fly
- Looks like
- Dark gray, golden hairs on the thorax, wings overlap at rest
- Peak season
- Entry in September, emerge on warm winter and spring days
- Where in Michigan
- Statewide, especially homes near lawns, pastures and fields
- Main risk
- Nuisance, dead flies attract other pests
- Risk level
- Low
Signs you have cluster flies
- Sluggish flies on sunny windows in winter or early spring
- Flies gathering in upper rooms, attics and around light fixtures
- Piles of dead flies on window sills and in attics
- Flies crawling out of outlets or ceiling fixtures
- Clusters of flies on the sunny side of the house in late summer
Where cluster flies come from
Cluster flies lay eggs in soil, and the larvae develop as parasites inside earthworms. Michigan lawns, pastures, hay fields and farm ground with healthy earthworm populations can produce a lot of them over the summer. That is why rural homes and homes surrounded by large lawns or fields tend to have the worst problems, often year after year.
They do not breed in your house, in garbage or in pet waste. They are not the same as house flies or blow flies, and seeing them does not mean something is dead or rotting in the walls. The flies in your attic grew up in the soil outside last summer and came in to wait out the winter.
Why cluster flies end up in your attic
In late summer and early fall, adult cluster flies gather on warm, sunny walls, especially the south and west sides. They crawl into gaps around siding, soffits, window trim and roof vents and settle into attics and wall voids to overwinter. Older farmhouses and homes with lots of gaps can host thousands.
On warm, sunny winter days, the heat inside the walls can wake them up. They crawl toward light and end up in living spaces, often upstairs bedrooms and rooms on the sunny side. In March and April they try to get back outside and gather on windows, buzzing loudly and moving slowly, which is how they got the nickname attic flies.
Large numbers of dead flies in the attic can attract carpet beetles and larder beetles, which then wander into the house. Vacuuming attic floors and window sills each spring cuts that secondary problem down. Weatherstrip the attic hatch too, so flies that wake up in the attic stay there instead of drifting down into the hallway.
Cluster fly prevention and treatment
Seal gaps in late August and September, before flies move in. Screen attic and soffit vents, caulk around window and door trim, and repair loose siding. Many Michigan homes see good results combining sealing with an exterior treatment applied by a licensed applicator in late summer to the sunny walls where flies land.
Inside, use a vacuum, fly swatter or window light traps. Avoid spraying insecticides inside wall voids or attics without professional guidance, since dead flies there feed other pests. Sealing for flies also helps keep out stink bugs and Asian lady beetles, which use the same entry points. A shop vacuum makes quick work of a sunny window covered in flies.
What you can do yourself
- 1Caulk gaps around windows, doors, siding and trim in late August or early September.
- 2Screen attic, gable and soffit vents.
- 3Vacuum flies from windows and attics.
- 4Use window fly traps or sticky traps on sunny windows.
- 5Seal around light fixtures and outlets on exterior walls.
When to call a licensed pro
- You get large numbers of flies every winter.
- Flies keep coming out of ceilings or upper walls.
- You need exterior treatment or sealing high on the house.
What cluster flies treatment costs in Michigan
$150 to $400per treatment
Typical Michigan ranges, not quotes. Price depends on the size and height of the home, how many sides are treated, and whether exclusion work is included.
Typical ranges, not quotes. See the full Michigan pest control cost guide.
Cluster flies: common questions
Are cluster flies harmful?
No. They do not bite, they are not known to spread disease, and they do not breed indoors. They are a nuisance because of their numbers, their buzzing at windows, and the dead flies that pile up in attics and on sills.
Why do I see flies in winter?
Cluster flies overwinter in attics and wall voids. A warm, sunny day heats the walls and wakes them up, and they crawl toward light, which often leads them into rooms instead of back outside.
When should I treat for cluster flies?
Late August to mid-September, before they move into the walls. Treating after they are inside does little. See the pest calendar for timing of all the fall invaders, and plan the work in early August.
How do I tell cluster flies from house flies?
Cluster flies are larger and slower, with short golden hairs on the thorax. Their wings overlap over the back when resting. House flies are smaller, quicker and have clear stripes on the thorax.