When do cluster flies come into Milwaukee homes?
According to the UW-Madison entomologist the site relies on, cluster flies start coming into homes from about the middle of September, sometimes in very large numbers. The entry runs through the autumn, so the window for sealing is short and falls early.
The date is the most useful fact a homeowner can use. A visit or a sealing job scheduled for the first week of September is ahead of the entry, while the same job booked for the first week of October is behind it. The sources give mid-September as the start, not a fixed day, so the plan should be set by the first warm week in early September rather than by the calendar alone.
The entry is usually noticed through the flies themselves rather than through the gaps. A homeowner sees a few on a sunny wall, then dozens in a room, and by then the path is already in use. That pattern is why this page treats the first sighting as an alarm, not a count to watch.
- Early September: inspect and mark the outside gaps
- Mid-September: the sources describe the start of the entry
- October: remove the flies inside and record the points for next year
Why does sealing come before cleaning up?
Sealing stops the next entry and cleaning removes what is already in. The sources point to sealing first because the flies that are inside are a result of the gaps outside, and the gaps are the part a homeowner can change.
A clean-up on its own is a short-lived job. The flies that are removed from a room are replaced by the next group arriving through the same gap, and a homeowner who keeps vacuuming without sealing will be doing the same work every week for the length of the entry. Sealing shortens the season for that building, because it closes the points the flies use.
Sealing is also more precise than a general spray. A spray put on the inside of a room reaches only the surfaces the spray lands on, while a sealed gap is closed on the path itself. The Milwaukee source for boxelder bugs recommends the same order, and it applies just as well to cluster flies: find the entry, close it, then deal with what is left inside.
- Find the gap on the outside first
- Close it with a material suited to the joint
- Remove the flies already inside
- Record the point for the next year
What happens to the flies after they come inside?
Cluster flies that come indoors do not need food or a host in the house to survive the autumn, so they gather in warm, sunny rooms and stay quiet until they are disturbed. The sources say the eggs are laid outdoors in soil, which is why the outside of the house is the lasting place to work.
Inside, the flies gather where it is warm and sunny, often upstairs or in a room with a south-facing window. They move slowly and often settle on curtains, frames and light fixtures. Because they are quiet, a room can look clean one day and fill with flies the next after a warm spell wakes them.
The Milwaukee sources say the next generation is laid as eggs in soil outdoors the following spring. This is a reason to set the outside of the building as the main site of work, and it is also a reason to ask about the soil around the foundation when the inspection is done. This page does not claim the flies breed in the house, because the sources do not say that.
- Warm, sunny rooms and window frames
- Quiet in cool weather, active after a warm spell
- Eggs laid in soil outdoors, per the UW-Extension agent
How should a homeowner remove cluster flies inside?
Remove cluster flies with a vacuum or a soft brush and a closed container, and avoid crushing them on the wall or the glass. Crushed flies leave a smear and a smell, and they do not stop the next arrivals.
A handheld vacuum is the simplest tool. Empty it outside, away from the windows, and then clean the frame and sill with a damp cloth. Dead flies in a window track can be swept up with a dry brush. Keep the vacuum's bag or canister closed afterward, because a dead fly left in a reservoir attracts more of the same.
Avoid spraying the room with household insecticide as a first step. A spray on a window or a curtain does not reach the flies that are still moving, and it puts a chemical in a room where people and pets sleep. The technician will say what, if anything, is appropriate for the room once the sealing points are known.
- Use a vacuum or soft brush, not a hand
- Empty the vacuum outside
- Wipe the frame and sill with a damp cloth
- Do not crush the flies on the glass
What does a cluster fly visit cover at the outside of the house?
The outside visit covers the foundation, the siding joints, the trim around windows and doors, the soffit and eaves, and the vents and utility entries. Each gap found is marked and sealed, and the result is recorded for the next season.
The technician works from the lowest point up. The foundation and the joint between the foundation and the siding are checked first, because a gap there can carry water and insects alike. Then the window and door trim, the soffit, the eaves and any vent or pipe that enters the wall are examined in turn.
Each gap is measured against the material that would close it. A narrow crack can take a flexible caulk, a larger opening may need a fine mesh or a metal cover, and a vent needs a screen that keeps insects out without blocking the airflow. The report names the material used and the point where it went.
- Foundation and the foundation-to-siding joint
- Window and door trim
- Soffit and eaves
- Vents and utility entries
How does the plan change in a spring after an autumn entry?
The spring visit checks the sealed points for failure and looks at the outside soil where the sources say the eggs are laid. It does not treat the lawn or the soil for cluster flies, because the sources do not describe that as a method.
A seal that held through the winter is the first thing to check. Freeze and thaw cycles can open a caulk joint, and a vent screen can be damaged by snow. The spring visit compares each marked point with its earlier condition and resets any seal that has failed.
The soil around the foundation is noted, not treated. The Milwaukee source describes the egg-laying but does not describe an outdoor treatment for cluster flies, so the report records the condition of the soil and the drainage at the foundation as a matter of observation, and it leaves any outdoor treatment decision to a later visit with the technician.
- Check each sealed point for failure
- Record the soil and drainage at the foundation
- Reset any seal that has opened
