Cluster flies come into Milwaukee homes in large numbers from about the middle of September, according to a UW-Madison entomologist, and UW-Extension says the females lay their eggs in soil outdoors the next spring. The homeowner's work is a September seal, a clean-up of the flies that get in, and a spring check of the soil and the seals.
Key takeaways
- The entry starts about the middle of September and can be large.
- Sealing before the entry is the first step; a clean-up of the flies inside is the second.
- Flies inside are removed by vacuum or soft brush, not crushed.
- The next generation is laid in soil outdoors the following spring, so the outside of the building is the lasting place to work.
- The sources do not describe an indoor breeding site, and this guide makes no such claim.
What does the autumn entry look like from the homeowner's side?
From the homeowner's side the entry looks like a few flies on a sunny wall, then a sudden group in an upstairs room. The sources describe a movement that can reach the millions across a season, and most homeowners see it first as a window full of slow-moving flies.
The first sign is usually a few flies on a window in an upper room or on the sunny side of the house. They move slowly and do not seem to be trying to get out, which is one reason they are often left alone until the number grows. The sources say the entry starts in mid-September, so the first sighting in that week is the start, not a passing oddity.
A homeowner who keeps a count in the first two weeks has better information than one who waits. A note of the date, the room and the number of flies lets the technician see whether the movement is growing and where the nearest gap is likely to be.
- A few flies on a sunny wall or upper window
- A sudden group in an upstairs room
- A count noted with the date and room
What should a homeowner do in September, before the flies arrive?
In September, before the flies arrive, the homeowner should book the sealing visit, clear the outside gutters and walls, and check the window and door frames for light from inside. These steps are the work that the entry is timed against.
The best preparation is a sealing visit scheduled in late August or the first week of September. The technician walks the outside, marks each gap and seals it. The homeowner can help by clearing the gutters and removing debris from the foot of the walls, so the technician can see the gaps clearly.
A homeowner can also check the window and door frames from inside on a bright day. A line of light at a frame is a gap, and the homeowner can mark it with painter's tape so the technician finds it quickly. The marks are not a substitute for a survey, but they make the survey faster.
- Book the sealing visit for late August or early September
- Clear gutters and debris from the foot of the walls
- Check frames from inside on a bright day and mark them
What is the right way to remove flies that are already inside?
The flies inside should be removed by vacuum or a soft brush into a container, and not crushed on the wall, because a crushed fly leaves a smear and a smell and does not stop the next arrivals. The vacuum is emptied outside.
Walk the room slowly and remove the flies as they are found. A flat, handheld vacuum does the job quickly. Hold the nozzle near the fly, let the suction take it, and continue along the window frame and the curtain. A soft brush can gather the flies on the sill and sweep them into a dustpan.
Do not use a household spray in the room as a first step. A spray does not reach the flies still moving in the window frame, and it puts a chemical in a room where people and pets are. The sealing is the work that stops the next group, and the clean-up is the work that deals with the current one.
- Vacuum or brush the flies into a container
- Work along the window frame and the curtain
- Empty the vacuum outside
- Avoid a spray as the first step
What happens in the spring after an autumn entry?
The UW-Extension agent says the flies that came in lay their eggs in soil outdoors the next spring. The homeowner's spring work is a check of the seals that held over the winter and a note of the condition of the soil and drainage at the foundation.
The eggs are laid outside, which is why the outside of the building is the lasting place to work. A homeowner who sees the flies again in the next autumn should not assume the same room is the source. The sealing record and the entry points are the evidence for where the flies came from.
The spring check is short. The technician or the homeowner looks at each marked point from inside with a light, notes any seal that has failed over the winter, and records the soil and drainage at the foundation. Any seal that has opened is reset before the next autumn.
- Check each marked point from inside with a light
- Reset any seal that has failed over the winter
- Record the soil and drainage at the foundation
What does this guide not claim?
This guide does not claim that cluster flies breed indoors, that they damage the structure, or that a particular product will stop them. The sources describe the entry, the outdoor egg-laying and the sealing advice, and the guide keeps to those.
A homeowner who reads a claim about cluster flies on another website should check it against the publisher's own page. The sources this site relies on are the UW-Madison entomologist's account of the entry and the UW-Extension description of the egg-laying. Neither describes an indoor breeding site or a product, so the guide does not present one.
The guide also does not name a local count or a neighbourhood with more flies than another. The sources do not give those figures for Milwaukee, and a number offered without a source would be a guess.
- No indoor breeding claim
- No product claim
- No local count or neighbourhood ranking
Why do sealing and clean-up both matter?
Sealing stops the next group and clean-up deals with the group already inside. Doing only one leaves the homeowner doing the same job every week, so the two are planned together.
A homeowner who vacuums the flies every day without sealing is working on the effect. The flies that are removed are replaced by the next arrivals through the same gap, and the work never ends until the gap is closed. Sealing shortens the season for that building, because it closes the path the flies use.
A homeowner who seals and then ignores the flies already inside is also left with work. Those flies are still in the room, and they can be removed quickly with a vacuum. The two steps together make the autumn shorter, and the spring check keeps the next season short too.
- Sealing closes the path
- Clean-up removes the flies already in
- Together they shorten the season
What does a household routine look like in October?
In October a household can keep a short routine: a walk of the sunny walls on a warm afternoon, a count of the flies in the rooms that have them, and a vacuum of the window frames. The routine takes little time and gives the technician a record.
A warm afternoon walk around the house takes a few minutes. The homeowner looks at the walls that face the sun, notes any cluster of insects, and checks whether the window frame on that wall has a line of light from inside. The note goes into the same record as the autumn's count.
The window frames inside are cleared each morning or evening during the peak. The flies on the sill are removed with a vacuum or a brush, and the vacuum is emptied outside. The routine is not a cure, but it keeps the room usable and it makes the record consistent.
- Walk the sunny walls on a warm afternoon
- Count the flies in each room with a note
- Clear the window frames morning or evening
Frequently asked questions
When do cluster flies start in Milwaukee?
From about the middle of September, according to the UW-Madison entomologist the site relies on. The sources describe a large entry through the autumn.
Do cluster flies lay eggs in my house?
The sources describe the eggs as laid in soil outdoors the following spring. This guide does not claim an indoor breeding site.
Should I vacuum or swat the flies?
Vacuum or brush them into a container. Swatting leaves a smear and a smell, and it does not stop the arrivals.
Will one visit end the flies?
A sealing visit closes the points that were found, and it reduces the number that get in. The spring check confirms that the seals held.
Is it normal to see flies in a room after sealing?
Some flies can still be in the house after sealing, because they came in before the work. Fewer arrivals over time is the sign that the sealing is working.
Sources
- UW-Madison news on cluster flies entering homes (entomologist's account, undated)
- UW-Extension, cluster fly egg-laying account (Marinette County agent, undated)
This guide is general information for Milwaukee County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.