Cluster flies in Wisconsin show up the same way every year. A warm October afternoon, a dozen slow flies bumping around an upstairs window, and no obvious reason for them. There’s no garbage nearby, nothing rotting, and the house is clean. People find it unsettling for exactly that reason, because the usual explanation doesn’t fit.

It doesn’t fit because these flies have nothing to do with sanitation. Here’s where they actually come from, how to tell them apart from the flies that do mean something, and what reduces them.

What are cluster flies?

They’re a group of flies that spend their larval stage inside earthworms.

The adult female lays her eggs in soil. The larvae that hatch out find an earthworm, enter it, and develop inside it over a few weeks. Several generations run through that cycle over a Wisconsin summer, entirely outdoors and entirely out of sight.

So the population building up around your house all season has been living in your lawn, your garden beds, and the field behind you. Properties with a lot of turf, open ground, or pasture nearby get the heaviest pressure, which is why this is more common in the suburbs and out toward the county lines than it is downtown.

Why are there flies at my window in October?

Because the last generation of the year is looking for somewhere to spend the winter, and your house is the warmest thing in the neighborhood.

On sunny fall afternoons they gather on south and west facing walls, the same warm surfaces that collect box elder bugs and Asian lady beetles. From there they work into gaps around window and door frames, under siding, at soffit and fascia joints, and through attic and gable vents. What they’re aiming for is an unheated but protected space, so attics, wall voids, and rooms nobody uses are where they settle.

Then they go quiet. What brings them back out is a warm spell, which in Wisconsin can be a sunny January afternoon as easily as an October one. The warmth wakes them, they move toward light, and they end up at the nearest window, which is usually upstairs. That’s why this problem tends to announce itself in a spare bedroom or a bathroom on the top floor.

They’re slow and clumsy when they emerge, which is a useful tell. A fly you can catch with a cup is not a house fly.

Are they a sign the house is dirty?

No, and this is the part worth saying plainly, because it’s the first thing most people assume.

They don’t breed in garbage, they don’t come up from drains, and they don’t feed on waste. They can’t reproduce indoors at all, because their entire life cycle requires soil and earthworms. A house with cluster flies in the attic has good soil outside it and gaps in the building envelope. That’s the whole story.

We’ve found them in immaculate homes, brand new construction, and houses that had been deep cleaned the week before. It’s a building and landscape situation rather than a housekeeping one.

How do you tell them from house flies and blow flies?

This matters more than it sounds, because one of the three means something is genuinely wrong.

Cluster flies are slightly larger than a house fly, dark gray, and sluggish. The reliable marks are short golden or yellowish hairs on the thorax, just behind the head, and wings that overlap across the back when they’re at rest. They show up in groups, at windows, on warm days.

House flies are smaller, faster, and hold their wings apart in a V shape when resting. If you’re seeing quick flies around a kitchen, a trash can, or a drain, that’s a different problem with a different cause.

Blow flies are the ones to pay attention to. They’re metallic, shiny blue or emerald, and unmistakable once you’ve seen one. A sudden appearance of blow flies indoors usually means something died in a wall void, a chimney, or an attic, most often a mouse or a bird. If the flies at your window are shiny and metallic rather than dull gray, the fly isn’t the problem you need to solve.

Do they cause any harm?

Very little directly, and a bit indirectly.

They don’t bite, they don’t damage the structure, and they don’t carry the contamination risk that flies breeding in waste do. What they do leave behind is small dark spots on walls, window frames, and curtains, and a greasy smear if you swat one against a painted surface.

The indirect problem is the same one that comes with box elder bugs. A large number of flies dying inside a wall void or attic becomes a food source for carpet beetles and larder beetles, and those do damage wool, stored fabric, and dry goods. That’s the real argument for keeping them out rather than letting them in and vacuuming the survivors every spring.

What reduces cluster flies in Wisconsin homes?

An exterior treatment applied before they get in, and sealing after they’re gone.

The window for exterior work is late summer into early fall, while the flies are still massing on the outside walls. A treatment on the siding, eaves, soffits, and around window and door frames reduces how many make it into the structure. Once they’re inside a wall void or an attic, that opportunity has passed for the year, because spraying doesn’t reach them in there and treating a cavity full of insects creates the dead-insect problem described above.

Sealing is the durable half. Soffit and fascia gaps, siding seams and corners, the openings around vents and utility penetrations, gaps at window and door frames, and attic or gable vents with damaged screening. That’s the same list that keeps mice and box elder bugs out, so one pass handles several problems.

Being straight about the result: even a well-timed exterior treatment on a property with heavy pressure won’t get you to zero. It reduces the number substantially. Any company promising a fly-free attic is setting you up to be disappointed.

What should you do with the ones already inside?

Vacuum them, and use something you can empty outside.

Don’t swat them against walls or window trim, because of the spotting. And don’t spray insecticide along interior window sills for this, since it accomplishes very little against insects that are emerging from a wall cavity a few at a time over several months.

If you want to know where they’re coming from, pay attention to which room and which window. Flies concentrating at one upstairs window on warm days point at the wall or attic space right behind it, and that’s the information a technician needs to find the entry point outside.

What to do next

If you’re seeing them now, note the room and the wall, then walk outside and look at that same section of the house for gaps at the soffit, the siding seams, and around the window frames. The exterior window for treatment is open in early fall and it closes once they’ve moved into the walls.

Ehlers Pest Management serves southeastern Wisconsin, and you can reach our experienced team at (414) 303-6926 to schedule service. A certified technician will confirm what kind of fly you actually have, which matters more than people expect, find where they’re getting in, and treat the exterior. If it turns out you have blow flies rather than cluster flies, we’ll tell you that too, because that’s a different job entirely.

If you’ve been finding centipedes in Wisconsin basements this fall, you’ve probably also found a few other things down there. Millipedes curled up on the floor, crickets that hop instead of chirp, and something silver that darts under the water heater when the light comes on. Those four show up together for the same reason, and once you know what that reason is, the answer to all of them is mostly the same.

Here’s what each one actually is, which of them can do damage, and what finding them repeatedly is telling you about the building.

What are these things in my basement?

Four species account for almost every call we get about basement bugs in southeastern Wisconsin.

House centipedes are the fast ones with the long legs, grayish yellow with dark stripes down the back, roughly an inch of body and a lot more leg. They move in quick bursts and stop dead. People find them genuinely alarming and they’re the most harmless thing on this list.

Millipedes are rounded, dark, slow, and segmented, with two pairs of tiny legs on each segment. When they’re disturbed or dying they curl into a tight spiral, which is how most people find them, motionless on the floor near a wall.

Camel crickets, sometimes called cave crickets, are the humpbacked, long-legged, wingless crickets that live in damp basements and crawl spaces. They don’t chirp, and they jump toward you when startled rather than away, which is why they have a reputation. The crickets you hear chirping in October are field crickets, a different insect that comes in from outside.

Silverfish are teardrop shaped, silver gray, with three bristles trailing off the back end. They move with a wriggle that looks like a fish swimming, they’re nocturnal, and they run for cover the moment a light goes on.

Are house centipedes in Wisconsin dangerous?

No, and they’re arguably working for you.

House centipedes are predators. What they eat is spiders, silverfish, small roaches, firebrats, and other soft-bodied insects living in the same damp spaces. They have venom for subduing prey, but their mouthparts struggle to break human skin, and bites are rare and comparable to a mild sting when they happen. They don’t damage the building, they don’t get into food, and they don’t carry disease.

Here’s the part worth acting on though. Unlike most things in this article, house centipedes can live indoors for years, and they only stay where there’s food. A basement with a persistent centipede population is a basement with a steady supply of other insects in it. The centipede is a symptom worth reading rather than the problem itself.

Which of these actually damage anything?

Two of the four, and it’s not the ones people worry about.

Silverfish eat starch. That means paper, cardboard, book bindings, wallpaper paste, photographs, and the glue in box seams, along with dried goods in a pantry. Boxes of stored documents, photo albums, and anything on a basement shelf are what they go after, and the damage is slow enough that people usually find it long after it started.

Camel crickets chew too. Fabric, paper, cardboard, and stored clothing are all fair game, and a large population in a crawl space can do noticeable damage to anything boxed up down there.

Millipedes and centipedes damage nothing. Millipedes do release a defensive fluid that can stain and has an unpleasant smell, which is a reason not to squash them on a light-colored floor.

Why do they all show up in fall?

Because the moisture outside changes, and moisture is the only thing all four of them care about.

Millipedes and field crickets live outdoors in leaf litter, mulch, and damp soil. When fall brings the first cold nights, or when a heavy rain floods their harborage, they move. A foundation wall is warm and the gaps along it lead somewhere sheltered, so they end up in the basement by accident. Most of them die within a few days, because there’s nothing down there for them to eat and the air is too dry to survive in for long.

Centipedes, silverfish, and camel crickets are a different case. They were probably already there. What changes in October is that the furnace comes on and dries out the upstairs, which concentrates them in whatever damp space is left. That’s usually the basement, the crawl space, or the bathroom, and it’s also where you’ve started going more often to pull out storage bins.

What does it mean if you keep finding them?

That the building has more moisture in it than it should.

One millipede on the floor in October is normal in Wisconsin and means nothing. Finding them every few days through the fall means something along the foundation is staying wet. Same with silverfish. Same with camel crickets.

The usual culprits are a downspout dumping against the foundation, grading that slopes toward the house rather than away, mulch or leaf litter piled against the siding, a crawl space with no vapor barrier, a dehumidifier that quit, or a slow plumbing leak nobody has found yet. That last one is worth ruling out, because an unexplained silverfish population in one corner of a basement has more than once turned out to be a leaking supply line inside a wall.

What actually reduces them?

Drying the space out does more than any product, so start there.

• Run a dehumidifier in the basement or crawl space and keep it running through fall. Getting relative humidity down is the single most effective thing available.
• Extend downspouts well away from the foundation, and check that grading carries water away from the house rather than toward it.
• Pull mulch, leaf litter, and stacked firewood back off the siding and foundation. That’s where millipedes and crickets live before they come inside.
• Get stored items off the floor and out of cardboard. Plastic bins with lids remove both the food source and the harborage for silverfish.

Sealing helps too, and it’s the same list of gaps that keeps mice out: the sill plate, utility penetrations, basement window frames, and the gaps around a bulkhead or garage door.

When is it worth calling someone?

When the moisture work is done and they’re still there, or when the population is clearly established rather than wandering in.

A perimeter treatment around the foundation reduces the wanderers, since millipedes and crickets have to cross treated ground to reach the building. Silverfish and camel crickets need interior work, because they breed indoors and drying the space alone takes a long time to catch up with an established population.

One honest note on centipedes. Treating for centipedes specifically accomplishes very little. They’re there because other insects are there, so the work that matters is reducing the population they’re feeding on and taking the moisture away. Handle those and the centipedes leave on their own.

What to do next

Go down to the basement and look at the floor along the walls rather than in the middle of the room. That’s where you’ll find them, and the corner they’re concentrated in usually points straight at the moisture source. Check what’s directly outside that corner while you’re at it, because the answer is often a downspout.

Ehlers Pest Management works across southeastern Wisconsin, and you can reach our experienced team at (414) 303-6926 to schedule service. A certified technician will identify what you actually have, look at the moisture conditions feeding it, and tell you whether this needs a treatment or a dehumidifier and a downspout extension. Sometimes it’s the second one, and we’ll say so.