Millipedes in Your Basement? Why They Show Up in Fall (and How to Get Them Out)

Quick answer: Millipedes come into Ontario basements in September and October because the ground outside gets too wet and too cold for them. They can’t survive indoors — they need constant moisture, so most die within a day or two on your basement floor. Sweeping them up is not the fix. Sealing the gap under the basement door, pulling mulch and leaf litter back from the foundation, and drying out the crawlspace is. If you’re finding dozens at a time, week after week, the real problem is grading or moisture at the foundation, and that’s what a $95 inspection — credited when you book the treatment — is meant to find.

Why September and October are millipede season in Ontario

Millipedes live in the top inch or two of soil, under mulch, beneath leaf litter, and inside the damp layer of thatch in your lawn. They eat decaying plant matter. All summer that habitat suits them fine and you never see one.

Then two things happen at once in early fall. The autumn rains saturate the ground, and the overnight lows start dropping. Waterlogged soil pushes millipedes up and out — they will literally drown in it — and the cooling nights send them looking for somewhere stable. Your foundation is warmer than the soil, and the gap under your basement door or the crack where the walkway meets the wall reads to a millipede like exactly the sort of dark, sheltered crevice it’s been looking for.

That’s why homeowners across Durham and York call us the same week: heavy rain Tuesday, and by Thursday there are twenty millipedes curled up on the basement floor. It isn’t an infestation that built up over months. It’s a migration that happened over 48 hours.

Older homes in Oshawa — block foundations, original weeping tile, mature trees dropping a thick leaf layer against the wall — get hit harder than newer builds. But newer subdivisions in Markham have their own version: fresh sod over compacted clay holds water at the surface, and the landscaping mulch runs right up to the siding.

Millipede or centipede? The difference decides what you do next

People use the words interchangeably. They shouldn’t, because the two pests mean completely different things about your house.

Millipedes are slow. Hard, rounded, segmented body, usually dark brown or greyish, and when disturbed they curl into a tight coil like a watch spring. They don’t bite. They don’t damage anything. They eat dead leaves.

House centipedes are fast. Flattened body, long trailing legs, and they sprint across a wall when the light comes on. They’re predators, and they’re in your basement because there is something down there to hunt.

That distinction matters more than the ID itself. Millipedes in the basement mean a moisture and entry-point problem. Centipedes mean a prey problem — silverfish, springtails, or something else living down there year-round. If you’re seeing both, you’ve got both. Our house centipede library page covers that side; the millipede identification guide has close-up photos if you want to be sure.

Why they die on your floor — and why that’s not good news

Here’s the part that confuses people. Millipedes need near-constant humidity to survive. Your basement, even a damp one, is a desert compared to the soil they came from. Most of the millipedes that get inside will be dead within 24 to 72 hours, curled up in the middle of the floor or along a baseboard.

So the problem appears to solve itself. Every morning you sweep up yesterday’s, and every morning there are new ones.

That’s the tell. If millipedes are dying indoors but the count doesn’t drop, they aren’t breeding inside — they’re still coming in. The supply is outdoors and the door is still open. This is the most common reason homeowners tell us they “tried spraying” and it didn’t work: the spray went where the dead ones were instead of where the live ones are entering.

One exception worth knowing. If you have a crawlspace with an exposed dirt floor, a chronically wet corner, or a persistent plumbing leak, millipedes can survive indoors — usually alongside springtails and sowbugs. If your millipedes aren’t dying, treat that as a moisture problem first, not a bug problem.

The fixes that actually stop them, in order of what works

Work from the outside in. Everything on this list is more effective than anything you can spray on a basement floor.

1. Pull the mulch back. Mulch, decorative bark, and leaf litter held against the foundation is a millipede nursery with a door directly into your house. Leave a 15–20 cm dry strip of bare soil or gravel between any organic material and the wall. This one change does more than the rest of the list combined.

2. Clear the leaf layer before it mats down. In October, wet leaves packed against the north side of a house hold moisture for weeks. Rake them off the foundation strip even if you leave the rest of the yard alone.

3. Fix where water sits. Downspouts discharging within a metre of the wall, a walkway that has settled and now slopes toward the house, a window well without drainage — each creates a permanently saturated pocket right at the entry points. Extending a downspout is a $20 fix for a recurring pest problem.

4. Seal the entry points. Door sweeps on basement and garage doors are usually the biggest single gap. Then the sill plate where the foundation meets the framing, gaps around dryer vents and hose bibs, unsealed basement window frames, and the expansion joint where a poured slab meets the wall. Backer rod and exterior-grade sealant, not spray foam alone.

5. Dry the inside. A dehumidifier held at 50% relative humidity makes a basement uninhabitable for millipedes, springtails, sowbugs and silverfish at once. Repair any leak first — a dehumidifier fighting an active leak just runs forever.

6. Then, and only then, treat the perimeter. A properly applied exterior barrier around the foundation intercepts millipedes before they reach the wall. It’s genuinely effective, and it’s also the step that’s useless on its own if steps 1 through 4 haven’t been done. This is what our residential pest control service does — the treatment goes on the outside, where the millipedes are, not on the basement floor where the dead ones are.

The University of Minnesota Extension’s guidance on sowbugs, millipedes and centipedes and the University of Wisconsin’s millipede fact sheet both land in the same place: this is a habitat problem, not a spray problem.

When dozens means something bigger

A handful of millipedes after a big rain is normal in an Ontario fall and doesn’t need a professional.

Hundreds is a different conversation. Mass migrations — a dustpan’s worth every morning for two weeks — almost always trace back to something structural: grading sending surface water at the foundation, a failed or clogged weeping tile, a crawlspace with no vapour barrier, or thick landscaping installed hard against the wall. Lakeshore properties in Oakville with high water tables see this pattern regularly.

The bugs are the symptom. Treat only the symptom and you’ll do it again next September, and the September after that.

That’s what the $95 professional inspection is for — credited in full when you go ahead with the treatment. A technician walks the perimeter, finds the moisture source and the entry points, and tells you which are $20 fixes and which need real work.

You can book a one-time General Insect Control treatment — from $395 exterior, or $475 interior and exterior — with our 60-day guarantee. For a single bad fall that’s a reasonable call, and it’s the usual entry point.

But be honest about what a one-time treatment is. It clears this year’s migration. It doesn’t change the fact that next September the rain will come, the soil will saturate, and the millipedes will move toward your foundation again. Millipedes, springtails, earwigs and sowbugs come back on a schedule.

That’s why we point most homeowners to the annual plan instead. Insect Control at $845/year covers the perimeter on a scheduled cycle so the barrier is fresh going into fall rather than reapplied after you’ve already found them inside. Essentials — Home Protection at $1,195/year adds rodent coverage, which matters because the same gaps millipedes use in September are the ones mice use in October.

Compare that to the alternative: a spring foundation repair after two years of water sitting against the wall runs into the thousands. A year of protection is cheaper than one season of ignoring what the millipedes are telling you about your drainage.

See current rates on our plans and pricing page, or call (705) 302-1887 and we’ll tell you honestly whether you need us or just a longer downspout.

Sani IQ is BBB Accredited, carries $5M liability coverage, is licensed under the Ontario Pesticides Act and registered with the MECP, and holds a 5.0 Google rating. Our technicians are employed, licensed and background-checked — never subcontractors — and we use reduced-risk products.

Frequently asked questions

Are millipedes dangerous to people or pets? No. They don’t bite or sting or carry disease. Some species secrete a defensive fluid that can stain a surface or irritate skin, so use a paper towel rather than bare hands. A dog that mouths one may drool but won’t be harmed.

Do millipedes damage a house? No. They don’t chew wood, fabric, wiring or stored food. They’re a nuisance pest. The damage risk isn’t the millipedes — it’s the moisture condition that brought them, which does affect a foundation.

Will they lay eggs in my basement? Almost never. They need moist soil to reproduce, and a normal basement is far too dry. If they’re persisting and multiplying indoors, you have an exposed dirt crawlspace or an active water problem to deal with first.

Why do I only get them on one side of the house? Usually the shaded or north-facing side, where soil stays damp longest and leaf litter mats down. Check the grading, downspout and mulch depth on that side specifically.

Does spraying inside the basement help? Barely. The millipedes indoors are already dying. Interior treatment doesn’t stop new ones entering. The effective work is exterior — the perimeter barrier plus sealing the entry points.

How long does the fall migration last? Typically two to four weeks, tracking the autumn rains, and it winds down with the first hard frost. If you’re still seeing live millipedes in December, that points to an indoor moisture source rather than migration.

Do I really need an annual plan for millipedes? Not for a handful of them. But if you’re getting a fall migration every year, a one-time treatment resets the clock without changing the outcome. The plan exists so the barrier is in place before the rains, not after.