House Centipedes in Ontario Basements: Should You Worry?

Quick answer: A house centipede is a predator. It is in your basement because something else is living down there for it to eat — usually silverfish, spiders, or carpet beetle larvae. One sighting is not a problem. Repeated sightings are a reading on two things: your basement’s moisture level and the size of the insect population feeding on it. Fix those and the centipedes leave on their own, because the food goes away.

What you are actually looking at

They are hard to mistake for anything else. A flattened, yellowish-grey body an inch to an inch and a half long, fifteen pairs of long legs — and on adult females, a final pair nearly twice the body’s length. They move in bursts, stop dead, and vanish behind the water heater.

The reaction is almost universal. It is also, as pest problems go, misdirected.

House centipedes are among the few things in a basement genuinely working in your favour. Penn State Extension describes them plainly as harmless and of little concern to homeowners, “despite their long legs and scary appearance.” They hunt silverfish, firebrats, carpet beetle larvae, cockroaches, and spiders. The University of Minnesota Extension says the same: they are beneficial predators using venom-filled jaws on prey much smaller than themselves.

So a centipede in a Scarborough bungalow basement or an Oakville finished rec room is not the discovery. It is the receipt for a discovery you have not made yet.

A centipede sighting is a message about something else

This is the part almost no article says out loud: a predator population is capped by its prey population.

House centipedes do not eat drywall, insulation, wood, food, or fabric. They cannot establish on nothing. So the number you see is roughly a function of how much there is to eat. Both extension services above make the same point from the other direction — abundant centipedes indicate that other insects are present, and the recommended response is to deal with the broader insect population rather than the centipedes.

That reframes your own observations:

One centipede, once, in September or October. Almost certainly a single individual in from outside or up from a crawl space. Nothing to act on.
Two or three over a few weeks, same room. Normal for an Ontario basement with any humidity in it. Worth a look at moisture, not worth a treatment.
Several a week, or sightings spreading to the main floor. Now it is information. Something is feeding them, and it has reached a population large enough to support predators. Silverfish are the usual answer; carpet beetle larvae in stored textiles are the expensive one, because they damage wool and felt while you are watching the centipedes.
Daylight sightings, repeatedly. House centipedes are nocturnal and shy. Routine daytime encounters usually mean individuals are being displaced from preferred harbourage.

The useful move is to stop counting centipedes and start looking for what they are hunting.

Why fall drives basement sightings in Ontario

Two things happen at once in October, and they stack.

First, outdoor shelter stops working. House centipedes need moisture and do not survive an Ontario winter exposed outdoors. Individuals living under mulch, stone, and deck boards through the summer either find a heated structure or they do not make it. A foundation crack, an unsealed sill plate, a window well, or a floor drain is a direct route in.

Second, the inside is at its most hospitable. Summer humidity is still sitting in block walls and slabs, the dehumidifier has often been off since Labour Day, and leaf-blocked downspouts are pushing fall rain toward the foundation. Penn State Extension lists their preferred harbourage precisely: beneath concrete slabs, inside cement block walls, in floor drains without water traps, and in unexcavated crawl spaces.

Ontario housing has a lot of that — older block and fieldstone foundations, cold rooms under front porches, partially excavated crawl spaces. Cottage country has its own version: a seasonal building in Muskoka or around Orillia that sits closed and unheated accumulates exactly the cool, damp, undisturbed conditions both centipedes and their prey want — and nobody notices until the fall visit.

Do house centipedes bite?

Technically they can. Practically it almost never happens.

The University of Minnesota Extension is direct about it: house centipedes are very shy, and their jaws are too small to easily break human skin. In the rare case of a bite, expect localized swelling, and the pain should not be severe. There is no medically significant venom concern for people or pets in Ontario.

They do not transmit disease, contaminate food, damage structures, or bite while you sleep. The honest answer to “should I worry” is no — not about the centipede. Worry, mildly, about what it tells you.

For identification photos and biology, see our pest library entry on centipedes.

Why squishing isn’t a strategy

Killing the one you see removes one predator. It does not change the conditions that brought it in, and it does not touch the prey population — which, if anything, benefits.

The same logic undercuts the usual DIY responses:

A perimeter spray of the baseboards knocks down whatever crosses it for a few weeks. It does nothing about humidity, and the prey in the wall cavities and under the slab is not crossing that line.
Sticky traps are genuinely useful — as a diagnostic, not a control. Put a few along basement walls and behind the furnace for two weeks and read what you catch. Silverfish and carpet beetle larvae alongside the centipedes means you have found the real problem.
A dehumidifier alone often helps substantially, but only if the water getting in is condensation rather than bulk water from grading or gutters.
Doing nothing is a defensible choice for one or two sightings a season, and we will tell you so.

The moisture fixes that actually reduce sightings

Everything that controls house centipedes is a moisture or harbourage fix, and most of it is not pest control at all:

Get the dehumidifier back on and target roughly 50% relative humidity or lower. An inexpensive hygrometer tells you where you actually are, which is often a surprise.
Fill floor drains that have dried out. A trap without water in it is an open door from the drain system. Pour a litre of water down any drain you do not use regularly.
Clear gutters and extend downspouts well away from the foundation before freeze-up — the single highest-value fall task for basement pests generally.
Seal the obvious penetrations: the sill plate gap, pipe and conduit entries, basement window frames, the gap under an exterior basement door.
Remove the harbourage. Cardboard boxes on a concrete floor hold moisture against the slab and shelter prey insects underneath. Plastic bins on shelving solve two problems at once.
Deal with stored textiles. Wool, felt, down, and natural fibres in cardboard or open bins in a damp basement are carpet beetle habitat — and carpet beetle larvae are centipede food.

Do those six things and sightings typically drop off without anything being sprayed, because the basement stops supporting the food chain.

When sightings justify an inspection

Call when the answer has stopped being about centipedes:

You are seeing them weekly or more, across more than one season.
Sticky traps are producing silverfish, carpet beetles, cockroaches, or spiders in volume.
You have found actual damage — holes in wool or felt, shed larval skins in stored textiles.
The moisture fixes above are done and sightings have not changed, which usually means water is entering somewhere you have not found.
The basement is finished, so the harbourage is behind drywall and you cannot inspect it yourself.

An inspection gets you the diagnosis, not a spray. A technician reads the moisture pattern, identifies what is actually breeding down there, finds the entry routes, and tells you whether this is a gutter problem you can fix yourself for nothing or a real insect population behind a finished wall.

If there is a live insect population feeding the centipedes, a one-time general insect treatment is the right entry point — inspection, identification, targeted treatment, with a 60-day guarantee on one-time service. Every service begins with a professional inspection, and its cost is credited in full when you proceed with the treatment. No setup fee, no monthly billing.

But a basement is a set of conditions, not an event. The moisture comes back every spring, grading shifts, gutters fill again, and the insect population that supported the centipedes re-establishes from outside. That is why the annual protection plan is the path we recommend for homes with recurring basement activity: scheduled seasonal visits, monitoring so a population is caught while small, exclusion work maintained rather than done once. The one-time treatment solves this autumn. The plan stops you doing this again next autumn.

See what the plans cover on our plans and pricing page, or answer a few questions in the free quote quiz and we will tell you which one fits your building.

Sani IQ is BBB Accredited, licensed under the Ontario Pesticides Act, registered with the Ministry of the Environment, Conservation and Parks, and carries $5M in liability coverage. Technicians are employed, licensed, and background-checked — never subcontractors — and we use reduced-risk products wherever they will do the job. Call (705) 302-1887.

Frequently asked questions

Are house centipedes dangerous to children or pets? No. They are shy, nocturnal, and their jaws struggle to break human skin. A bite is rare and produces local swelling at worst. They carry no disease of concern and are not toxic if a pet eats one.

Should I kill house centipedes or leave them alone? If you are seeing one occasionally, leaving them alone is the rational choice — they are reducing your silverfish and carpet beetle population for free. If you are seeing them often, the answer is not to kill them more efficiently but to find out what they are eating.

Why do I only see them in the bathroom and the basement? Moisture. Those are the two rooms in most Ontario homes with reliable humidity, and the drains in both are direct routes from where centipedes harbour.

Do house centipedes mean my house is dirty? No. They respond to moisture and prey availability, not housekeeping. Immaculate homes with damp block basements get them, and they are common in new builds where concrete is still curing.

Will they go away on their own in winter? Sightings usually drop sharply because the indoor population retreats to the dampest, least disturbed harbourage. That is a change in visibility, not necessarily in population. If sightings resume in spring, they overwintered inside.

I found one in my bed. Does that mean there are more? It means one wandered — centipedes are not attracted to people or bedding. If it happens repeatedly, the population below is large enough that individuals are being pushed out of preferred harbourage.