Millipedes in Ontario

Overview

Millipedes are slow, dark, worm-like arthropods that live outdoors in damp soil and leaf litter, quietly recycling decaying plant matter. Most of the year, Ontario homeowners never notice them. The exception is when conditions shift suddenly — after heavy rain saturates the ground, during a dry spell that drives them to seek moisture, or in fall when they look for shelter. Then millipedes migrate in mass numbers, and homes near thick mulch, damp foundations, or wooded lots can find dozens or even hundreds of them creeping into basements, garages, and along foundation walls. Indoors they’re harmless and short-lived — they dry out and die within a day or two — but a mass migration is unsettling and messy, and it points to a moisture problem right against the house.

Identification

A millipede has a hard, cylindrical, worm-like body 25–40 mm long, dark brown to blackish, made of many segments. The defining feature is two pairs of legs per body segment — dozens of short legs that ripple in a wave as it crawls. Millipedes move slowly, and when disturbed or dead they coil into a tight, flat spiral. They have short antennae and no visible pincers or fangs.

The look-alike is the house centipede, but the two are opposites in almost every way: millipedes are slow, round, and coil up; centipedes are fast, flat, and flee. The two-leg-pairs-per-segment rule is the definitive test.

Life Cycle

Millipedes develop through gradual metamorphosis. Females lay eggs — often dozens to a hundred — in damp soil, and the young hatch with only a few body segments and legs, adding more with each moult until they reach adult size. Development takes several months to a couple of years depending on species and conditions, and adults can live for several years, moulting periodically throughout life. They overwinter as adults in the soil, below the frost line, which is why a mild Ontario winter and a wet spring can leave large surviving populations poised to migrate. Reproduction happens outdoors in soil; millipedes do not breed inside homes.

Habitat & Behaviour

Millipedes are creatures of damp darkness. They live in the top layer of soil, in leaf litter, under mulch, logs, stones, and boards, and in compost — anywhere organic matter is decaying and moisture is steady. They’re nocturnal, hiding by day and foraging at night, and they’re highly sensitive to moisture: too dry and they desiccate, too wet and they flee. That sensitivity drives their signature behaviour, mass migration. When heavy rain floods their habitat, or a dry spell parches it, or fall temperatures signal the need for shelter, large numbers move together, often uphill and toward the vertical surfaces of homes, streaming in through any nearby gap.

Diet

Millipedes are decomposers, and beneficial ones. They feed on decaying leaves, rotting wood, mulch, compost, and other dead organic matter, breaking it down and returning nutrients to the soil — a useful role in gardens and forests. Occasionally they’ll nibble seedlings or soft, damp roots, but they cause little garden harm. Indoors they find nothing to eat: they don’t feed on wood framing, fabric, paper, or stored food, which is part of why they can’t survive inside for long. Their diet is strictly the damp, decaying material of the outdoor world.

Signs of Infestation

• Mass migrations of millipedes on foundations, driveways, patios, and basement floors, especially after heavy rain or in fall.
• Coiled dead millipedes in basements, garages, and along baseboards — they dry out quickly indoors.
• Live millipedes in damp low areas — basement corners, around floor drains, in garages and crawl spaces.
• Clusters under outdoor harbourage — mulch, leaf litter, boards, and stones against the foundation.

Damage Caused

Millipedes cause no structural or property damage. They don’t chew wood, wiring, fabric, or paper, and they don’t infest stored food. In the garden they’re net beneficial, breaking down decaying matter, with only rare, minor nibbling of seedlings. The only real cost of a millipede problem is the cleanup and unpleasantness of a mass migration — sweeping up scores of dead and dying millipedes from a basement floor — and the staining their defensive secretions can leave if crushed on light surfaces. It’s a nuisance and a signal of excess moisture, not a source of damage.

Health Risks

Millipedes pose minimal health risk. They don’t bite, sting, or transmit disease. Their one defensive trick is a mild fluid, secreted from glands along the body, that can smell unpleasant and — with some species — cause slight skin irritation or temporary staining if you handle them and then touch your eyes. The simple precaution is not to pick them up bare-handed; use a broom, dustpan, or gloves. For people and pets, millipedes are effectively harmless, and there’s no allergy or contamination concern of the kind associated with cockroaches.

Seasonal Activity in Ontario

Millipede problems in Ontario are strongly seasonal and weather-driven. Spring brings the first surge, as melting snow and wet weather saturate the soil and push overwintered adults to move. Summer is usually quiet unless a heavy rain follows a dry spell, which can trigger a sudden migration. Fall is the second big window: cooling temperatures and the search for overwintering shelter send large numbers toward homes in late summer and autumn, seeking cracks to shelter in or escaping excess ground moisture. Homes in wetter regions and near wooded lots — cottage country around Orillia, and low-lying areas across Barrie and Simcoe County — see the heaviest migrations. Winter drives survivors below the frost line until spring.

Where They Hide

Outdoors: in soil, leaf litter, mulch, and compost; under logs, boards, stones, flowerpots, and mulch piled against the foundation. Indoors, where they’ve migrated: damp basement and garage corners, around floor drains and sump pits, in crawl spaces, under boxes in low storage areas, and along foundation walls — always the dampest, lowest spots, because they can’t survive the dry air of a heated home for long.

How They Enter Homes

Millipedes enter through foundation cracks, gaps around basement windows and doors, garage doors, weep holes, utility penetrations, and under thresholds with worn weatherstripping. Because they migrate along the ground toward vertical surfaces, homes with mulch, leaf litter, or dense plantings right against the foundation give them a launch pad straight to the wall. Sliding basement and garage doors are common entry points during a mass migration, when sheer numbers overwhelm small gaps.

Prevention Tips

1. Clear a dry band at least 30 cm (1 ft) around the foundation — remove mulch, leaf litter, and organic debris.
2. Improve drainage so water flows away from the foundation; fix downspouts and grade soil to slope outward.
3. Run a dehumidifier in basements and crawl spaces to keep low areas dry.
4. Seal cracks around the foundation, basement windows, garage doors, and utility penetrations.
5. Replace worn door sweeps and weatherstripping on basement and garage doors.
6. Move woodpiles, compost, and dense plantings away from the house.
7. Rake and remove fallen leaves in fall before the autumn migration begins.

A dry, debris-free perimeter removes the damp harbourage that stages millipede migrations right at your walls.

DIY vs. Professional Treatment

For most millipede events, DIY is enough: sweep or vacuum up the ones indoors (they’ll die on their own regardless), then dry out and clear the perimeter to stop the source. Because millipedes can’t survive indoors, you’re rarely fighting an established infestation — you’re managing a migration. Where DIY falls short is a recurring problem: a chronically damp foundation, a wooded lot, or repeated seasonal surges that overwhelm sealing efforts. There, a professional can identify and treat the outdoor harbourage, apply a residual perimeter barrier before migration season, and pinpoint the moisture conditions driving it. Sani IQ’s integrated approach targets the source and stands behind it with a Pest-Free-Or-It’s-Free guarantee. See general insect control pricing or request a quote.

References

• University of Minnesota Extension — Sowbugs, Millipedes and Centipedes
• University of Missouri Extension — Millipedes and Centipedes

Last updated: July 16, 2026 · Reviewed by Sani IQ licensed technicians