Why do I suddenly have millipedes in my basement after it rains?

Millipedes live in moist soil and leaf litter around your foundation. When heavy rain saturates the soil, they are driven out of their habitat and seek drier conditions — which leads them into your basement through gaps in the foundation. This is a predictable pattern: millipede invasions almost always follow periods of heavy rainfall. Improving drainage around the foundation and sealing entry points prevents this.

Are millipedes the same as centipedes?

No — they are very different. Millipedes are slow, round in cross-section, have two leg pairs per body segment, feed on decaying plant matter, and are completely harmless. Centipedes are fast, flat, have one leg pair per segment, are active predators, and can technically bite. If it is moving fast and looks alarming, it is probably a centipede. If it is slow and curls into a ball, it is a millipede.

Do millipedes damage plants or food?

Millipedes primarily feed on decaying plant matter and are generally beneficial in the garden. In large numbers, they can occasionally damage seedlings and soft plant tissue, but this is uncommon. Indoors, they do not eat food, fabric, or wood. They die within a few days indoors without access to moisture and decaying organic matter.

Millipede — Identification, Biology & Prevention

Overview

Millipedes are occasional invaders in Montreal homes, particularly in late summer and fall when heavy rainfall saturates the soil and drives them to seek drier conditions. Unlike centipedes, millipedes are slow-moving detritivores — they feed on decaying plant matter, not other insects. They are completely harmless to humans and cause no structural damage, but large numbers invading a basement or crawl space are a significant nuisance. Their presence almost always indicates excessive moisture around the foundation and an abundance of decaying organic matter nearby.

How to Identify Millipedes

Body length: 20–100 mm depending on species — cylindrical and worm-like
Colour: Dark brown to black; some species have reddish or orange markings on the sides
Legs: Two pairs of legs per body segment — appear to have hundreds of legs moving in a wave
Movement: Slow, deliberate, and smooth — very different from the fast, erratic movement of centipedes
Defence: Curl into a tight coil when threatened; some species secrete a mildly irritating fluid
Antennae: Short, club-shaped
Distinguish from centipedes: Millipedes are slower, rounder in cross-section, have two leg pairs per segment (centipedes have one), and do not bite
Habitat: Soil, leaf litter, under logs and stones, and in damp basements

Biology & Behaviour

Lifespan: 5–10 years for large species
Eggs: Laid in soil in clusters of 20–300
Development: Hatch with 3 pairs of legs; add segments and legs with each moult
Diet: Decaying plant matter, leaf litter, fungi, and occasionally living plant roots
Activity: Primarily nocturnal; most active after rain
Preferred conditions: Moist soil, high humidity, abundant decaying organic matter
Seasonal invasion: Move indoors in late summer and fall when soil becomes saturated after heavy rain, or in spring during thaw
Indoor survival: Cannot survive long indoors — die within a few days without moisture and food

Risks

No health risk: Millipedes do not bite, sting, or transmit disease
Defensive secretion: Some species secrete a mildly irritating fluid when threatened — can cause skin irritation or discolouration; wash with soap and water
Eye irritation: Avoid touching eyes after handling millipedes
No structural damage: Millipedes do not chew wood, fabric, or food
Nuisance: Large numbers in basements and crawl spaces are unpleasant
Indicator: Their presence indicates excessive moisture and decaying organic matter around the foundation

Signs of Infestation

Millipedes found on basement floors, in crawl spaces, and along foundation walls
Increased numbers after heavy rainfall or during spring thaw
Dead millipedes on basement floors — they die quickly indoors without moisture
Millipedes curled into tight coils when disturbed
Abundance of leaf litter, mulch, or decaying wood near the foundation
High moisture readings in the basement or crawl space

Prevention

Reduce moisture around the foundation — ensure proper grading so water drains away from the house
Remove leaf litter, mulch, and decaying wood from within 30 cm of the foundation
Keep mulch depth to less than 5 cm near the foundation
Seal gaps under doors, around utility penetrations, and through foundation vents
Install a dehumidifier in the basement — target 40–50% relative humidity
Fix leaky gutters and downspouts — direct water away from the foundation
Remove firewood, compost bins, and debris piles from near the foundation
Apply exterior perimeter insecticide in late summer before fall invasion begins

Professional Treatment

Millipede invasions are best addressed through habitat modification rather than chemical treatment. Removing moisture and organic matter from around the foundation eliminates the conditions that attract them. For large invasions, professional exterior perimeter spray applied in late summer creates a barrier that prevents entry. Interior treatment involves vacuuming millipedes and applying crack-and-crevice insecticide to entry points. Call 514-809-1999 for a free inspection if millipede invasions are recurring.

Call 514-809-1999 Pest Control Montreal