Porcellio scaber Latreille, 1804

Sowbug

A land-dwelling crustacean, not an insect, that needs damp places to survive.

Order: Isopoda
Family: Porcellionidae
Group: Occasional invaders
Ontario status: Established, common

Why it matters

A nuisance and a moisture indicator. Sowbugs indoors signal damp conditions that can also support mold, silverfish, and other moisture pests. They damage nothing and threaten no one.

Origin

Native to Europe; now cosmopolitan, one of the most widespread terrestrial crustaceans on earth, and long established in Ontario.

Behavior

A nocturnal scavenger that feeds on decaying leaves, wood, and organic matter. Cannot roll into a ball, its body is too flat (unlike its cousin the pillbug). Harmless: no bite, no sting, no venom.

Living conditions

Needs constant moisture. It breathes through gill-like structures under its abdomen, so dry air kills it. Outdoors it lives under logs, stones, leaf litter, mulch, and doormats; indoors it appears in damp basements, bathrooms, and ground-level rooms.

Signs of presence

Small gray oval creatures under doormats, planters, and stored items
Sightings in damp basements, laundry rooms, and bathrooms
Clusters under logs, stones, or mulch near the foundation
Most active at night or after rain

Weaknesses

Desiccation, remove moisture and it cannot survive
Ground-level harborages: leaf litter, mulch against the foundation, boards, stones
Entry gaps at ground level: door sweeps, foundation cracks, weep holes

Myths and facts

Myth Sowbugs are insects.
Reality They are terrestrial crustaceans, relatives of crabs and shrimp. They have seven pairs of legs and breathe with gills.
Myth They infest homes the way cockroaches do.
Reality They cannot breed in dry indoor conditions. Indoor sightings are wanderers from outside that die without moisture.

Research notes

Morphology

Adults 8-15 mm long, 4-6 mm wide; dorsoventrally flattened oval body of seven pereon segments; slate-gray with a rough, granular dorsal surface; two pairs of antennae (one pair minute); seven pairs of walking legs; uropods project visibly beyond the rear margin.

Diagnostics

Flat (not convex) profile that cannot conglobate; granular dorsal texture; uropods extending past the telson. Distinguished from Armadillidium by the flattened body and inability to roll into a ball.

Easily confused with

Pillbug, Armadillidium vulgare The pillbug is convex and rolls into a complete ball; the sowbug is flat and cannot.

Life cycle

Adult: Lives 2-3 years; 1-3 broods per year, roughly 20-50 young per brood
Egg: Carried 3-4 weeks in the female's marsupium
Egg to adult: Several months, temperature-dependent
Generations per year: 1-2 outdoors in Ontario
Nymph or larva: Mancae; about 5 molts to maturity over several months

Development

Direct development, no metamorphosis. Eggs carried in a ventral brood pouch (marsupium); young hatch as mancae and gain the seventh leg pair after the first molt.

Reproduction

Sexual reproduction; females brood fertilized eggs in the marsupium until hatching. Brood size roughly 20-50.

Distribution

Global: Cosmopolitan in temperate regions worldwide
Native range: Europe
Ontario: Established, common province-wide in leaf litter, gardens, and damp structures

Ecology

Detritivore and key decomposer of leaf litter and decaying wood. Aggregates in moist microhabitats via thigmotaxis and humidity response; activity peaks at night and after rain. Prey for ground beetles, centipedes, and spiders.

Resistance

Unknown, not a pesticide target; no documented resistance.

IPM implications

Moisture management is the entire strategy: fix leaks, ventilate crawlspaces, grade soil away from the foundation, and remove ground-contact harborages (mulch, boards, leaf litter) within a meter of the building. Exclusion at ground level, door sweeps, sealed weep holes, foundation crack repair, stops indoor wanderers. Indoor sightings are a diagnostic signal for dampness, not a breeding population.

Chemical ecology

Aggregation is driven by touch (thigmotaxis) and moisture gradients rather than long-range pheromones; no defensive chemistry of note.

Genetics and DNA

No reference genome published as of 2026.

Taxonomy

Order Isopoda, suborder Oniscidea (woodlice), family Porcellionidae. Porcellio scaber Latreille, 1804.

Open questions

Population genetics of introduced North American lineages are poorly mapped; desiccation-tolerance thresholds across populations are understudied.

References

Penn State Extension, Sowbugs and Pillbugs
University of Minnesota Extension, Sowbugs and pillbugs in the home
NC State Extension, Woodlice: sowbugs and pillbugs