Pillbugs and sowbugs in your BC home: the moisture diagnostic and structural fix

Pillbug vs sowbug — key differences

Feature	Pillbug (Armadillidium)	Sowbug (Oniscus / Porcellio)
Roll into ball	Yes — defensive posture	No — flattens only
Body shape	Rounded, convex	Flatter, more oval
Tail appendages	None visible	Two small tail-like appendages
Colour	Grey, sometimes banded	Grey to brownish
Moisture need	High, but slightly more tolerant	Very high — desiccates quickly indoors
Common location	Basement floors, crawlspace access	Crawlspace, under-slab moisture zones

Why they're in your home

Pillbugs and sowbugs are outdoor detritivores — they live in garden soil, mulch, leaf litter, and damp wood debris, breaking down organic matter. They enter homes through foundation gaps, under doors, through crawlspace openings, and at utility penetrations when outdoor conditions are wet enough that indoor humidity (particularly in BC basements and crawlspaces) is suitable for survival. In Metro Vancouver, these entries peak in autumn (rain-saturated soil plus cooling temperatures) and in spring after wet winters. High water-table areas like Surrey, Delta, and parts of Richmond see more persistent populations because the groundwater creates ongoing subslab and crawlspace humidity regardless of surface conditions. The key diagnostic fact: pillbugs and sowbugs cannot survive in properly conditioned indoor spaces. If you find them regularly in your basement or crawlspace, ambient humidity there exceeds 60–70% RH. If you find them in upper-floor living areas, there's an active moisture pathway — either rising damp or a plumbing issue — moving moisture up through the structure.

How to

Pillbug and sowbug elimination protocol

Focuses on moisture as the root cause. Chemical treatment is a secondary layer, not the primary fix.

1
Confirm moisture conditions
Place hygrometer in basement, crawlspace access, and any room with sightings. If readings exceed 60% RH, moisture management is the primary intervention. Check under slab for visible moisture seepage, and inspect crawlspace vapour barrier for tears and gaps.
2
Exterior grading and drainage
Inspect the foundation perimeter. Ground should slope away from the building at minimum 2% grade for 1.8 m. Standing water or low spots within 2 m of the foundation sustain the outdoor populations that migrate in. Address grading, improve downspout extensions, remove dense mulch touching the foundation wall.
3
Seal entry points
Foundation cracks, door bottoms with inadequate sweeps, basement window frames, and utility penetrations are the primary entry routes. Seal with polyurethane or hydraulic cement for foundation cracks, replace door sweeps, foam utility penetrations.
4
Improve interior moisture conditions
Mechanical dehumidifier in basement targeting 50% RH. Improve crawlspace vapour barrier if absent or damaged. Ensure crawlspace ventilation is functional. Indoor populations die off as conditions deteriorate — usually within 1–2 weeks.
5
Exterior perimeter granule treatment if population is large
For significant exterior populations at the foundation, a granular residual product in a 30 cm foundation band reduces the pressure migrating in. This buys time while structural improvements are made. Not a permanent fix alone.

Frequently asked questions

Are pillbugs dangerous to garden plants?+

Outdoor pillbug populations occasionally damage tender seedlings, particularly in wet springs when seedlings are in direct soil contact. They prefer dead organic matter and are mostly beneficial decomposers. Not significant garden pests in established beds, but can be an issue in seeding trays and direct-sown rows.

Do sprays work on pillbugs?+

Pyrethroid residual sprays kill pillbugs on contact and leave some residual effectiveness. They work for the current population but don't address the moisture conditions that make the habitat suitable. Without moisture management, populations rebound from outdoor sources. Use chemical treatment as a short-term measure while making structural improvements.

Are pillbugs related to insects?+

No. They're crustaceans — class Malacostraca — the same class as shrimp, crabs, and lobsters. They evolved from marine ancestors and retain gill-based respiration, which is why they require high humidity. This also explains why they die quickly in dry conditions and why desiccation (reducing humidity) is so effective as a control strategy.