Sowbug vs Pillbug

📐 FIELD GUIDE ILLUSTRATION

Millipede and pill bug identification illustration with labeled anatomical features — PestControlBasics.com

Original illustration by PestControlBasics.com. Use the labeled features above to confirm your identification.

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PestControlBasics Editorial Team

Reviewed by Derek Giordano · Updated 2026

🔍 Identification

Pillbug (Armadillidium vulgare): 10-18mm; dark grey; smooth dorsal surface; rolls into a perfect sphere when disturbed — the defining 'roly-poly' or 'rollie pollie' behavior. Sowbug (Oniscus asellus): 10-15mm; lighter grey; 2 tail-like appendages extending beyond body; CANNOT roll into a ball. Both are crustaceans (more closely related to lobsters than to insects).

🧬 Biology & Behavior

Both species are non-insect crustaceans that have fully adapted to terrestrial life — they must stay in moist environments to breathe (they have modified gills). They breathe through humidity, not just moisture. Found anywhere with persistent dampness: basement edges, under landscaping materials against foundation, in decomposing wood, and in crawl spaces. Both are completely harmless decomposers.

⚠️ Damage & Health Risk

Nuisance only when they enter structures. No damage, no biting, no disease. Their presence in basements in large numbers confirms excessive moisture.

🔧 DIY Treatment

Reduce foundation moisture (improve drainage, remove mulch from foundation contact). Bifenthrin perimeter spray. Vapor barrier in crawl space. Door sweeps. Indoor individuals die within 24-48 hours without moisture — no indoor treatment needed.

👷 When to Call a Pro

Never warranted alone.

❓ FAQ

Are pillbugs insects?

No — pillbugs and sowbugs are crustaceans (Isopoda), more closely related to crabs and shrimp than to any insect. They're the only crustaceans to have fully adapted to terrestrial life. They still breathe through modified gills and require humidity to survive.

Why do I have hundreds of them in my basement?

Large numbers indicate excessive moisture in or near the basement — from a drainage problem, plumbing leak, high water table, or inadequate vapor barrier. Fix the moisture source and populations self-resolve, since they die indoors without adequate humidity.

Factor	Details
U.S. Range	All or most U.S. states
Regional Detail	Distribution varies — consult your local extension service for regional prevalence data.

Factor

Details

U.S. Range

All or most U.S. states

Regional Detail

Distribution varies — consult your local extension service for regional prevalence data.

📅 Treatment Timing Guide

Treating at the right time dramatically improves results. Pest control timed to the life cycle uses less product and achieves better long-term control.

Period	Action
Spring	Inspection and perimeter treatment before pest season starts.
Summer	Active monitoring and targeted treatments as needed.
Fall	Preventive treatment before overwintering pests seek entry.

Period

Action

Spring

Inspection and perimeter treatment before pest season starts.

Summer

Active monitoring and targeted treatments as needed.

Fall

Preventive treatment before overwintering pests seek entry.

💰 Professional Treatment Costs

Service Type	DIY Cost	Professional Cost
Initial inspection	Free (self-inspect)	$75–$150 (often credited to treatment)
One-time treatment	$30–$100 in materials	$150–$500
Annual service contract	N/A	$400–$900/year
Severe infestation	Often ineffective alone	$500–$2,500+

Service Type

DIY Cost

Professional Cost

Initial inspection

Free (self-inspect)

$75–$150 (often credited to treatment)

One-time treatment

$30–$100 in materials

$150–$500

Annual service contract

N/A

$400–$900/year

Severe infestation

Often ineffective alone

$500–$2,500+

Prices vary by region, property size, and infestation severity.

❓ Common Questions About 🐛 Sowbug & Pillbug

How do I confirm I actually have this pest (not something similar)?

The most reliable confirmation is a physical specimen — capture one and compare to reference images on this page. For cryptic pests (bed bugs, termites), look for secondary signs: frass, shed skins, mud tubes, or bites with a specific pattern. When uncertain, a professional inspection is faster than months of misidentification.

Can I treat this myself or do I need a professional?

DIY is effective for small, accessible infestations caught early. Professionals are worth the cost when: the infestation is inside wall voids or structural elements, multiple rooms are affected, you have health-risk pests (hantavirus, venomous species), or DIY has already failed twice.

How long until the infestation is completely gone?

Expect 3–8 weeks for most infestations with proper treatment. Insects with dormant life stages (pupae, eggs) extend the timeline because those stages are impervious to most insecticides. Follow-up treatments at 2 and 4 weeks catch each new cohort as they emerge.

What's the most common mistake people make treating this pest?

Treating only the visible pest population while ignoring the harborage site, entry point, or breeding location. Killing adults provides temporary relief but the population rebuilds from hidden egg cases, pupae, or new arrivals through unaddressed entry points.

🐛 Sowbug & Pillbug