All pests

Pillbug (Armadillidium vulgare)Pillbug

Armadillidium vulgare (Latreille, 1804)

Pillbug

The roly-poly that rolls into a perfect ball when disturbed.

Order
Isopoda
Family
Armadillidiidae
Group
Occasional invaders
Ontario status
Established, common

Why it matters

A harmless nuisance and, like the sowbug, a moisture indicator. Indoors it signals dampness; outdoors in large numbers it can nibble seedlings and soft plant tissue in gardens.

Origin

Native to Europe; now cosmopolitan and long established in Ontario gardens, greenhouses, and damp structures.

Behavior

Nocturnal scavenger of decaying plant matter. Its signature move is volvation, rolling into a tight ball (conglobation) when disturbed, which protects its soft underside and gills. Harmless: no bite, no sting.

Living conditions

Same needs as the sowbug: constant moisture, darkness, and decaying organic matter. Found under pots, stones, mulch, and logs; indoors in damp basements and ground-floor rooms.

Signs of presence

  • Round gray 'balls' under pots, stones, and doormats
  • Sightings in damp basements and bathrooms
  • Large numbers under mulch or leaf litter near the foundation
  • Active at night and after rain

Weaknesses

  • Desiccation, dry conditions are lethal
  • Damp harborages against the building: mulch, leaf litter, stacked wood
  • Ground-level entry gaps: doors, vents, foundation cracks

Myths and facts

  • Myth Pillbugs and sowbugs are the same thing.

    Reality Close cousins, but the pillbug rolls into a ball and the sowbug cannot, the single best field character for telling them apart.

  • Myth They are baby armadillos or some kind of insect.

    Reality They are terrestrial crustaceans, relatives of shrimp. The 'ball' is a defense, not a shell.

Research notes

Morphology

Adults 12-18 mm long; strongly convex, almost hemispherical body; dark gray to blackish, smooth dorsal surface; seven pereon segments; uropods short, not extending beyond the rear margin; capable of complete conglobation with no gaps.

Diagnostics

Convex profile plus complete volvation is diagnostic. Uropods hidden (vs. projecting in Porcellio). Smooth rather than granular dorsum.

Easily confused with

  • Sowbug, Porcellio scaber The sowbug is flattened and cannot roll into a ball; the pillbug is rounded and can.

Life cycle

Adult
Lives 2-3 years; 1-3 broods per year, roughly 20-60 young per brood
Egg
Brooded roughly 3-4 weeks in the marsupium
Egg to adult
Several months, temperature-dependent
Generations per year
1-2 outdoors in Ontario
Nymph or larva
Mancae; several molts over months to maturity

Development

Direct development. Eggs brooded in the marsupium; mancae emerge and molt about 5 times to maturity.

Reproduction

Sexual; females carry eggs in the marsupium. Brood size roughly 20-60.

Distribution

Global
Cosmopolitan in temperate regions
Native range
Europe (Mediterranean basin)
Ontario
Established, common in gardens, compost, greenhouses, and damp structures

Ecology

Detritivore; important decomposer in garden and forest-floor food webs. Conglobation reduces water loss and deters predators. Nocturnal; aggregates under cover objects.

Resistance

Unknown, not a pesticide target; no documented resistance.

IPM implications

Same moisture-and-exclusion logic as the sowbug: eliminate damp harborages against the foundation, correct drainage and ventilation, seal ground-level entries. Indoor individuals are incidental wanderers; persistent indoor presence warrants a moisture investigation, not a pesticide application.

Chemical ecology

No significant defensive chemistry; defense is mechanical (conglobation). Aggregation via thigmotaxis and humidity response.

Genetics and DNA

A reference genome exists: a 1.72 Gb ZW-female assembly (Chebbi et al.; NCBI GCA_004104545.1), a model resource for terrestrial-isopod sex determination and Wolbachia-symbiosis research.

Taxonomy

Order Isopoda, suborder Oniscidea, family Armadillidiidae. Armadillidium vulgare (Latreille, 1804), parentheses indicate original description in a different genus (Armadillo vulgaris).

Open questions

Genetic structure of introduced populations and the physiological limits of conglobation as a desiccation defense are not fully characterized.

References

  • Penn State Extension, Sowbugs and Pillbugs
  • University of Kentucky Entomology, Pillbugs and Sowbugs
  • UC IPM, Sowbugs and Pillbugs