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Woodlice

Also known as Sowbugs, Pillbugs, Slaters, Roly-polies

Beneficial or harmlessHarmless
A pillbug (Armadillidium vulgare), which can roll into a ball Photo: Franco Folini · CC BY 2.5

In short

Woodlice are small land crustaceans that recycle rotting plant material. They are abundant in healthy gardens, compost heaps and greenhouses and cause little or no damage to healthy plants, though large numbers can nibble very soft tissue such as seedlings.

How it looks

Oval, flattened bodies covered in overlapping armour-like plates, usually slate grey or brownish-grey, sometimes with yellowish or pinkish-brown markings. Adults are roughly 1–2 cm long, with seven pairs of legs and two antennae. Pillbugs can roll into a tight ball; sowbugs cannot and have two small tail-like projections at the rear. They hide by day under pots, stones, mulch and bark and come out at night.

Signs

Usually no damage. Occasionally small holes in seedlings, very soft shoots or strawberry fruits lying on damp soil; often they are simply enlarging damage started by slugs.

What favours it

Damp, sheltered places with decaying organic matter: under pots and saucers, in thick mulch, in compost heaps and in humid greenhouses. They dry out easily and cannot survive long in dry air.

Prevention

  • Keep mulch thin enough that its surface dries between waterings.
  • Raise seed trays and seedlings off the ground and grow seedlings on quickly.
  • Lift strawberry fruits off the soil with straw or a mat.
  • Clear rotting debris from around pots in greenhouses.

Treatment

Start with the first step and move down only if the problem persists.

  1. Usually none: they are part of a healthy garden and should be encouraged.
  2. Indoors, sweep or vacuum strays and reduce the damp hiding places that attract them.
  3. Protect vulnerable seedlings with the prevention steps rather than killing woodlice.
  4. Insecticide barrier treatments are rarely warranted; if used, follow the product label.

Pros and cons

Pros

  • Break down decaying plant material and help recycle nutrients, especially in compost.
  • Food for toads, centipedes, some spiders, ground beetles, shrews and parasitoid flies.
  • Harmless to people and pets.

Cons

  • Can occasionally damage seedlings, very soft growth and ripening fruit lying on soil.
  • A nuisance when they wander indoors.

Look-alikes

Pill millipedes
Also roll into a ball, but have many more legs (two pairs per segment) and a glossier, smoother body.
Vine weevil adults
Beetles with six legs and a snout, matt black with pale flecks.
Earwigs
Slender insects with pincers at the rear.

Where you find it

Seedlings, soft young growth and strawberry fruits can be nibbled; established plants are not harmed.

Sources

  1. Woodlice — RHS
  2. Millipedes, centipedes, sowbugs and pillbugs — Colorado State University Agricultural Biology
  3. Millipedes, Centipedes, Sowbugs and Pillbugs (IG093) — UF/IFAS

Written in our own words from these sources. Check a product label before using any treatment.