Sowbugs & pillbugs
Oniscidea — Porcellio and Armadillidium spp.
also called Woodlice, Roly-polies, Slaters
Land crustaceans that eat dead plant matter, not living plants. They turn up in damage they did not cause, and the honest page is the useful one.
How to know it
A flat gray-brown armored creature 8–15 mm long with seven pairs of legs and a segmented plated back. A pillbug rolls into a tight ball when touched; a sowbug cannot, and has two small tail projections. Both are crustaceans, not insects — closer to a crab than to a beetle.
What you'll see
- Crowds of them under pots, boards, mulch and stones, scattering when the cover is lifted.
- Shallow grazing on the underside of a strawberry or a melon already lying on damp ground.
- Feeding at the edge of an existing slug hole or a rot, which is what gets them blamed.
- Seedlings in a very damp, over-mulched propagation area grazed at the stem — the one case where they do real damage.
- Nothing at all on a plant that is healthy and off the ground.
Where it comes from
They breathe through gill-like structures and must stay damp, which governs everything they do: they hide by day under anything moist and come out at night to feed on decaying leaves, wood and compost. Females carry the eggs in a fluid-filled pouch on the underside. Two generations a year and a lifespan of two or three years.
Damp, sheltered, heavily mulched gardens, greenhouses and cold frames. Numbers can be enormous in a compost heap or under a board, and in almost all of those cases they are doing useful work rather than harm.
Dealing with it
- Establish what actually caused the damage first; nine times in ten the sowbug is feeding on an existing wound.
- Raise strawberries and melons onto straw, mats or a tile so the fruit is not lying on damp ground.
- Reduce damp harborage close to seedlings — lift boards, thin the mulch, space the pots.
- Trap them under a hollowed half-melon or a rolled damp newspaper and empty it each morning if numbers around seedlings are genuinely a problem.
- Do not use pesticide on them. They are recyclers, and there is almost never anything to control.
Keeping it out
- Keep mulch a few centimeters clear of stems and seedlings.
- Improve ventilation and drainage in greenhouses and frames.
- Water in the morning so surfaces are dry overnight.
- Keep the compost heap away from the seedling bench.
- Welcome them in the compost heap, where they are one of the fastest workers you have.
What it goes for
Nothing living, as a rule. Soft fruit lying on damp ground and seedlings in an over-damp frame are the exceptions.
Who hunts it for you
- CentipedesChilopoda Flat, fast, one pair of legs to a segment — and a predator, unlike the slow round millipede it is constantly confused with. It hunts slugs, cutworms, springtails and woodlice by night under mulch and stones. Learn the difference and leave this one alone.
- Salamanders & newtsCaudata Damp-loving hunters of slugs, worms, woodlice and beetle larvae that spend the day under a log or a stone and work the beds at night. Companions to the toads: they want the same damp shade, the same log pile, and the same absence of slug pellets.
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com