Invertebrates
Pill Bug
Armadillidium vulgare · also called roly-poly, woodlouse, potato bug
The pill bug is the small grey armoured creature under every flowerpot and log, the one children love because it rolls into a perfect ball. It is one of the most familiar animals in the garden, and almost nothing people assume about it is right: it is not an insect, it breathes with gills, and it carries its babies in a pouch.
Overview
Armadillidium vulgare is a terrestrial crustacean, an isopod, related to crabs and shrimp rather than to any bug. Now found nearly worldwide, it lives in leaf litter and damp soil and eats decaying plant matter, breaking it down and returning it to the soil.
A crustacean on land
It breathes with gill-like structures on its underside that must stay moist, which is why it lives in damp places and dies if left in the open. Threatened, it rolls into a ball — the trait that names it. The similar sowbug cannot roll up fully and has two little tail spikes.
A pouch and a soil cleaner
The female carries her eggs and young in a fluid-filled brood pouch, like a tiny marsupial. And pill bugs take up copper, lead, and cadmium from polluted soil and store it as crystals in the gut, cleaning the ground as they feed.
Status
Least Concern and abundant almost everywhere, carried around the world by trade — a helpful recycler, and at worst a minor garden nuisance.
Pill Bug: common questions
Is a pill bug an insect?
No. A pill bug is a crustacean, specifically a terrestrial isopod, and its closest relatives are aquatic isopods and, more broadly, crabs, shrimp, lobsters, and barnacles. Insects have six legs, three body sections, antennae, and usually wings; a pill bug has fourteen legs, a segmented armoured body, two pairs of antennae one of which is tiny, and never any wings. It also breathes with modified gills rather than the tracheal tubes of insects. Pill bugs are among a small number of crustaceans, along with some crabs and hermit crabs, that have left the water entirely, and they still carry the marks of their marine ancestry.
What is the difference between a pill bug and a sowbug?
They are close relatives and both are land isopods often found together under the same log, but there are quick ways to tell them apart. A pill bug, in the family Armadillidiidae, can roll into a complete, tight ball and has a smoothly rounded rear end with no projections. A sowbug, in families such as Porcellionidae and Oniscidae, has a flatter body, cannot roll up fully, and has two small pointed tail appendages sticking out the back. If you poke it and it becomes a perfect sphere, it is a pill bug; if it just flattens and scuttles away, it is a sowbug.
Are pill bugs harmful to a garden?
Mostly they are helpful. Their main job is breaking down dead leaves, wood, and other plant litter into finer material that enriches the soil, and in doing so they also recycle nutrients and even remove some heavy-metal pollution. They occasionally nibble soft seedlings, strawberries lying on the ground, or the roots of stressed plants, and in very large numbers in a greenhouse or a damp mulch bed they can do minor damage, but they are not a serious pest. If they become too numerous, reducing the amount of damp mulch and debris near vulnerable plants is usually enough.
Why do some pill bugs look blue or purple?
A vivid blue or purple pill bug has been infected by an iridovirus, a virus that fills the animal's tissues with crystalline particles that scatter light and produce the metallic colour, in the same physical way that a blue jay's feathers or a butterfly's wing get their colour. The infection is usually fatal to the pill bug within a few weeks and does not spread to people, pets, or plants. Blue pill bugs are uncommon but not rare, and finding one is a sign that the virus is circulating in the local population. Some pill bugs also carry a bacterium, Wolbachia, that converts genetic males into functional females.