What Do Pill Bugs Eat? Decomposers of Dead Leaves and Wood

Dead leaves and rotting wood

The common pill bug is a detritivore, an animal that feeds on dead organic matter. Its main food is leaf litter, the layer of fallen leaves that covers the ground in woods, hedges and gardens. Pill bugs have been recorded eating the leaves of many trees, including maples, willows, oaks and limes, as well as dead grass, rotting wood, bark and plant roots.

They are fussy about freshness, but in the opposite way to people. Pill bugs much prefer leaves that have been lying on damp ground for a while, softened and enriched by fungi and bacteria. Fresh, tough leaves full of bitter tannins are eaten slowly or left alone. Like other woodlice, pill bugs scrape and chew with strong mouthparts and leave leaves skeletonized, with only the veins remaining.

Fungi, microbes and algae

Much of the nutrition a pill bug gets from rotting leaves comes not from the leaf itself but from the fungi and bacteria living on it. These microbes are richer in protein than dead leaves and help break down tough plant fibers. Pill bugs graze fungal threads, mold and algae directly, and they generally favor litter that fungi have already colonized.

By eating fungi, pill bugs influence how fast leaf litter decomposes and how much carbon dioxide the soil releases. Ecologists study them partly for this reason.

Why pill bugs eat their own droppings

Pill bugs regularly eat their own feces, a habit called coprophagy. One estimate suggests that almost a tenth of a pill bug’s food can come from its own droppings. This is less strange than it sounds: a dropping is finely chopped plant material that still contains nutrients, and after a day or two on damp ground, fungi and bacteria have started to digest it further.

Biologists have also suggested that re-eating droppings helps woodlice keep enough copper. Their blood carries oxygen with hemocyanin, a copper-based pigment, so copper is an essential nutrient for them.

Do pill bugs eat living plants?

Sometimes. Pill bugs prefer dead material, but when they are very numerous or the ground is dry, they may feed on tender seedlings, young shoots, roots and ripe fruit lying on the soil, such as strawberries. Damp mulch and heavy leaf cover can allow large numbers to build up close to young plants.

In most gardens the damage is minor and local. In some farming regions where crop residues are left on the surface, very large pill bug populations have damaged seedlings of crops such as soybeans and sunflowers. It is a reminder that a pill bug is a generalist that eats whatever soft food is close at hand.

Meat, molts and minerals

Pill bugs are not hunters, but they scavenge. They feed on dead insects, dead earthworms and other carrion, and in drought they become more scavenger-like. Woodlice often eat their own shed skin after molting, which helps them recover the calcium needed to harden a new shell.

Calcium is a constant concern for an animal that wears armor. The common pill bug is most common on chalky and limestone soils in places such as Britain, where calcium is plentiful, and it stores calcium from its old shell before each molt.

How pill bugs help the soil

Pill bugs do not finish the job of decomposition, but they speed it up. By chewing leaves into tiny pieces and passing them out as droppings, they multiply the surface area that fungi and bacteria can attack. Their tunnels and movements also mix organic matter into the top of the soil.

Woodlice share this work with other decomposers. Earthworms drag leaves underground and grind them finely, while millipedes, which eat a similar diet of rotting plants, chew litter much as woodlice do; see what do millipedes eat? Together they help turn autumn leaves into the dark, crumbly humus that feeds plants.

Storing heavy metals

Pill bugs and other woodlice have a striking ability: they absorb heavy metals such as copper, zinc, cadmium and lead from their food and lock them away in small storage packages in their digestive gland, the hepatopancreas. They can hold higher concentrations of these metals than most other land animals without being poisoned.

This makes woodlice useful to scientists as bioindicators, living measures of soil pollution near mines, smelters, roads and copper-treated vineyards and orchards. The amount of metal stored in their bodies reflects how much is in the leaf litter around them, although the link becomes less reliable at very high pollution levels.

Who eats pill bugs?

Pill bugs are themselves an important food. They are eaten by spiders, centipedes, ground beetles, toads, frogs, salamanders, shrews and many birds, and they are regular prey for hedgehogs; see what do hedgehogs eat? The woodlouse spider specializes in them and can pierce their armor with its long fangs. By turning dead leaves into living food for these animals, pill bugs form a vital link between the leaf litter and the wider food web. Their main defense against becoming a meal is explained in why do pill bugs roll into a ball?