Woodlice and pill bugs are recyclers. They live mostly on dead and decaying plant material rather than fresh green plants. What they are often after is the fungi and bacteria growing on that rot, which do much of the digesting for them. And isopods as a whole group eat far more than leaves, from algae in ponds to whale carcasses on the deep seafloor.
What Woodlice and Pill Bugs Eat in the Wild
Fallen leaves on the ground, often called leaf litter, are the main food of land isopods.1PubMed Central. Terrestrial isopods as model organisms in soil ecotoxicology: a review They prefer leaves that are already decaying over freshly fallen ones, because decayed leaves are softer and easier to digest.2Cambridge University Press. Nutrition in terrestrial isopods (Isopoda: Oniscidea): an evolutionary-ecological approach They are not fussy about tree type, either. Native woodlice readily ate oak leaves from trees not native to their area, and some species preferred them.3Europe PMC. Feeding preferences of native terrestrial isopod species (Oniscoidea, Isopoda) for native and introduced leaf litter
Turn over a damp log or a pile of old leaves and you will usually find isopods clustered there, since rotting wood is both food and shelter. The leaves they work on look darker and softer, often eaten down to a lacy network of veins.
Wild isopods in one dry habitat ate about three times more of the thin living crust on the soil surface than plant litter.4PubMed Central. ‘Dust you shall eat’: The complex nutritional and functional considerations underlying a simple diet That crust supplies calcium and helpful microbes even though it is low in protein and energy.5PubMed Central. ‘Dust you shall eat’: The complex nutritional and functional considerations underlying a simple diet When you see isopods pressed against bark, a rock or damp soil, they are usually scraping off a thin film of microbes, algae or fungus rather than eating the surface itself.
Land isopods also eat their own droppings, partly because those droppings are coated with dense microbes.6Cambridge University Press. Nutrition in terrestrial isopods (Isopoda: Oniscidea): an evolutionary-ecological approach Passing food through the gut twice lets microbes work on it again, so the isopod can take up more nutrients from poor food. It is normal behaviour, not a sign of hunger or illness. They will also eat dead insects and other small animal remains when they find them. They generally scavenge rather than hunt, so a dead beetle or worm is food but a healthy live insect is not usually a target.
Gardeners often worry about them. Because isopods mainly eat dead material, healthy plants are rarely the target. They may nibble very soft seedlings or ripe fruit touching damp soil, but damage blamed on them is often caused by slugs, which leave slime trails.
Why Fungi and Bacteria Matter So Much
Land isopods cannot digest the tough woody fibre in plants by themselves.7PubMed Central. Lignocellulose degradation in isopods: new insights into the adaptation to terrestrial life They rely on microbes living in their gut to do part of the work.8PubMed Central. Lignocellulose degradation in isopods: new insights into the adaptation to terrestrial life In one experiment, isopods that ate soil crust with its microbes killed ate more but grew more slowly, which shows the live microbes help them digest.9PubMed Central. ‘Dust you shall eat’: The complex nutritional and functional considerations underlying a simple diet
They also seek out microbe-rich spots. Woodlice on maple leaves ate the fungus-infected spots first, then plain leaf, and the tough veins last.10Pedobiologia. Selective feeding on a maple leaf by Oniscus asellus (Isopoda) The infected spots were richer in nitrogen, a building block of protein.11Pedobiologia. Selective feeding on a maple leaf by Oniscus asellus (Isopoda) Microbes make poor food more attractive, especially when the food itself is low quality.12Pedobiologia. Selective consumption and digestion of litter microbes by Porcellio scaber (Isopoda: Oniscidea)
For feeding, this means pre-rotted leaves are better food than fresh-picked green ones. Fresh leaves still hold tough fibre and plant chemicals, and microbes have not yet softened them. Offering only fresh greens can leave an isopod underfed. Leaves that sat outside for a few weeks over winter are ideal. A little natural decay in an enclosure is useful, not a problem.
You can give microbes a head start by dampening dried leaves a few days before adding them, so fungi and bacteria begin to colonise them. Keep part of the enclosure damp but not soaked.
Some keepers bake or boil leaves to remove pests. That kills the helpful microbes too. Some accept the trade-off, and adding untreated soil or rotting wood restores microbes.
White, thread-like fungus growing on rotting wood or leaves is normally harmless, and the isopods actively graze it. The mould worth worrying about is a thick, slimy or foul-smelling growth on leftover fresh food, which is a sign the food should come out.
Calcium and Protein Needs
Isopods tightly control how much calcium they eat.13PubMed Central. ‘Dust you shall eat’: The complex nutritional and functional considerations underlying a simple diet They store calcium in white patches on their underside before they moult, which means shedding the old shell.14PubMed. Analysis of CaCO3 deposit formation and degradation during the molt cycle of the terrestrial isopod Porcellio scaber (Crustacea, Isopoda) An isopod’s shell is a hard outer layer stiffened with calcium, so each time it grows it needs a fresh supply to harden the new one. Land isopods reuse calcium by eating their own shed skin.15PubMed. Calcium translocations during the moulting cycle of the semiterrestrial isopod Ligia hawaiiensis (Oniscidea, Crustacea) Leaving shed skins in the enclosure helps them.
Good calcium sources include cuttlebone, the most common choice, which can stay in the tank. Crushed eggshell and limestone or chalk also work. Leave it available all the time rather than in occasional doses.
Isopods also regulate their protein intake, so they eat more protein-rich food when protein is short. In captivity, protein is usually given as fish flakes, dried shrimp or insects in small amounts. Offer calcium and protein as separate options so the isopods can pick what they need. Leaf litter alone is low in both, so a colony fed only leaves can slowly run short without obvious early signs.
When something is missing, isopods may show it. Eating each other, or moulting ones in particular, may signal low protein, and cannibalism of moulting animals is a known isopod behaviour. Poor or failed moults can point to too little calcium.
What to Feed Pet Isopods
- Leaves: dried, decaying hardwood leaves such as oak, beech or maple as a constant base.
- Wood: rotting hardwood and white-rotted wood.
- Vegetables: carrot, squash, cucumber and sweet potato as treats.
- Calcium: cuttlebone or another calcium source.
- Protein: small amounts of fish flakes or dried shrimp.
Keep leaf litter on the floor at all times so they always have something to eat. Offer a small portion of fresh food every few days, and adjust by what disappears in a day. A portion that vanishes overnight tells you the colony can handle a bit more. With a new colony, start with a familiar food, such as leaf litter and a slice of soft vegetable, so you can see they are eating before adding extras.
Pesticides and weedkillers are poisonous to isopods and can last years in the environment.16PostPods. What do Isopods eat? Collect leaves only from unsprayed areas, and wash shop-bought produce, ideally organic, especially peels.17PostPods. Can Isopods Eat Kitchen Scraps? Safe Foods Guide Remove uneaten fresh food within about 24 hours,18PostPods. What do Isopods eat? because leftovers go mouldy and attract grain mites.19PostPods. Can Isopods Eat Kitchen Scraps? Safe Foods Guide
Some foods are best avoided: anything salted, seasoned, cooked with oil or processed, and bread or sugary food that moulds fast. Keepers commonly avoid citrus and onion too.
Isopods also absorb and store toxic metals from their surroundings very efficiently.20ScienceDirect / Elsevier (Sci Total Environ.). Cadmium assimilation in the terrestrial isopod, Porcellio dilatatus–is trophic transfer important? Leaves from roadsides or industrial areas are a poor choice.
What Water-Dwelling Isopods Eat
Freshwater isopods grow well on young water plants, algae coating plants and stones, and decaying oak leaves.21Freshwater Biology. Feeding and growth of <i>Asellus aquaticus</i> (Isopoda) on food items from the littoral of Windermere, including green leaves of <i>Elodea canadensis</i> In an aquarium or pond they act as cleaners of algae and dead leaves. They are a separate group that lives fully in water, with gills that work there, while land species depend on damp air. Woodlice and pill bugs will drown or struggle if put into an aquarium, and aquatic isopods need water conditions very different from land species. Fish in the same tank may eat them or keep them hiding.
In a tank, oak or other hardwood leaves soaked until they sink give freshwater isopods both food and cover. As the leaves soften and grow a film of microbes, the isopods graze them, so replace a few leaves as they break down. Algae-covered surfaces feed them too.
Among sea species, seagrass isopods ate kelp at 20 times the rate of their usual algae and seagrass when offered both.22Hakai Institute. The Kelp Brings All the Isopods to the Yard Kelp is tastier and richer in nutrients.23Hakai Institute. The Kelp Brings All the Isopods to the Yard
Eating also has to be weighed against safety. Seagrass isopods prefer food over shelter until a predatory fish is nearby,24PubMed. The relative importance of food and shelter for seagrass-associated invertebrates: a latitudinal comparison of habitat choice by isopod grazers and then give up the better feeding spots. Some stream isopods detect fish by smell and stop moving out to feed, losing feeding time as a result.25Oikos. Habitat Use and Fish Avoidance Behaviors by the Stream-Dwelling Isopod Lirceus fontinalis
Giant Deep-Sea and Parasitic Isopods
Giant isopods are meat-eaters that scavenge on the deep seafloor.26Wikipedia. Giant isopod They eat fish, crabs, shrimp, squid, sponges and whale carcasses, and sometimes attack live prey.27Aquarium of the Pacific. Giant Isopod Food reaching the deep seafloor is unpredictable, mostly dead animals sinking from above, so a giant isopod travels slowly and relies on smell to find carcasses. When a large carcass does arrive, it can eat until its body is visibly swollen, storing energy for the wait ahead.
Because food is scarce in the deep sea, they can go years without eating, over five years in captivity.28Wikipedia. Giant isopod Their cold, deep home keeps their metabolism low, so they burn very little energy. They are not dangerous to people, and live far from any swimmer.
A smaller sea scavenger eats fish or crustacean remains with no preference.29Ophelia. Biology of feeding in the scavenging isopod Natatolana borealis (Isopoda: Cirolanidae) It often eats injured or moulting members of its own species.30Ophelia. Biology of feeding in the scavenging isopod Natatolana borealis (Isopoda: Cirolanidae)
Parasites work differently, because they feed on a living host over time and must avoid killing the fish they depend on. The tongue-eating louse attaches to a fish’s tongue and feeds on its blood and mucus.31Wikipedia. Cymothoa exigua As the tongue wastes away the louse takes its place, and the fish can then use the parasite like a tongue.32Wikipedia. Cymothoa exigua Different fish parasites specialise, some living 90-95% on blood, others mainly on mucus or muscle.33Asian Pacific Journal of Tropical Disease. Study of the functional morphology of mouthparts of parasitic isopods of marine fishes These parasites target fish and are not a danger to people who handle or eat the fish.
Why Their Eating Habits Help Soil
Isopods’ leaf eating plays an important part in recycling nutrients in soil.34PubMed Central. Terrestrial isopods as model organisms in soil ecotoxicology: a review With isopods present, leaves broke down much faster and topsoil gained nitrogen, phosphorus and carbon.35Geoderma. Effects of terrestrial isopods on soil nutrients during litter decomposition They chew leaves into small pieces, which gives microbes far more surface to work on, and their droppings carry those nutrients into the soil.
You can use this. In bioactive terrariums they clean up dead leaves and droppings, and outdoors a leaf pile supports them. Avoid spraying where you want them to work.
There is a downside to eating soil, leaf litter and droppings. Isopods swallow tiny plastic fragments from their surroundings,36bioRxiv. Effects of microplastic ingestion on hydrogen production and microbiomes in the gut of the terrestrial isopod Porcellio scaber without being able to tell them apart from food. Some plastic types disrupt the gut microbes that help them digest.37PubMed. Microplastic ingestion affects hydrogen production and microbiomes in the gut of the terrestrial isopod Porcellio scaber Keepers can lower the risk by avoiding plastic mulch, synthetic decorations that shed fibres, and leaves from heavily littered ground.