Out in the wild, isopods sit near the bottom of the food chain, so plenty of animals eat them. Birds, amphibians, reptiles, small mammals and a range of predatory invertebrates all take woodlice - and one kind of spider, the woodlouse spider, has the jaws to get past their armour. This guide runs through the main predators of isopods and the defences that help woodlice survive despite them.
It's a useful bit of background for keepers too: the same defences and vulnerabilities shape how isopods behave in captivity, and why they're so valued as a natural live food. If you keep a bioactive setup, our look at isopods as feeders covers the captive side.
The Woodlouse Spider: Built for Woodlice
The standout isopod predator is the woodlouse spider (Dysdera crocata) - also called the woodlouse hunter, sowbug killer or slater spider - whose long jaws suit it to catching woodlice. Most predators are put off by a woodlouse's tough, sometimes chemically-defended shell, but this spider gets around that with a pair of disproportionately large jaws (chelicerae) tipped with long fangs. One fang spears the soft underside while the other grips the armoured back, and the spider injects venom. Females reach around 11-15 mm, with a reddish front body and a pale abdomen, and they hunt at night under logs, stones and bark wherever woodlice are common. A close relative, Dysdera erythrina, looks very similar and also eats woodlice, but in Britain it is less common, and it is found more often in old grassland and ancient woodland than around houses.
It's worth noting the woodlouse spider isn't a strict specialist - in laboratory tests it takes other invertebrates too - but woodlice are the prey its jaws are built for.
Other Invertebrate Predators
Several other small hunters take a steady toll on woodlice, mostly under cover of darkness in the leaf litter and soil:
- Centipedes - fast nocturnal predators like the common centipede (Lithobius forficatus) regularly hunt woodlice and other soil arthropods.
- Predatory beetles - the devil's coach horse (Ocypus olens), a large rove beetle found under stones and debris, counts woodlice among its prey, and various ground beetles take woodlice too.
- Parasitic flies - the larvae of woodlouse flies (Rhinophoridae) develop inside living woodlice, and they are the only insect parasitoids of woodlice known.
- Other woodlice - woodlice will cannibalise their own kind, particularly freshly-moulted individuals, which are soft and defenceless until the new exoskeleton hardens.
Birds
Many garden and woodland birds eat isopods as part of a varied invertebrate diet. In the UK and Europe, robins, blackbirds, tits and nuthatches all take woodlice, foraging through leaf litter and turning over debris. Birds are also listed among the predators of the woodlouse spider itself - so they eat both the hunter and the hunted.
Amphibians
Amphibians that live in damp woodland are well placed to feed on woodlice, since they share the same moist microhabitats. Toads are particularly good consumers of woodlice, and salamanders take them too - the eastern red-backed salamander, for example, hunts woodlice along with millipedes and other small invertebrates under logs and leaf litter. Their need for damp conditions puts amphibians right where isopods congregate.
Reptiles
Various small reptiles include isopods in their diet. Ground-foraging lizards such as the common wall lizard pick off ground-active arthropods including woodlice, and some small snakes will take them as part of a broader invertebrate diet. (A note on a common mix-up: newts are sometimes listed here, but newts are amphibians, not reptiles - it's lizards that are the reptile woodlouse-hunters.)
Mammals
Small insectivorous mammals round out the list. Shrews, with their famously high metabolism and near-constant need to eat, take woodlice along with beetles, snails and other small prey. Hedgehogs, omnivorous foragers of gardens and hedgerows, also eat woodlice as part of a varied diet. For these animals, the abundance of woodlice in leaf litter makes them a reliable, if small, food source.
How Do Isopods Defend Themselves?
Given all these predators, woodlice rely on several defences. The best known is conglobation - the ability of pill bugs (and Cubaris, and other "rollers") to curl into a tight armoured ball, protecting their soft undersides and legs. Many species also have tough, sometimes distasteful or chemically-defended shells that deter casual predators. Behaviourally, they stay hidden by day and forage at night, shelter under logs, bark and stones, and aggregate in groups, which reduces any single individual's chance of being the one that's caught.
The weak point in all this is moulting. When a woodlouse sheds its exoskeleton it's briefly soft and pale, and far more vulnerable - which is exactly when it's most likely to fall prey, even to other woodlice. It's also why, in captivity, isopods need plenty of hides and a calcium source to moult quickly and safely.
Frequently Asked Questions
What eats isopods in the wild?
Birds (robins, blackbirds, tits), amphibians (toads, salamanders), small reptiles (lizards), small mammals (shrews, hedgehogs), and many predatory invertebrates - centipedes, ground and rove beetles, parasitic flies, and the woodlouse spider. Other woodlice will also cannibalise freshly-moulted individuals.
What is the woodlouse spider?
The woodlouse spider (Dysdera crocata) is a night-hunting spider with long jaws suited to catching woodlice. One of its long fangs spears the woodlouse's soft underside while the other grips the armoured back, which gets it past the shell that deters other predators.
How do woodlice protect themselves from predators?
Many roll into a ball (conglobation) to shield their soft parts, have tough or chemically-defended shells, hide by day and forage at night, and gather in groups. They're most vulnerable just after moulting, when the new shell hasn't yet hardened.
Do isopods get eaten by other isopods?
Yes. Woodlice will cannibalise their own, especially individuals that have just moulted and are soft and defenceless until their new exoskeleton hardens.
Why does it matter what eats isopods?
Isopods are an important link in the food chain, feeding a wide range of animals while recycling decaying matter. The same role makes them a valued natural live food for captive reptiles and amphibians.
In a home enclosure none of these pressures apply, which is exactly why colonies grow the way they do. Our captive-bred isopods arrive without the predators, the parasites or the quarantine a wild-caught culture brings with it.
Leave a comment