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 spider, the woodlouse spider, has evolved specifically to hunt them. 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: The Specialist Hunter
The standout isopod predator is the woodlouse spider (Dysdera crocata) - also called the woodlouse hunter, sowbug killer or slater spider - which is specifically adapted to hunt 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 fangs (chelicerae) that pierce the soft underbelly, injecting venom while avoiding the defensive chemicals. 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, the red woodlouse spider (Dysdera erythrina), does much the same in leaf litter.
It's worth noting these spiders aren't strict specialists - in the lab they'll take silverfish, earwigs and other invertebrates too - but woodlice are simply the prey they're built for and most often catch.
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 woodlouse-eating rove beetle, or devil's coach horse (Ocypus olens), is a notable ground predator found under stones and debris, and various ground beetles take woodlice too.
- Parasitic wasps - some Ichneumonidae wasps target woodlice, albeit at a lower rate than the larger predators.
- 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. Interestingly, birds like tits and nuthatches are also the main 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 wasps, and the specialist woodlouse spider. Other woodlice will also cannibalise freshly-moulted individuals.
What is the woodlouse spider?
The woodlouse spider (Dysdera crocata) is a hunting spider specifically adapted to prey on woodlice. Its oversized fangs pierce the woodlouse's soft underside, getting past the armoured shell and defensive chemicals that deter 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.
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