Are Isopods Poisonous? What the Science Actually Says

No. Isopods are not poisonous. They have no venom and no sting, and nothing in an isopod is toxic to a person or a pet. They are chemically defended, which is a different thing, and until 2025 nobody knew what the chemical was. A woodlouse carries about forty nanograms of a sticky deterrent aimed at spiders. Your gecko can eat them. The real risks attach to wild-collected animals, and they are parasites and heavy metals.

What a woodlouse actually makes

This has been an open question for as long as people have kept isopods, and in 2025 it was answered for one species. Fischer and colleagues published the first identification of a defensive chemical from any crustacean, working on Porcellio scaber, the common rough woodlouse in every British garden.

They found four compounds, three of them new to science, all quinoline derivatives, secreted from the tegumental glands. The quantity is the part worth sitting with: about forty nanograms per animal. To swallow a single gram of it, something would need to eat twenty-five million woodlice.

Then they tested it. False widow spiders ate fifteen of twenty mealworm beetles but only six of twenty woodlice. When they painted the four-compound blend onto beetles, the spiders ate one out of nineteen, against sixteen of twenty water-treated controls. Each compound also deterred the spider alone, though every one was tested at forty nanograms, which is the natural total for all four together and so between two and ten times what a woodlouse actually carries of that single compound. The two most abundant were the weakest on their own, leaving five and seven of twenty beetles eaten against one of nineteen for the full blend. The mixture is doing more than any part of it.

So it works, and it works on a spider. No vertebrate was tested, and at that dose it is hard to see how it could do anything to one. The compounds sit on the outside of the animal, and spiders simply did not eat prey carrying them. Whether they act by contact, taste or smell was not established, and nothing in the work points to a swallowed poison.

The glue matters more than the chemistry

Alongside that, the isopods anyone actually keeps have repugnatorial glands opening along the lateral plates and the uropods, at the back end. A 2025 survey looked at six species and found the uropod secretion in all of them: viscous, sticky, slightly opaque and odourless. It comes out as droplets that can be drawn into threads. It is a glue.

The trigger takes real force. Rough handling with tweezers did not make the 2025 team's animals discharge their lateral plate glands at all, and only chilling did it. Older work records uropod droplets appearing after a spider bites a leg or after an experimental pinprick, and the chemists who collected the secretion had to use a heated probe. This is an animal holding an expensive secretion back until something has actually damaged it.

Two species out of the six also released a foul smell, from the lateral plate glands instead of the uropods, and only when chilled: Armadillidium klugii and Armadillidium gestroi, which are also the two that look most like warning-coloured animals. Nobody has analysed what that smell is. The chemistry above is P. scaber only, and it does not transfer.

What a bird does about it

The other 2025 paper is the one I keep coming back to. Ďurajková, Veselý and Tuf offered isopods to eighty wild-caught great tits, each bird tested once and then released, with mealworms and a Blaptica dubia roach as edible controls. Five isopod species: Porcellio scaber, Oniscus asellus, Armadillo officinalis, Armadillidium versicolor and A. gestroi.

Three findings, and the third is the one that matters.

The roach was attacked significantly more often than every isopod, and eaten more often than every isopod. Birds could tell them apart and hesitated over them. Among the isopods, P. scaber and O. asellus, the two least conspicuous species, were eaten more often than the other three.

A. gestroi stood out on one measure. Birds that attacked it showed far more discomfort behaviour than with any other prey: wiping the bill, drinking, flushing the feathers. Those are the standard avian tells for distasteful prey, and gestroi provoked them more than anything else in the trial.

And the finding that unsettles a great deal of received wisdom: colour made no difference to how often birds attacked. The conspicuous species fared no better than the drab ones. The authors are careful about how they put it, saying they did not prove that conspicuously coloured isopods are better protected, which is not the same as proving colour is irrelevant, and eating rates did track colour to some extent. They are also clear that isopods are protected against birds, since every one was attacked and eaten less than the roach. What was weak was the disgust reaction afterwards, in every species except gestroi. Their reading is that these glands are aimed primarily at invertebrates and small insectivorous mammals.

So can my gecko eat them

Yes. There is no published record of any vertebrate being poisoned by eating an isopod, and I looked hard for one. Harm from parasites carried by wild-collected animals is a separate matter, and it is documented; that is further down.

There is also real evidence they are valuable food. An aviary study on flycatchers found breeding success depended on woodlice and millipedes being available, that each supplied roughly three times more calcium than the snail shells in a natural nestling diet, and that birds deprived of woodlice produced defective eggshells in twenty-five of fifty-three eggs. Putting woodlice back doubled to tripled breeding performance. That is a bird study and not a reptile one, and I would not stretch it further than it goes, but it is far stronger evidence than anything the trade normally cites for isopods as a calcium source.

Two caveats. Large spiny Porcellio may be harder for a small lizard to swallow, an assumption made in 1975 and untested fifty years on. And keepers report isopods grazing on animals that are shedding, torpid or unwell. Neither is documented, and both are worth knowing.

The two risks that are real

Both attach to wild-collected isopods, and neither is toxicity.

Acanthocephalans. Spiny-headed worms use isopods as an intermediate host. Centrorhynchus cystacanths have been recorded encysting in amphibians and reptiles that ate infected isopods, which is the real mechanism behind the standing advice not to feed wild woodlice to your reptiles. The related Plagiorhynchus cylindraceus does something stranger: it infects pillbugs and changes their behaviour so they sit out on pale, exposed surfaces instead of hiding, which gets them eaten by starlings, which is exactly where the parasite needs to go. A captive-bred culture several generations from the wild, never exposed to bird droppings, has no route to any of this. Our guide to mites and pests covers what does turn up in a closed culture.

Heavy metals. Woodlice are used as pollution biomonitors because they are exceptional metal accumulators, storing zinc, cadmium, lead and copper in the hepatopancreas. Hopkin's survey around the Avonmouth smelter found cadmium above background in the woodlouse hepatopancreas twenty-five kilometres to the east of the works. The important caveat is that at individual sites near disused mining areas, rubbish tips or busy roads, the metal levels in woodlice could not be predicted accurately from the leaf litter or the soil. Nobody has fed metal-loaded woodlice to a captive reptile and measured what happens, so I will not claim a proven hazard. But a clean-up crew is a breeding population your animal grazes on for years, captive-bred stock is easy to come by, and not collecting from a roadside is a cheap precaution.

Four things people repeat that are wrong

"Woodlice spray acid." I can find no published pH measurement of an isopod secretion, and nothing in the literature calls it acidic. It is described as proteinaceous, viscous and smelly.

"They defend themselves like millipedes." Millipedes are myriapods and make benzoquinones, or hydrogen cyanide in the flat-backed groups, in quantities that stain skin and sting eyes. Isopods are crustaceans, and the identified active compounds in a woodlouse come to about forty nanograms. Even allowing that the whole secretion is a droplet of fluid rather than forty nanograms of neat chemical, the two defences are not comparable.

"Armadillidium klugii mimics the black widow." Levi noticed in 1965 that a pillbug, a pill millipede and a black widow near Dubrovnik were all glossy black with red spots, and suggested mimicry. It has never been tested. The 2025 work makes it more interesting, not less, because klugii does turn out to smell foul, but the mimicry itself remains a sixty-year-old idea.

"Woodlice bite." Not people. The mandibles are built for scraping decaying matter and cannot break human skin. The animal doing the biting is the woodlouse spider, Dysdera crocata, a specialist woodlouse predator with oversized jaws, and that one does bite.

Common questions

Are isopods safe for reptiles and amphibians to eat?

Yes, on the evidence available. Nothing published records harm, and woodlice are a well-documented calcium source for insectivorous birds. Use captive-bred stock and the parasite question does not arise.

Can isopods hurt a person?

No. No venom, no sting, and nothing that can break human skin. I have found no published allergy case either, though isopods are crustaceans, so anyone with a serious shellfish allergy might reasonably wear gloves when handling cultures.

Why has my woodlouse turned bright blue?

An iridovirus, and it is fatal. It shows up most obviously on pale morphs such as the Whiteout. Isolate anything that changes colour and do not breed from it.

Does rolling into a ball protect them?

Nobody has run a direct predation trial. There is indirect evidence for it: a rolled animal gives a spider nowhere to bite, and pill woodlice stay rolled for longer when they can smell a predator. The only directly measured benefit is water conservation, at about a third less water lost while rolled in dry air.

Are brightly coloured isopods more toxic?

Not on the evidence. Tested against a real bird predator in 2025, colour made no difference to how often isopods were attacked, and the two species that actually smell foul are not the most gaudy ones in the hobby. Buy a bright morph because you like looking at it.

If you are choosing a first culture, the beginners guide is the place to start, what isopods eat covers feeding, and everything we breed is in isopods for sale.