--- title: "The cinnabar caterpillar: the poison, worn" slug: "cinnabar-caterpillar" canonical: "https://www.thebritographer.co.uk/journal/cinnabar-caterpillar/" date: "2026-07-30" subject: "Tyria jacobaeae" wordCount: 976 readingTime: "PT5M" author: "The Britographer" publisher: "The Britographer" license: "https://www.thebritographer.co.uk/privacy/#license" --- # The cinnabar caterpillar: the poison, worn *A field note — July 2026* The ragwort has been eaten down to a candelabra of bare stems, and the things that did it are still there, in no hurry to leave. Fat, ringed, black and gold, a dozen of them working the last of the leaves at the top where the yellow flowers were — chewing in the open, in full sun, on a plant that would put a horse in the ground. Nothing is troubling them. Nothing is going to. They are advertising, in the loudest colours the verge has, exactly how bad an idea it would be to try. This is the cinnabar — *Tyria jacobaeae* — though the name, like the fame, belongs to the adult it will become: a red-and-black moth named for the old red mineral, the vermilion of paint-boxes. You will meet the moth far less often than this. The gold-and-black caterpillar, stripping ragwort on a July bank, is the stage most people actually see and almost nobody can name. It is doing the one thing it was built to do — eat the plant that everything else has learned to leave alone. ## The poison, handed on [Ragwort](https://www.thebritographer.co.uk/journal/ragwort/) defends itself with a slow poison — the pyrrolizidine alkaloids that ruin a grazing animal's liver a quiet dose at a time. Most creatures taste the bitterness and walk away. The cinnabar does the opposite. It eats the leaves, keeps the poison down, and stores it in its own body. A bird tries one and remembers. > **The ragwort's defence does not stop at the ragwort.** ## The coat comes later What comes out of the egg is small and pale — a dull yellowish thing with no pattern on it at all, feeding in a huddle on the underside of the leaf it was laid on, indistinguishable at arm's length from any other small pale grub on any other plant. The black rings are not issued at birth. They arrive. A caterpillar grows by outgrowing itself. The skin will not stretch, so it is split and shed and left behind, and the animal that walks out of it is a size larger. Each instar larger than the last. The banding arrives with the third of them, and after that the black deepens with every skin, until the last of them — thirty millimetres of black and gold, out in the open sun. It is eating the poison from its first day. It does not wear the warning until it is big. Hidden while there is little of it to find; loud once there is enough of it to be worth a bird's attention. ## The warning that is also a hide Up close the rings could not shout louder: black, then a hard egg-yolk gold, black, gold — the pattern of something that means to be seen and remembered. But walk back a few paces and the stripes break up. Against the chewed stems and the broken light of the verge, the same loud caterpillar smears into nothing. Close to, a warning; far off, a smudge. It carries a *do-not-touch* for the predator already leaning in, and a *nothing-here* for the one still scanning from the hedge. One coat, two messages, depending on how near you stand. ## And then they eat each other There is no daintiness in how they feed. They arrive as a brood and they do not ration. A good ragwort can go from full leaf to bare stalk in a day or two — the flowers taken as they climb — and when the plant is gone they leave it: hundreds of metres of open ground for something an inch long, and most of them will not make the crossing. The ones that go nowhere eat the ones that can't. It is not cruelty; it is arithmetic: too many mouths, one finite plant, and a poison that guards them from everything except each other. ## The red you rarely meet By late summer the eating is done. The caterpillar leaves the plant, works its way into the litter and the top inch of soil, and stitches itself a rough shelter of silk there. Then it moults one final time, and what comes out from under the old skin is not a larger caterpillar but a hard brown case — the animal's own new outer wall, sealed, with the whole of next year folded inside it. Through the winter the caterpillar is taken apart in the dark and the moth is built from the pieces. Come spring it climbs out as the thing the name was always about: a moth in red and charcoal, flying by day, as poisonous as it ever was and just as unbothered about hiding it. The insect the guidebooks picture. The one you will hardly ever catch at it. We pull ragwort up — it is a listed injurious weed, and it does poison livestock — and every stem cleared is a table taken from an insect that eats almost nothing else. The cinnabar has fallen by seventy-one per cent in twenty-five years. Meanwhile, in New Zealand and on the Pacific coast of America, they have deliberately imported this same moth to fight this same plant, and are grateful for it. We pull up the one thing it lives on and ship it abroad to eat the thing we pulled up. None of which the caterpillar knows, or would mind if it did. It is on the verge right now, black and gold in the full sun, doing the only thing it does. Next ragwort you pass in high summer — the ragged yellow one nobody stops for — look at the stems. If they have been chewed down to bare, look closer at what is still there. The plant we are so keen to be rid of is the only larder it has ever had. --- Source: The Britographer — https://www.thebritographer.co.uk/journal/cinnabar-caterpillar/. Cite as: The Britographer, "The cinnabar caterpillar: the poison, worn".