The cinnabar moth and ragwort

David Chapman / 29 July 2015

Wildlife expert David Chapman on the beautiful black and red cinnabar moth and the toxic ragwort its survival depends on.



Common ragwort belongs to the same family of flowers as the benign and universally loved daisy. It shares the same flower structure as the daisy since it has lots of tiny flowers packed into a tight head; each head is surrounded by bracts, which resemble petals. The compound nature of this flower head is the reason for the daisy family being called ‘composite’, or ‘Compositae’ to use its scientific title.

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Unlike the daisy, the common ragwort is a tall plant and its flower head is entirely yellow. The yellow flowers are borne in clusters, which are usually flat-topped and its leaves are deeply lobed and toothed; they are usually hairy underneath and have a rather unpleasant smell particularly when crushed.

This unattractive smell hints at the ragwort’s sinister side; common ragwort is poisonous to livestock. This doesn’t usually present a problem because most animals avoid eating it but when dried in hay horses may eat it and the alkaloid toxins in ragwort can lead to liver damage.

The law requires landowners to remove ragwort from land where it might infest a neighbour’s property but only if a complaint is made. The ragwort is, after all a native plant and has become an integral part of our ecosystem, the cinnabar moth in particular depends entirely upon it and even makes use of its poisonous properties.

The life cycle of the cinnabar moth

The cinnabar moth was named after this association, its scientific name, Tyria jacobaeae, derived partly from the scientific name for common ragwort, Senecio jacobaea. 

The cinnabar moth lays its eggs in large batches, of up to 150, on the lower leaves of ragwort and when the caterpillars emerge (June to August) they eat their way up the plant. In their early stages the caterpillars are prone to attack from many insects but as they progress they store poison from their host plant in their bodies making them unpalatable to birds and they advertise this fact with bold orange and black stripes.

In August, the mature caterpillars leave the host plant and spin a cocoon in which to hibernate in the soil, at this stage they are sometimes eaten by moles. 

They emerge the following summer (May to July) as adult moths when their boldly coloured red and black wings still advertise the poisons they contain making them safe from attack by birds, though crab spiders do take their toll.

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