Ragwort is the main food plant of the cinnabar moth. It has evolved to use it as a food. It is not poisoned by it. This quote comes a book called The Horse and Pony Care Bible in Association with Horse and Hound published in 2007.

Professor Derek Knottenbelt is quoted as saying: -

`I would not normally advocate the eradication of any species, but this one has nothing to offer. I don't accept that eradicating ragwort would eradicate the Cinnabar Moth, which feeds on it. Ragwort is burgeoning and the Cinnabar is declining. In fact, I believe it is being poisoned. The moth was common throughout the years that ragwort was rare and now that ragwort is widespread, Cinnabar Moths are difficult to find. If we care about the moth, we have to find out why its population is declining in the face of an ad lib supply of "feed'.

This is a crazy thing to say. There are published scientific papers that show that the alkaloids are required for the moth to be stimulated to lay its eggs on the plant (1) and also that the caterpillars require the presence of the alkaloids in order to be stimulated to feed. (2) The evidence shows quite plainly that plant is essential for the moth's survival.

It is also worth mentioning that a proper government scientific survey published in 2007 actually showed significant declines in ragwort, so the evidence shows that ragwort is not burgeoning.

Professor Knottenbelt is a well known anti-ragwort campaigner. At the time of writing he is the Scottish chairman of the British Horse Society which is responsible for the publicising of many many falsehoods about ragwort. He is also responsible for the Ragwort South Africa myth claiming the plant is a problem there and may have given people cancer there, when the experts say there is no record of it ever being recorded.

References:
1. Pyrrolizidine alkaloids as oviposition stimulants for the cinnabar moth, Tyria jacobaeae Macel M, Vrieling K. J Chem Ecol. 2003 Jun;29(6):1435-46.
2. Gustatory responsiveness to pyrrolizidine alkaloids in the Senecio specialist, Tyria jacobaeae (Lepidoptera, Arctiidae) E. A. Bernays,T. Hartmann and R. F. Chapman Physiological Entomology Volume 29, Issue 1, pages 67–72, March 2004