The Enemy

The Cinnabar Moth larvae can strip a ragwort plant in no time at all. Both the larva and moth are brightly coloured to act as warning, so they are seldom eaten by predators except, strangely, some species of cuckoo. They are so voracious they even turn cannibal. As each female moth lays up to 300 eggs, plants often have a dozen on more caterpillers munching away. They've even been introduced in some countries to try to control ragwort. But they don't seem to eat the seeds.

Since 2003 there is a Ragwort Control Act, and DEFRA have strict guidelines to try to encourage land owners to seek and destroy these highly poisonous plants in an effort to reduce the number of slow and painful deaths of horses. Cattle, pigs and chickens are also susceptible, but deer, sheep and goats are more tolerant. There is no sure treatment, although milk thistle to support liver function sometimes helps.

Soapbox: it's infuriating to clear your own fields of ragwort only to find your neighbour has not done the same. Seeds can recontaminate an area from two fields away.

Late edit: should have made clear that Ragwort is the enemy, the caterpillar helps to control it, in small numbers. My husband for the last ten years has spent many many hot hours pulling up the damn stuff, only for seeds to float in and set up another crop for next year. If it gets into silage, that's a year's work ruined.