On a warm afternoon in late July, walk the edge of almost any rough field, verge or railway bank and you will find a plant that the law of the land would call a weed. Common ragwort stands waist high, its flat-topped blaze of yellow flowers loud against the drying grass. Look closer and the flowers are busy: hoverflies, solitary bees, soldier beetles, the odd gatekeeper butterfly. And on the leaves, if you are lucky, a row of caterpillars in black and gold hoops, feeding shou