Our Changing World - great white butterflies

They're large and green, move in packs and love their broccoli - and for the last year they've been the target of a search and destroy mission. They're the caterpillars of the recently-arrived great white butterfly, which has been declared biosecurity enemy number one in Nelson by the Department of Conservation. A crack team of caterpillar hunters have visited over 25,000 gardens so far, and Alison Ballance joins in the hunt with project leader Richard Toft from Entecol.