In message , mogga
writes
Whatever this is something is scoffing it as fast as it can grow. I
have seen a couple of caterpillas in the area but none on it yet.
(There was a very strange red streaked one that looked just the same
colour as the fence paint OH was using last week...)

Cinnabar moth caterpillar? (Normally eats ragwort.)
--
Stewart Robert Hinsley
http://florulaprima.blogspot.com