On Oct 16, 2008, bonitin from Gent Belgium (Zone 8a) wrote:
Its old name was Tenthredo scrophulariae, but according to Wikipedia that has been changed to Allantus scrophulariae.
Its larva looks very much like a caterpillar. They feed on plants of the Scrophulariaceae family. The larva is vegetarian but the adult wasp hunts on other small insects.