Robin's pincushion gall (Diplolepis rosae)

Noticed on Meg's walk yesterday. I've never seen or heard of this thing, but it is caused by a wasp laying its eggs in the stem of a wild rose. I'm still not clear exactly what I'm looking at; is this produced by the rose in response to infection (like other galls), or is it made by the wasps/grubs? Or something else?