Bedeguar gall? I always knew this as Robin's pin cushion so I was surprised to find that it has a different name in the reference books. All three books I have that include this gall use bedeguar. As a result I went looking for my dictionary but I could find no bedeguar there so off to my old friend Wikipedia and it seems to have originally been a Persian expresion that got currupted by the French and it means 'wind-brought'. The Robin in question is not our red-breasted friend but Robin Goodfellow, the woodland sprite of English Folklore.
The gall is formed by a tiny wasp, Diplolepsis rosae, laying its eggs in a bud of a wild rose, often the dog-rose or field rose, and a subsequent chemical reaction produces the gall. Inside the gall are multiple chambers each occupied by a single larva. The larvae overwinter in the gall and hatch in the spring. The hatching wasps do not mate, they are virtually all fertilised females and male wasps of this species are, apparently, very rare. The wonder of nature!