That must be sore!

The rose bedeguar gall, Robin's pincushion gall, or moss gall develops as a chemically induced distortion of an unopened leaf axillary or terminal buds, mostly on field rose (Rosa arvensis) or dog rose (Rosa canina) shrubs, caused by the inserted eggs of the parthenogenetic hymenopteran gall wasp (Diplolepis rosae (Linnaeus, 1758)).
According to Professor Google, that is!