Robin's pincushion gall

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 parthenogenetic hymenopteran gall wasp. The wasp lays eggs on the stem of a wild rose, in this case a dog rose, Rosa canina.

This structure gives the appearance of a ball of moss, and its filaments are often brightly coloured, being at their best around September; starting off green and then passing through pink and crimson to reddish brown. A large specimen can be up to 10 cm in width. The larvae develop and then over winter as pupae in the now brown and dry-looking structure, emerging in May. I have never opened up a Robin's pin cushion but it sounds interesting so to do. The unilarval chambers are set in a woody core which persists after the filaments have worn off. As the larva feeds and grows within this gall, it probably undergoes five larval instar stages, the growth stages between moults. The final instar stage is reached by late October.