I had to look this one up. It’s a ‘robin’s pin cushion’ on a rosa glauca. According to the RHS book of pests and diseases, the small gall wasp named above lays its eggs in buds in mid summer and these galls are subsequently caused to develop. Behind the dense mossy growth is a hard central core which is the equivalent of a 2-up-2-down to the wasp’s grubby offspring for the winter until they pupate in the spring.

 

When you think about it its quite amazing that the invasion of the wasp eggs causes the plant to react in such a helpful way. I am assuming the mossy covering is a proliferation of leaf stems or some such as the covering ‘dries up’ in the autumn. But what is the mechanism that provokes the plant to grow such a weird bauble? And why only species roses ( for such is the case)? Are all cultivars lacking some primeval component the wasps need?