A mossy rose gall aka Robin's pincushion or rose bedeguar gall in the field, which is the home of a wasp named Diplolepis rosea. The female lays up to 60 eggs within each leaf bud with her ovipositor, the grubs develop inside the gall and then hatch in the spring.
In the sunlight these galls look quite attractive I feel and interestingly only less than one percent of the grubs that will hatch will be males. Usually the galls appear only in plants that are under stress from drought, waterlogging or hedge cutting and so are an easy target for the wasp.
Medicinally they were thought to cure baldness, treat colic and even better induce sleep if placed under a pillow which is for me so I'll just pop out and harvest one for tonight then!