I was alerted to the possibility of finding these galls by a post I saw on Twitter – social media certainly is good for spreading the word about what to look for when and where.
These galls, these downward rollings of the leaf edges on rose species, are due to the larvae of the sawfly Blennocampa phyllocolpa. Presumably, they cause the leaves to roll in order to create shelters for themselves, as the leaves seem otherwise unharmed, though, when I carefully unrolled perhaps 10 leaves, I only found a larva in one of them. The adult sawfly is a tiny black creature, photos of which you can see on The Sawflies (Symphata) of Britain and Ireland website. If you have roses in your garden, you may already be aware of this sawfly’s activities on your plants.