I’ve spent a bit of time looking at buckthorn over the last few days. It seems that now is the right time to be looking for Dark Umber and Brown Scallop larvae, as I’ve found quite a few of them.
Dark Umber larvae appear to prefer sitting out on the tips of shoots, the dark streak at their rear matching the dark bud cases of the foodplant. Brown Scallop larvae spin leaves together, often forming pods, and hide up inside of these. They appear to be grazing the leaves on the inside of the pod, which then shrivels up. They probably make quite a few of these pods as I found plenty of vacated ones.
I’ll add some photos when I get a chance.