One of the benefits of checking the leaves of various trees, and other plants, for leafmines is that, even when I don’t find them, I do find other things just as interesting, and this is one of those. I didn’t know what it was, of course, until I got home and checked my Field Studies Council Guide to plant galls in Britain fold-out guide. (If you’re not familiar with these, check the FSC online shop here. I have a few of these and find them really useful.)
My guide informed me that these galls on the leaves of Ash trees were created by the larvae of Dasineura fraxini, a species of tiny midge. Few people have ever seen the adult midge but I did have a look at one of the galls that was partly open and spotted one of the miniscule orange larvae. These galls can be found from May through to October, at which time the leaves will fall to the ground, where the larvae will pupate and over-winter until emerging as adult midges in the springtime.