I saw this sign a few times on shoots of black walnut in 2016. In this case I tore into it, revealing a larva inside. Definite head capsule so not Diptera, but with these pictures, unfortunately, I can't quite tell if it's a lep or a beetle (can't rule out sawfly either I suppose ;)

I've heard that walnut is a tough plant, chemically speaking, and it always amazes me to find insects eating such plants.

Rearing in progress.