So our birch trees in the back yard have, for the last few summers, had kinda large greyish patches dried-out, that would cover about half of a leaf. I thought it was a watering-related thing. Today, I held one of the leaves up to the light, and discovered that the interior of the leaf is kinda hollowed out and inflated, and there's a larva wiggling around inside it.
Looks like leaf-mining sawflies based on googling it. I never even knew that leaf-mining was a thing; it's kinda crazy that it's inflating and hollowing out the internal space of a leaf. Good news is it doesn't sound like it's particularly harmful to the tree, but the bad news is that there doesn't seem to be any easy fix since the insects are protected from any pesticide by a layer of leaf.
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