Last year in May I had sawfly larva damaging my leaves.
They chew holes from the back side of the leaves, and sort of don't chew through the top tissue layer of the leaf. So it winds up looks translucent.
I heard that crushed crab shells doesn't KILL the larva, but it does feed the good things in the soil that eat (and thus kill) the chitinous organisms such as foliar nematodes and sawfly larva. That's why I put in the crab shells this year, mixed it in with the potting mix of every new hosta, and refurbished some of the older pots which had such damage last year.
I'm including a picture here of a pot of H. 'Heartache' which had a lot of silvery larval trails across the soil surface. BUT, not a single hole in any of the leaves. I'm liking that, and hope it continues. My DH is now asking for some crab shell to scratch around his tomato plants to help control nematodes. He harvested his first ripe tomato yesterday.
Later I'll post a picture from last year showing sawfly larva damage to some plants.