I'm not sure, but they are a caterpillar, so the Bt should do the trick for you. Probably the larva of some nondescript tiny brown moth or something. :) But you inspired me to go out and check my chard and see what was eating it in the night, thanks. A month ago I would have been sure I knew what it was, but I was surpised to find some sort of tiny green and orange bug/beetle in large numbers. I don't know their name, but hopefully neem will help keep my remaining chard alive to the cooler months when I'll more want to eat it.

Something has eaten some of my chard leaves so bad that there was just the veins left.

I think it might be slugs. No sign of caterpillars.

Cara
That's what I thought too, until i went out at night to look. I couldn't see anything in the daytime. If it is slugs, you will see them at night.