Exactly, that's why it's a good choice. It's also harmless to humans.BT is only for lepodoptera, if I spelled that right, as in moth and butterfly larva. Earth worms are in a different family called segmented worms.
And they have to ingest the product. They pick it up while eating, and is the most effective or quickest acting on those worms that eat a lot/very fast ie: cut worms, tomato horn worms.