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"Some bait worms (Eisenia, sold as "red wigglers") are toxic, so pieces of the larger night crawlers (Lumbricus terrestris) are best for small snakes." - The Garter Snakes: Evolution and Ecology by Rossman, Ford, and Seigal
This is a pretty definitive source, so don't go there. Stick to the nightcrawlers to feed your Thamnophis.