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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.
Garter snakes are pretty omnivorous and happy to eat what you put in front of them, as a rule. Unfortunately, not everything they will eat is good for them as a steady diet.
A garter snake on a steady diet of mice is at risk of fatty liver disease and a much shorter lifespan. Mice are a rich, fatty food that should be only an occasional treat. Goldfish is the usual diet for these animals, but even this is not ideal as goldfish contain substances that can cause vitamin deficiency if fed long-term. A much better garter snake diet consists mostly of minnows (buy them in the bait ship), earthworms and the occasional mouse or rat pup.