As much as your garters probably love the frogs you get for them, they're probably the worst thing to be feeding them. As a rule, you're pretty much better off feeding captive bred prey items, just due to the fact that they "tend" to have lower parasite loads (if any) when compared to wild caught prey. Also frogs can carry a number of parasites that can get into your snakes and cause problems. Since it's a natural prey item, parasites that affect snakes will be naturally found in them. That's how it works in nature, the only problem is that in captivity, the stresses that snakes have and the small area they live in, make it that the parasites can get out of control and can really harm the snakes.....where in the wild, they may be able to live with them.