any accidental wild Tham population in Europe

[zooplan]

establishing a population in Europe. I may be horribly wrong though.

I not sure about your European toad theory i would bet there is little, possibly no, difference in American/European toad toxins i remember one of Shannons threads where she reported huge eastern,s both size and population, that were apparently living on toads which were present in almost plague proportions[/QUOTE]
I agree that European enviroment offers enough prey for Garter Snakes. Some are proven yet by captive feeding, like earthworms, slugs, Silversides, furthermore like some newts, treefrogs, grasfrogs... mice and last but not least tadpoles have no poison or other protection than flight.
Although we don´t know any current pest population, each found "escapee" could be a specimen of an unknown habitat.

Remember the few San Francisco Garter Snakes, that were caught in official population studies in San Mateo County, CA.