While researching raising and keeping worms for feeding I came across some information that floored me.
Worms are not native to the Great Lakes region, nor through much of the Northern US and Canada. Glaciers pushed Native worm species South and west, and all species currently found in these northern areas are Non-native, invasive foreign species.

Night crawlers are European, Red Wigglers Eurasian. Forest ecosystems in the Great Lakes regions are being disrupted by introduced worm populations that are causing major changes in the ecology of the forest floor.

See;
Great Lakes Worm Watch :: Forest ecology and worms
bootstrap analysis: as the worms turn
Michigan Tech Media Relations Story#57 - Earthworms Endangering Rare Northern Ferns

What does this mean to us?

If I'm interpreting this information correctly, it means that garters and similar snakes in these areas would *not* have been eating worms since the area was recolonized after the last glaciation. Their primary natural diet *has* to be something else- worms are only common, even since introduction my man, in towns, areas agriculturally developed by man, and near roads where egg cases were probably brought in by vehicles.

So, here is what I'm left wondering; What have wild garters in these northern regions *actually* been eating for the last 10,000 years or so?
It seems we can't simply write it off as earthworms anymore.