- Ecological concerns of removing wild garter snakes.
I combine this with moral reason's to not remove from the wild. Reptiles in general are facing huge population pressures from roadkill, loss of habitat, habitat alteration, pollution/pesticides, disease etc. 50 years ago, the pressures of pet collection were a minor inconvenience; but now, anything minor is kicking a species when it is down. I would like us to not wait until gartersnakes are endangered before we consider them worth saving.
If wild caught animals are the best option to establish captive bred stock, it should be done with carefull planning and forethought. They should be taken from strong populations, and young of that year. This age group is the most expendable and likely to fall to predation anyways.

- Parasites.
All wild caught animals have parasites. They will accumulate in captivity due stress and an enclosed environment. Get a fecal done and deworm as appropriate.

- Stress levels of wild garter snakes.
They acclimate to captivity really well provided they are cared for properly.

- Getting wild garter snakes to eat pre-killed prey
Generally not that difficult. Consider feeding variety of live prey as a means of environmental enrichment.

Ian