I have noticed that my garter snake only eats after it has smelled it's prey to make sure it is a slug, and it doesn't matter whether it's moving or not. If I were to coat, say, a caterpillar with slug slime, would my snake try eating it?
Yes, changing the scent has been accepted by most snake breeders as a method to feed stubborn snakes (reference). For example, if your snake prefers chicken, and wont accept mice, rub a chick on a pre-killed mouse and this may trick it into eating the mouse (reference). With regard to garter snakes, the diet may change based on its habitat, its age and even being kept in captivity can change its diet. This article should provide you information with feeding your garter snake if it is stubborn (reference).