If you see eaten foliage, the culprit is most likely above ground such as a rabbit, deer, slugs, snails or most commonly, a caterpillar. Look on the back of the leaves and check with a flash light at night for worms or caterpillars. You do not need to uproot your cannas. You can scrape the soil away from the base of the plant to see if you find any fat larvae, slugs, snails or caterpillars hiding there during the day. I suggest you replant the canna you removed after raking the soil and looking for insects, snails or slugs. Use a slug bait if snails are common in your area.  Don't worry the leaves will grow back and a few eaten leaves will not kill your cannas. Keep growing, Marianne Binetti