What could be eating my leaves?

I would stake my left......... eyebrow that it is a caterpillar of some sort, probably green and a "cabbage moth" larvae. Look for poop, small rounded crusty cubes, black in colour, stuck to a leaf, the culprit will be one leaf above the poop. Since it is only one plant, the "resident caterpillar" theory holds water, if it were grasshoppers or locusts, all your plants would almost certainly show signs of "grazing". When found you can squash it by pinching the leaf around it like a taco, this seems to warn other moths not to lay their eggs there. If you are a "live and let live" type of person, throw it over your fence or relocate it away from your yard, the further the better, otherwise chickens/birds love eating them, thereby you will be enabling the "circle of life" in nature - eat or be eaten! Hope this helps, Organoman.