Parsleyworms are large, strikingly colored 2-inch caterpillars. Green with yellow-dotted black bands across each segment, they emit an odor and project orange horns when startled. They feed voraciously on parsley and fennel foliage, leaving only bare stems. Before destroying a parsleyworm, be aware that it is the larvae of the black swallowtail butterfly. To preserve both it and your parsley move it to another member of the parsley family that you can spare, such as carrot, dill, parsnips or the common weeds, wild carrot or Queen Anne's lace.