A healthy horn worm, one that will gobble up the leaves of your tomato plants, looks like this.
PHOTO: MARY APPELHOL
Many wholistic gardeners will—upon encountering a pest that's obviously in the process of dining on a "crop" plant—promptly pick the critter off and drown, "gish," or otherwise permanently dispose of it.
Most often, of course, such militant garden protection is the best policy. However, should you happen to espy a tomato horn worm that is covered with the white cocoons of the braconid wasp as opposed to a healthy and hungry specimen, your best course of action is to leave the worm alone. It will soon die and dehydrate as a result of its parasites. The wasp larvae will then mature and lay more eggs on more of the pests that threaten to rob you of your hard-earned harvest!