Black swallowtail caterpillars
Published in Yard & Garden Line News, Volume 6, Number 12, August 1, 2004
As caterpillars, black swallowtails feed on many plants in the Umbelliferae including parsley, carrots, dill, fennel, celery, and Queen Anne's lace. They prefer eating the flowers or small seeds. When the caterpillars first hatch they are black with a white band around the middle of its body. As they grow, they gradually turn light green with black, yellow, and white markings and are about 1 1/2 inches long when fully grown.
Sometimes when disturbed, black swallowtail caterpillars, like other swallowtail larvae, will displayed a forked appendage on the top of their head known as an osmeterium. This fleshy Y-shaped organ emits a foul smell and is used to help protect the caterpillars from natural enemies. It may look frightening but it is harmless to people.
If you encounter black swallowtail caterpillars in your garden, tolerate them whenever possible. Their feeding in many cases is not serious. If he do find that they are causing significant damage, the easiest solution it to handpick the caterpillars and destroy them.