Parsley Worm

'Parsley worm' is the familiar garden name for the caterpillar of the eastern black swallowtail butterfly, Papilio polyxenes, one of the most widespread and recognizable swallowtails in North America. The caterpillar earns its common name from its fondness for parsley and other plants in the carrot family, which it shares with garden vegetables like dill, fennel, and carrot tops.

Because it feeds on cultivated herbs and vegetables as readily as on wild relatives such as Queen Anne's lace, the parsley worm is a frequent and often welcome visitor to backyard gardens, where many gardeners deliberately plant extra host plants to support it.

When disturbed, the caterpillar displays one of the most distinctive defenses among North American butterfly larvae: an orange, forked, fleshy organ called an osmeterium that everts from just behind the head and releases a strong odor, a display that mimics the appearance of a small snake's tongue and can startle potential predators.

• Body is bright green with black bands encircling each segment, each black band dotted with small yellow-orange spots
• Reaches about 5 cm (2 in) in length at maturity
• Younger instars look quite different, resembling a bird dropping with mottled black and white coloring
• When threatened, everts a bright orange, Y-shaped osmeterium from behind the head that releases a noticeable odor
• Found on carrot-family (Apiaceae) plants including parsley, dill, fennel, carrot foliage, and Queen Anne's lace

Widespread across most of North America south of Canada's boreal zone, wherever host plants in the carrot family grow, including vegetable gardens, meadows, roadsides, and open fields. Caterpillars are present from spring through fall depending on latitude.

The caterpillar feeds exclusively on plants in the carrot family, chewing leaves and sometimes flower parts of its host. Its bold coloration functions as a warning display, while the eversible osmeterium provides an additional startle defense against ants, spiders, and other small predators. As a specialist herbivore, it plays a modest ecological role as a link between carrot-family plants and the wider community of parasitoids, predators, and birds that feed on caterpillars, while adults contribute to pollination as they visit flowers for nectar.

Females lay single, round, pale eggs on the leaves or stems of host plants. Larvae hatch and pass through five instars over roughly three to four weeks, starting out mottled black and white before transitioning to the bold green-and-black banded pattern of later instars. Mature caterpillars leave the host plant to form a mottled brown or green chrysalis attached to a nearby stem or structure by a silk girdle. Depending on latitude, two to three generations may occur per year, with the final generation of the season overwintering as a chrysalis.