Eastern Black Swallowtail Caterpillar

The Eastern black swallowtail caterpillar is the larval stage of one of the most familiar swallowtail butterflies in eastern North America, frequently found munching on garden herbs like parsley and dill. Gardeners often encounter this species directly since its host plants are common culinary and ornamental herbs from the carrot family.

As it matures through its instars, the caterpillar's appearance changes dramatically, starting as a small dark larva resembling a bird dropping and developing into the bold, banded pattern for which it is best known.

• Mature larvae are bright green with black bands encircling each segment
• Black bands are studded with rows of yellow-orange spots
• Smooth, plump, cylindrical body without spines or dense hair
• Younger instars are mostly black and white, mimicking bird droppings
• When threatened, it everts a fleshy orange forked organ called an osmeterium from just behind the head, releasing a strong odor
• Similar in general shape to other swallowtail larvae but distinguished by the yellow-spotted black banding

This species ranges across most of the eastern and central United States into southern Canada, extending westward through much of North America. Caterpillars are found wherever host plants in the carrot family grow, including cultivated gardens, meadows, prairies, and roadside patches of wild carrot or Queen Anne's lace, typically from late spring through early fall.

The caterpillar feeds exclusively on plants in the Apiaceae family, such as parsley, dill, fennel, carrot tops, and Queen Anne's lace, making it a familiar and sometimes unwelcome visitor to herb gardens. Its main defense is the osmeterium, a bright orange gland that it thrusts out and waves toward an attacker while releasing a pungent scent to deter predators. In the wider ecosystem it is preyed upon by birds, wasps, and other predatory insects, and it serves as an important pollinator in its adult butterfly stage.

Females lay small round pale yellow eggs singly on host plant leaves and flower heads. The larva passes through five instars, changing from a bird-dropping mimic to the familiar green-and-black banded form, over roughly three to four weeks. The mature caterpillar forms a mottled brown or green chrysalis attached to a stem by a silk girdle. The species overwinters as a chrysalis in northern parts of its range and can produce two to three generations per year further south.