Black Swallowtails – Caterpillars in the Herb Garden

Swallowtail butterflies belong to the family Papilionidae. They are considered large, beautiful, and found in rural, urban, and suburban areas wherever there are adequate food resources. A common swallowtail butterfly in Nebraska is the black swallowtail, Papilio polyxenes.

Eggs and Young Caterpillars

Female black swallowtail butterflies locate host plants and deposit a single, yellow, spherical egg on the foliage. Newly emerged caterpillars consume the eggshell from which it emerges. They resemble bird droppings and rest on the upper side of the leaves. This coloration and mimicry help to escape predation. Sometimes several eggs are found on the same plant, and it is not uncommon to find caterpillars of different stages of development on a single plant.

Growing Caterpillars

The larvae of black swallowtail butterflies are known as parsley worms and considered a feeder of herb and vegetable gardens that grow dill, fennel, parsley, caraway, and carrot. These plants belong to celery, carrot, or parsley family, Apiaceae. Also referred to as umbellifers, this family of flowering plants has more than 3000 species, and the leaves emit a strong odor when crushed. Other plants in this family used as a host plant for black swallowtail caterpillars include Queen Anne’s lace, golden Alexander, poison hemlock, wild parsnip, and sweet cicely.

As black swallowtail caterpillars mature and molt, they change dramatically in size, shape, and coloration, changing from a textured black color with orange and white markings, to having a smooth appearance with green and black bands and yellow spots. There are five larval instars or molts, and unlike the monarch or queen caterpillar, black swallowtail caterpillars do not possess any long black tentacles anywhere on its body.

The Osmeterium

All swallowtail larvae have an eversible, forked horn called an osmeterium which extends from a slit behind the head when the caterpillar is threatened. The osmeterium of the black swallowtail caterpillar is bright yellow-orange, sticky, and produces a sweet, putrid odor. If the caterpillar senses danger, it arches its thorax and rubs the osmeterium on the potential predator thus escaping danger. This has proven to be an effective defensive strategy against ants, spiders, praying mantises, and even humans.

Mature caterpillars stop feeding and wander around, sometimes several feet or a property away from the host plant. Caterpillars have been seen near, far, high, and low, just walking around. Black swallowtails form their chrysalis above ground in a protected place. This can be under an overhang, the side of a wall, attached to outdoor furniture, equipment, plants, pots, or basically anywhere!

The Pupa or Chrysalis

Once ready to pupate, the caterpillar produces silk and attaches its hind end to this silk pad. It also produces a silk girdle to suspend its midsection. The caterpillar molts a final time, leaves its exoskeleton behind, and produces a chrysalis. The new chrysalis is green, but as it dries, may turn brown and become camouflaged in the environment. The unique shape and way the pupa is suspended (horizontal or head up), make swallowtail pupae easy to identify.

Life Cycle of the Black Swallowtail

In Nebraska, there are often two generations of black swallowtails per year/season. The first generation emerge as butterflies about 10-14 days after pupation in the summer and start the life cycle again. The second generation may overwinter as pupae triggered by shorter daylight hours and a drop in temperature. Somtimes caterpillars get caught in a cold snap in fall and run out of food before they can adequately mature. They may perish on the host plant or go into pupation early, producing smaller butterflies in the spring.

Adult Butterflies

Adult black swallowtails have a wingspan of 3-1/4” to 4-1/4” inches long. The upper wings are black with two rows of yellow spots along the edges and a row of powdery iridescent blue spots between the rows. The hindwings have a red spot with a black bullseye in the center. The body is black, and the abdomen has a row of yellow spots on each side. Female butterflies typically have smaller or lighter yellow spots and a more prominent blue area compared to males. From the underside, wings of males and females are identical and have the characteristic narrow lobe or tail on the

Nectar Sources

As adults, butterflies feed on a variety of flowers that other butterflies use as nectar sources, including annuals such as zinnias, cosmos, lantana, blue salvia, sweet alyssum, petunia, sunflowers, and perennials like dianthus, coneflowers, milkweed, phlox, liatris, and several others. Removing spent blooms allows the plant produces more flowers, which extends the blooming season and provides more food resources for resident and migrating butterflies.

Friend or Foe?

Black swallowtail butterflies are not endangered or at risk, but are sometimes considered pests. Caterpillars seem to appear out of nowhere as they chew their way into vegetable gardens, raised beds, even potted plants in farmers markets or garden centers. They often feed, unnoticed for weeks, until they reach about an inch long or the damage to the host plant can no longer be ignored. Mature caterpillars may completely defoliate the food source because they have a huge appetite. Sometimes several eggs are laid on a single plant that cannot adequately sustain all the caterpillars until pupation.

If you want butterflies, it is necessary to plant intentionally for caterpillars AND the adults. If you’re upset about sharing your dill or parsley, the solution is simple – Plant more! If you don’t want the caterpillars, pick them off and transplant them to weedy hostplants in a ditch. That way, they can still grow up to become a beautiful butterfly and fly away.

  • parsley for caterpillar food

All photos taken by J. Green who grows dill specifically for caterpillars.

Jody Green, PhD
Extension Urban Entomologist at Nebraska Extension
Jody Green is the extension urban entomologist at Nebraska Extension in Douglas-Sarpy Counties. She specializes in structural, household, and health-related insect pests.
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