The Black Swallowtail caterpillar, Papilio polyxenes, is a common sight in gardens across North America, often referred to as the parsley worm. This creature is a subject of both fascination and concern for gardeners who wonder about its impact on their cultivated herbs. Understanding the specific dietary needs of this larva is the first step in appreciating its role in the ecosystem and coexisting with it in a garden setting. The caterpillar’s diet is highly specialized.
Key Features for Identification
Identifying the Black Swallowtail caterpillar is based on its distinct appearance, which changes as it grows. The earliest stages, or instars, are small, dark, and spiny, often featuring a white saddle-like marking that makes them resemble bird droppings for camouflage.
As the caterpillar matures into its final instars, its appearance transforms into the iconic, plump, striped form. The mature larva is bright green with transverse bands of black accented by rows of yellow or yellow-orange dots. This colorful pattern serves as a warning to predators, a signal reinforced by a unique defensive organ.
The larva possesses an orange-yellow, fleshy, forked gland called the osmeterium. When disturbed or threatened, the caterpillar quickly everts this organ, releasing a foul-smelling substance that deters many predators. The mature caterpillar can grow up to two inches long before it stops eating and prepares for its next life stage.
Primary Host Plants and Preferred Foods
The Black Swallowtail caterpillar feeds almost exclusively on plants belonging to the carrot family, Apiaceae. These plants contain chemical compounds that the caterpillars have evolved to tolerate and sequester for their own defense, which aids in predator deterrence. The female butterfly actively seeks out these specific host plants to lay her eggs.
Common garden herbs are frequently chosen, but the caterpillar also consumes root vegetables and wild relatives.
- Parsley (Petroselinum crispum)
- Dill (Anethum graveolens)
- Fennel (Foeniculum vulgare)
- Carrots (Daucus carota)
- Parsnips (Pastinaca sativa)
Wild relatives, such as Queen Anne’s lace (Daucus carota) and native alternatives like Golden Alexander (Zizia aurea), are also utilized. The caterpillar has successfully adopted non-native cultivated plants as hosts due to the shared chemical profile within the Apiaceae family.
How Diet Changes During Growth
The caterpillar undergoes five growth stages, and its feeding habits change as it develops. The tiny first instars consume small amounts of plant material, often nibbling on new growth shortly after hatching. As the larva progresses through the middle instars, its feeding volume increases steadily.
The most dramatic dietary shift occurs in the final, fifth instar stage, characterized by a voracious appetite. During this period, the caterpillar grows rapidly, consuming significant foliage to accumulate the energy needed for transformation. This heavy feeding phase typically lasts only a few weeks.
Once the caterpillar reaches its maximum size, it stops eating entirely and begins to wander away from the host plant. It prepares to find a secure spot for pupation, anchoring itself with silk and entering the prepupal stage. It forms a characteristic ‘J’ shape before becoming a chrysalis.