The Black Swallowtail caterpillar is a common sight in gardens, particularly those growing herbs like parsley and dill. These striking larvae, often called “parsley worms,” feature bright colors that can lead to public concern about their safety. Their presence on edible plants and their unusual appearance prompt many to question whether they pose a threat to humans or pets. Understanding the caterpillar’s defenses can help gardeners protect both their herbs and this species.
Are Black Swallowtail Caterpillars Toxic?
The definitive answer is no; the Black Swallowtail caterpillar is neither poisonous nor venomous to humans or common pets. It lacks the mechanism to inject venom and does not contain toxins harmful when ingested or touched. While not poisonous, they are considered largely unpalatable or distasteful to many predators. This mild unpleasantness is not a threat to human health, meaning accidental handling or ingestion by a pet is unlikely to cause serious harm. The caterpillar is medically harmless, and no special precautions are necessary when encountering one.
The Caterpillar’s Unique Defense Strategy
Instead of relying on true toxicity, the Black Swallowtail caterpillar employs a defense mechanism known as the osmeterium. This fleshy, Y-shaped gland is normally concealed just behind its head. When the larva feels threatened or is disturbed, it quickly everts this bright yellow-orange organ.
The osmeterium releases a strong, foul-smelling chemical repellent that deters small predators like ants, spiders, and mantids. The chemicals give off a pungent, disagreeable odor. For humans, the scent is often described as smelling like a concentrated version of the caterpillar’s host plant, such as parsley or dill.
This defensive smell signals that the caterpillar is unappetizing rather than deadly. The caterpillar gains this unpalatability through chemical sequestration, absorbing compounds from its host plants and storing them in its body tissue. While this makes the caterpillar distasteful to birds and other predators, the compounds are not dangerous to people.
Identifying the Black Swallowtail Larva
Identifying the Black Swallowtail larva, or “parsley worm,” confirms its harmless nature in a garden setting. The caterpillar goes through several developmental stages, but the older larvae are the most recognizable. These late-stage instars are smooth-skinned and grow to be about two inches long. They display a pattern of bright green, with transverse black bands and yellow or orange spots.
Early instars appear quite different, often being spiny and mostly black with a white saddle, mimicking bird droppings for camouflage. The Black Swallowtail exclusively feeds on plants in the carrot family (Apiaceae). Common garden favorites include parsley, dill, fennel, and carrot tops, along with wild plants like Queen Anne’s Lace. Gardeners finding a larva with the characteristic banding on these herbs can be confident they have encountered a harmless Black Swallowtail caterpillar.