The Black Swallowtail caterpillar (Papilio polyxenes) is a colorful butterfly larva commonly found eating parsley, dill, fennel, celery, and related plants. Young caterpillars are dark and resemble bird droppings, while mature larvae become green with black bands and yellow-orange spots. They are sometimes called parsley caterpillars because of their strong association with garden herbs. Although they may consume noticeable foliage, they eventually transform into beautiful Black Swallowtail butterflies. Understanding their stages, diet, defenses, chrysalis, and care makes them easier to identify and protect.
What Does a Black Swallowtail Caterpillar Look Like?
Black Swallowtail caterpillars change dramatically as they grow, which sometimes causes gardeners to mistake different instars for separate species. Early larvae are mostly dark with a pale central patch, helping them resemble bird droppings. Later caterpillars develop the familiar green body, black transverse bands, and yellow or orange spots associated with mature parsley caterpillars.
Black Swallowtail Caterpillar Identification
A mature caterpillar typically has:
- Bright green body
- Repeating black bands
- Yellow or orange spots inside the black bands
- Smooth-looking body without venomous spines
- Small true legs near the head
- Fleshy abdominal prolegs
- Retractable orange-yellow osmeterium behind the head
- Length approaching about 2 inches when fully grown
The Eastern Black Swallowtail caterpillar is the larval form of Papilio polyxenes asterius, the widespread eastern North American subspecies.
Young Black Swallowtail Caterpillar
A very young Black Swallowtail looks quite different from the mature green caterpillar.
Early instars are dark brown or black with a conspicuous whitish patch across the middle of the body. This pattern provides camouflage by making the tiny caterpillar resemble bird droppings.
As it molts repeatedly, its appearance gradually changes until green becomes the dominant background color.
Black Swallowtail Caterpillar Stages
The larval period consists of several growth stages called instars. The caterpillar cannot simply stretch its outer skin indefinitely, so it periodically stops feeding and molts. Its appearance changes after these molts. The complete larval stage generally lasts about 10–30 days, although temperature and the host plant being eaten influence development speed.
Black Swallowtail Caterpillar Instars
The developmental sequence generally looks like this:
- First instar: Tiny, very dark caterpillar with a pale saddle-like area.
- Early instars: Dark body remains dominant as the caterpillar grows.
- Middle instars: Green coloration and contrasting bands become increasingly obvious.
- Later instars: Green body develops prominent black bands and yellow-orange spots.
- Final instar: Large green caterpillar feeds heavily before leaving the host plant to pupate.
Why Is My Caterpillar Not Moving?
A caterpillar that suddenly stops eating and remains still is not necessarily sick.
Before molting, caterpillars commonly become inactive while the new exoskeleton develops underneath the old one. Disturbing them during this period can interfere with the molt.
A mature larva may also stop eating and begin wandering when it is ready to find a location for its chrysalis.
What Do Black Swallowtail Caterpillars Eat?
Black Swallowtail caterpillars primarily specialize on plants in the carrot family, Apiaceae. This feeding preference explains why they frequently appear unexpectedly in vegetable and herb gardens. Parsley is particularly associated with the species—“parsley caterpillar Black Swallowtail” is one of the strongest related topics in the supplied keyword set.
Best Host Plants
Common foods include:
- Parsley
- Dill
- Sweet fennel
- Celery
- Caraway
- Queen Anne’s lace
- Wild parsnip
- Mock bishopweed
- Water cowbane
- Common rue
UF/IFAS also records several native Apiaceae as hosts. Common rue is unusual because it belongs to the citrus family rather than Apiaceae.
Black Swallowtail Caterpillars on Parsley and Dill
Parsley and dill are among the easiest host plants to grow in a butterfly garden.
A large final-instar caterpillar can eat surprisingly quickly, so several caterpillars may strip a small herb plant.
If you want to support the butterflies while still harvesting herbs, grow extra parsley, dill, or fennel specifically for caterpillars.
Never move them onto an unknown plant simply because it looks similar to their original host.
Are Black Swallowtail Caterpillars Poisonous?
Black Swallowtail caterpillars may look threatening because of their bold coloration and strange orange “horns,” but those horns are not stingers. They lack the venomous spines associated with dangerous caterpillars. They do, however, have an effective chemical defense called an osmeterium that is characteristic of swallowtail caterpillars.
Do Black Swallowtail Caterpillars Sting?
No. They do not have a venomous sting.
When frightened, the caterpillar may raise the front of its body and suddenly extend a bright yellow-orange forked organ from behind its head.
This structure can look like a pair of antennae or horns, but it is actually an osmeterium.
What Are the Orange Horns?
The osmeterium releases a strong-smelling chemical secretion intended to discourage predators.
UF/IFAS notes that this defense can be effective against ants. Once the threat passes, the caterpillar pulls the organ back inside its body, making the “horns” disappear.
The actual antennae are tiny structures on the caterpillar’s head and are completely different from the orange osmeterium.
Black Swallowtail Caterpillar vs Monarch
Black Swallowtail and Monarch caterpillars are sometimes confused because both have bold striped patterns. Host plant is one of the simplest ways to distinguish them. Monarchs depend on milkweed, while Black Swallowtails mainly eat members of the parsley and carrot family.
Main Differences
| Feature | Black Swallowtail | Monarch |
| Main color | Green with black bands | Black, white and yellow |
| Spots | Yellow-orange spots | No similar spot pattern |
| Long filaments | No permanent long filaments | Two pairs |
| Defensive organ | Orange osmeterium | No osmeterium |
| Main food | Parsley-family plants | Milkweed |
| Adult | Black swallowtail | Orange-and-black Monarch |
A caterpillar eating dill, parsley, or fennel is much more likely to be a Black Swallowtail.
Black Swallowtail vs Anise Swallowtail Caterpillar
The Anise Swallowtail (Papilio zelicaon) and Black Swallowtail are close relatives, and mature caterpillars can look extremely similar. Both can use fennel and other Apiaceae, particularly where their geographic ranges overlap.
How to Tell Them Apart
Caterpillar appearance alone may not always provide a reliable identification.
Geographic location, the host plant, detailed markings, and eventually the adult butterfly are more useful together. The Anise Swallowtail is primarily western, while the Eastern Black Swallowtail occurs throughout much of eastern and central North America.
Avoid identifying either species based solely on green-and-black stripes.
Black Swallowtail Caterpillar to Chrysalis
Once the caterpillar finishes growing, it leaves its feeding area and searches for a secure place to pupate. It attaches itself to a surface with silk at the rear and produces a silk girdle around the middle of its body. It then molts one final time into a chrysalis.
What Does the Chrysalis Look Like?
The chrysalis may be green or brown, helping it blend into its surroundings.
During warm weather, the pupal period generally lasts around 9–18 days. However, a late-season chrysalis may enter diapause and remain in that stage throughout winter.
Black Swallowtails therefore make a chrysalis rather than a conventional silk cocoon.
How to Care for a Black Swallowtail Caterpillar
Black Swallowtails can be observed through metamorphosis if they are provided with suitable food and a clean, ventilated environment. The most important requirement is a constant supply of fresh leaves from an appropriate host plant. Caterpillars also produce considerable waste, called frass, particularly during their final stages.
Basic Caterpillar Care
- Supply fresh parsley, dill, fennel, or another confirmed host.
- Use only pesticide-free plants.
- Replace wilted foliage regularly.
- Provide good ventilation.
- Remove frass daily.
- Avoid overcrowding.
- Do not disturb a caterpillar while molting.
- Provide branches or upright surfaces for pupation.
- Give a wandering mature caterpillar room to select a pupation site.
Never use poison hemlock or water hemlock as convenient captive food plants; UF/IFAS warns that these host plants are extremely poisonous to humans.
Are Black Swallowtail Caterpillars Bad for the Garden?
Black Swallowtail caterpillars can eat significant amounts of parsley, dill, or fennel, but they are rarely serious landscape pests. Their feeding is temporary, and the caterpillars eventually become pollinating adult butterflies that feed on flower nectar.
Should You Remove Them?
In a butterfly-friendly garden, leaving them alone is usually the best choice.
Instead of spraying the caterpillars, consider planting additional host plants. Moving them to another suitable pesticide-free plant can also protect a valued herb.
Avoid broad-spectrum insecticides around host plants because they can kill both caterpillars and beneficial insects.
Black Swallowtail Caterpillar Predators
Despite their camouflage and chemical defenses, many Black Swallowtail caterpillars never reach adulthood. Birds, predatory insects, spiders, and parasitoids can attack them. UF/IFAS records multiple tachinid fly and parasitic wasp species associated with Black Swallowtail larvae.
Caterpillar Defense Mechanisms
The caterpillar uses several strategies to survive.
Young larvae resemble bird droppings, making them less obvious to predators. Older larvae display bold patterns and can expose their foul-smelling osmeterium when threatened.
Black Swallowtail caterpillars even have an unusual sanitation behavior: they may use their mandibles to throw fecal pellets away from their resting area, potentially making it harder for enemies that use fecal odors to locate them.
FAQs
1. What does a Black Swallowtail caterpillar look like?
Mature caterpillars are green with black transverse bands and yellow-orange spots. Young caterpillars are much darker and resemble bird droppings.
2. What do Black Swallowtail caterpillars eat?
They eat parsley, dill, fennel, celery, Queen Anne’s lace, caraway, and several other plants, primarily in the carrot family.
3. Are Black Swallowtail caterpillars poisonous to touch?
They are not venomous stinging caterpillars. Their main active defense is a retractable orange osmeterium that releases a repellent odor.
4. How long does a Black Swallowtail stay a caterpillar?
The larval stage generally lasts about 10–30 days, depending on temperature and the host plant.
5. Can you tell a male from a female caterpillar?
Not reliably from ordinary external caterpillar markings. The clear male-versus-female differences become much easier to recognize after the butterfly emerges.