The Black Swallowtail (Papilio polyxenes) is a striking North American butterfly known for its dark wings, yellow markings, and colorful caterpillars. Often called the parsley swallowtail, its larvae commonly appear on parsley, dill, fennel, celery, and other plants in the carrot family. Young caterpillars look dramatically different from mature ones, while adults also show noticeable differences between males and females. From tiny yellow eggs to green or brown chrysalises, the Black Swallowtail has a fascinating life cycle that gardeners can easily observe.
What Does a Black Swallowtail Look Like?
Adult Black Swallowtails are large, predominantly black butterflies with tails extending from their hindwings. Yellow spots, blue scaling, and orange-red markings create distinctive patterns. Their wingspan is about 6.9–8.4 cm, or roughly 2.7–3.3 inches. Females are generally larger than males, and the markings on the upper wings provide useful clues for determining sex.
Black Swallowtail Identification
- Scientific name: Papilio polyxenes
- Family: Papilionidae
- Wingspan: about 2.7–3.3 inches
- Predominantly black wings
- Rows of yellow spots
- Blue scaling on the hindwings
- Red-orange eyespot near the inner hindwing
- Distinctive swallowtail extensions on the hindwings
The Eastern Black Swallowtail, Papilio polyxenes asterius, occurs throughout much of southern Canada and the eastern and central United States, extending west toward the Rocky Mountains and southwest into northern Mexico.
Black Swallowtail Male vs Female
Males typically have two prominent rows of bright yellow spots across their upper wings. Their blue scaling is relatively limited.
Females usually have smaller and less conspicuous yellow spots but much stronger powdery blue coloration on the hindwings. Females also tend to be somewhat larger.
When their wings are closed, male and female Black Swallowtails look much more similar.
Black Swallowtail Caterpillar Identification
Black Swallowtail caterpillars change dramatically as they grow. This can make different stages appear to be completely different species. Young caterpillars are dark and resemble bird droppings, an effective camouflage against predators. Mature larvae develop the familiar green, black, yellow, and white pattern commonly seen eating parsley and dill in home gardens.
Black Swallowtail Caterpillar Stages
The caterpillar passes through several instars as it grows.
- Early instars: Mostly black or dark brown with a noticeable white saddle.
- Middle instars: Begin developing stronger bands and lighter markings.
- Later instars: Become increasingly green.
- Mature caterpillar: Green with black transverse bands and yellow or orange-yellow spots.
Each stage ends with a molt, during which the caterpillar sheds its old skin to accommodate its growing body.
Black Swallowtail Caterpillar vs Monarch
Mature Black Swallowtail and Monarch caterpillars can both appear striped, but several features separate them.
Monarch caterpillars have smooth-looking black, white, and yellow bands and long fleshy filaments at both ends.
Black Swallowtail caterpillars are predominantly green with black bands containing yellow spots. They also feed mainly on carrot-family plants, whereas Monarch caterpillars depend on milkweed.
What Do Black Swallowtail Caterpillars Eat?
Black Swallowtail caterpillars specialize mainly on members of the carrot family, Apiaceae. This explains why gardeners regularly discover them eating culinary herbs. Although a caterpillar may remove considerable foliage from an individual parsley or dill plant, it eventually becomes a beneficial adult butterfly that visits flowers for nectar.
Black Swallowtail Host Plants
Common host plants include:
- Parsley
- Dill
- Sweet fennel
- Celery
- Caraway
- Queen Anne’s lace
- Wild parsnip
- Mock bishopweed
- Water cowbane
- Spotted water hemlock
- Common rue
Common rue is unusual because it belongs to the citrus family rather than the carrot family.
Some wild host plants, particularly poison hemlock and water hemlock, are dangerously toxic to humans and should not be planted simply to attract caterpillars.
Parsley Caterpillar
The name parsley caterpillar is frequently used for Black Swallowtail larvae because parsley is one of their best-known garden host plants. Your keyword set strongly reflects this association.
Females may lay individual eggs on parsley foliage. Once they hatch, caterpillars begin eating and can become surprisingly large within a few weeks.
Gardeners who want both parsley and butterflies can plant several extra herbs specifically for the caterpillars.
Black Swallowtail Eggs and Life Cycle
The Black Swallowtail undergoes complete metamorphosis through four stages: egg, larva, pupa, and adult. Development speed varies with temperature and food quality. Multiple generations can occur during a growing season, while the final generation in colder regions commonly spends winter as a pupa before adults emerge the following year.
Four Life Stages
- Egg: Pale yellow eggs are laid singly on host plants.
- Caterpillar: Larvae hatch, eat foliage, and pass through several instars.
- Chrysalis: The mature caterpillar pupates.
- Adult: The butterfly emerges, feeds on nectar, mates, and lays eggs.
UF/IFAS reports that eggs generally last about 4–9 days, larvae about 10–30 days, and non-overwintering pupae about 9–18 days, although temperature and host plants affect timing.
Black Swallowtail Eggs on Parsley
Eggs are pale yellow and usually deposited individually on new host-plant growth rather than in large clusters.
Parsley, dill, and fennel are particularly convenient places to search.
A newly hatched caterpillar is tiny and dark, so gardeners may notice leaf damage before recognizing the insect responsible.
Black Swallowtail Chrysalis
Instead of making a silk cocoon, a Black Swallowtail forms a chrysalis. The caterpillar attaches itself in an upright position using silk at the rear of its body and a thin silk girdle around its middle. The chrysalis can be either green or brown, providing camouflage against surrounding stems and vegetation.
Green vs Brown Chrysalis
Non-overwintering chrysalises may be green with yellowish markings or brown, partly depending on their surroundings.
Overwintering pupae are generally brown.
A summer butterfly may emerge after roughly 9–18 days, but an overwintering chrysalis can remain dormant for months before completing development in spring.
Are Black Swallowtail Caterpillars Poisonous?
Black Swallowtail caterpillars are not considered dangerous stinging caterpillars. Their conspicuous appearance is sometimes mistaken for a warning that they have venomous spines, but their defense works differently. When disturbed, a caterpillar can suddenly extend an orange-yellow, forked organ from behind its head called an osmeterium.
What Are the Orange “Horns”?
The orange horns are the osmeterium, not permanent horns or stingers.
When threatened, the caterpillar raises its front end and pushes this organ outward. It releases an unpleasant chemical odor that can discourage predators such as ants.
After the danger passes, the osmeterium retracts and becomes hidden again.
What Do Adult Black Swallowtails Eat?
Adult Black Swallowtails no longer chew parsley or other host leaves. Instead, they use a long proboscis to drink flower nectar. Gardens containing both nectar flowers and caterpillar host plants can therefore support multiple stages of the butterfly’s life cycle.
Butterfly Food
Adults use flower nectar as their main food source.
A successful butterfly garden should combine sunny nectar-producing flowers with parsley, dill, fennel, or suitable native Apiaceae for caterpillars.
Avoiding unnecessary insecticides is particularly important because products applied to host plants can kill the caterpillars gardeners are trying to attract.
Black Swallowtail vs Spicebush Swallowtail
Both butterflies are predominantly black and occur across parts of eastern North America, but their caterpillars and host plants are completely different. Black Swallowtail caterpillars commonly live on parsley, dill, and other Apiaceae, while Spicebush Swallowtail larvae primarily use spicebush and sassafras.
Key Difference
Adult female Black Swallowtails have noticeable blue scaling and yellow spots, while Spicebush Swallowtails have a different arrangement of blue and orange markings.
The easiest identification may come from the caterpillar. A mature Black Swallowtail caterpillar is green with black bands and yellow dots. A Spicebush Swallowtail caterpillar has large false eyespots that give it a snake-like appearance.
FAQs
1. What is the scientific name of the Black Swallowtail?
The scientific name is Papilio polyxenes. The widespread eastern subspecies is Papilio polyxenes asterius.
2. What do Black Swallowtail caterpillars eat?
They primarily eat carrot-family plants, including parsley, dill, fennel, celery, Queen Anne’s lace, and several native Apiaceae.
3. Are Black Swallowtail caterpillars poisonous?
They are not considered dangerous stinging caterpillars. Their main active defense is an orange-yellow osmeterium that releases repellent chemicals.
4. How long does a Black Swallowtail chrysalis last?
A summer chrysalis commonly lasts about 9–18 days, but an overwintering chrysalis may remain dormant for months.
5. How can you tell a male from a female Black Swallowtail?
Males usually have larger, brighter yellow spots. Females generally show more extensive iridescent blue coloration between the yellow spot rows on their hindwings.