The black swallowtail (Papilio polyxenes) completes its entire life cycle in about 30 days during warm months. Egg, caterpillar, chrysalis, adult. Four stages, one continuous process, and a transformation that still surprises people who watch it for the first time.
This is one of the most commonly encountered swallowtails in North America, partly because its caterpillars feed on plants most gardeners already grow. Dill, parsley, fennel, and carrot are all fair game. If you’ve ever found a striped caterpillar on your herb garden and wondered what it was, there’s a good chance you were looking at a black swallowtail larva in the middle of its life cycle.
Understanding each stage helps you know what to expect, when to expect it, and how long the process actually takes. This guide walks through the full black swallowtail life cycle from egg to adult, with timing for each stage and notes on the variations that catch people off guard.
Key Takeaways
- The black swallowtail life cycle spans roughly 30 days in summer, broken into four stages: egg (3-5 days), caterpillar (3-4 weeks), chrysalis (10-14 days in summer), and adult (1-2 weeks).
- Caterpillars pass through five instars, starting as tiny brown-and-white larvae that mimic bird droppings, then transforming into the familiar green, yellow, and black striped form in later instars.
- The chrysalis comes in two color forms, brown or green, and late-season chrysalises can overwinter for several months before the adult emerges in spring.
- Females lay eggs singly on the foliage of plants in the carrot family (Apiaceae), making dill, parsley, fennel, and wild Queen Anne’s lace common host plants across most of the range.
Stage 1: Eggs
A female black swallowtail lays her eggs one at a time, placing each one individually on a leaf or stem of a host plant. She doesn’t cluster them. A single plant might end up with several eggs if she returns multiple times, but each egg gets its own spot.
The eggs are small, round, and pale yellow when first laid. Over the next few days they develop a darker ring or spot as the embryo inside grows. By the time the egg is close to hatching, you can often see a dark head capsule through the shell. The egg stage lasts about three to five days in warm weather.
Host plant selection is deliberate. Females are strongly drawn to plants in the carrot family, botanically known as Apiaceae. In gardens, this means dill, parsley, fennel, and cilantro are common targets. In wild areas, Queen Anne’s lace (Daucus carota) and native plants like golden Alexanders (Zizia aurea) serve the same role. If you want to support black swallowtails, planting a patch of dill or parsley is one of the most reliable ways to do it. The broader guide to caterpillar host plants covers the best options and how to establish them for a full season of use.
Females tend to prefer actively growing foliage over older, tougher growth. They’ll inspect a plant carefully before landing to lay, running their legs over the leaf surface to detect the right chemical signals. Temperature plays a role in egg-laying activity as well. On cooler days, females are less active and may skip laying entirely.
Stage 2: Caterpillar
The caterpillar stage is the longest part of the black swallowtail life cycle, typically lasting three to four weeks. During that time, the larva passes through five distinct growth phases called instars. The changes between instars are dramatic enough that early and late caterpillars look like completely different animals.
Instars 1 and 2: The Bird-Dropping Phase
First-instar caterpillars are tiny, about 2-3 mm when they hatch, and look nothing like the colorful larva most people picture. They are dark brown or black with a white or cream saddle marking in the middle of the body. The resemblance to a bird dropping is intentional from an evolutionary standpoint. A predator scanning a leaf for something worth eating is likely to pass right over something that looks like waste.
The second instar is similar in pattern but larger. The white markings may become more prominent, and the caterpillar starts to look slightly less convincing as a bird dropping as it grows. Both instars are vulnerable and spend most of their time feeding near the leaf they hatched on.
Instars 3 Through 5: The Color Transformation
The third instar is where the real change begins. The caterpillar develops a green base color with narrow black bands crossing the body. Small yellow spots appear within those bands. The bird-dropping disguise gives way to a new strategy, one that relies on the bright colors being associated with chemical defense.
By the fourth and fifth instars, the caterpillar has its full adult coloration. The background is green, crossed by wide black bands, each containing a row of yellow-orange spots. The pattern is bold and distinctive. At this stage, a fully grown fifth-instar caterpillar can be around 45 mm long, and it is actively feeding throughout the day.
All instars share one defensive feature: the osmeterium. This is a forked, orange gland tucked behind the head that the caterpillar everts when threatened. It releases a pungent, unpleasant smell meant to deter predators. Most people who handle black swallowtail caterpillars trigger it at least once. It disappears back into the head when the threat passes.
What they eat changes slightly by instar. Early instars tend to feed on the flowers and softer parts of the host plant. Larger caterpillars will eat foliage more aggressively and can strip a small dill plant in a matter of days. A fifth-instar caterpillar has a noticeably large appetite compared to its early days. For context on how this fits into the broader picture of butterfly development, the guide to the butterfly lifecycle covers larval development across multiple species.
Stage 3: Chrysalis
When a fifth-instar caterpillar is ready to pupate, it stops eating and begins wandering. This is sometimes called the wandering stage, and it can last a day or two. The caterpillar is looking for a suitable spot to form its chrysalis, usually a stem or twig away from the host plant, often a few feet off the ground.
Once it finds the right spot, the caterpillar spins a silk pad and attaches its rear end to it. It then spins a silk girdle around its midsection, anchoring itself upright against the support. This silk harness holds the pupa in place through the entire pupal stage. Within 24 hours, the caterpillar sheds its final skin and the chrysalis is revealed underneath.
Brown vs Green Chrysalis
One of the more interesting things about the black swallowtail chrysalis is that it comes in two color forms: brown and green. The color is not random. Research suggests that the chrysalis responds to light conditions and the color or texture of the surrounding surface during formation. A caterpillar pupating against a brown twig is more likely to produce a brown chrysalis. One against green foliage is more likely to produce a green one.
Both colors provide camouflage. A brown chrysalis attached to a woody stem looks remarkably like a broken twig or a dead bud. A green chrysalis in vegetation blends into the surrounding foliage. The color is set during the first day of pupation and doesn’t change after that.
For a detailed look at the structure and biology of the pupal stage across swallowtail species, the swallowtail chrysalis guide covers what’s actually happening inside during metamorphosis.
Overwintering
Summer chrysalises produce adults in about 10 to 14 days. But caterpillars that pupate in late summer or early fall follow a different path. The chrysalis enters diapause, a kind of suspended development triggered by shorter days and cooler temperatures. In diapause, the pupa can remain viable through months of winter cold.
These overwintering chrysalises don’t emerge until the following spring, when warming temperatures and longer days signal that conditions are right. In the northern part of the range, this means the first adults of the year are emerging from chrysalises that have been attached to their sticks since the previous August or September. It’s a strategy that lets the species survive winter without any adult form needing to overwinter.
Stage 4: Adult Butterfly
When the adult is ready to emerge, the chrysalis darkens noticeably over the preceding day. The wing pattern becomes visible through the pupal case. Emergence itself takes only a few minutes. The butterfly pushes out, hangs from the empty case, and pumps fluid through its wing veins to expand them fully. Within a few hours, the wings are hardened and the butterfly is ready to fly.
Adult black swallowtails are sexually dimorphic, meaning males and females look different. Males have bright yellow spots arranged in two rows across the upper wings, with minimal blue scaling. Females have smaller yellow spots, extensive iridescent blue scaling across the hindwings, and an orange eyespot near the tail. Both have the characteristic black base color and hindwing tails that define the species.
Adults live for roughly one to two weeks in the wild, though this varies with temperature and predation pressure. During that time, they nectar on a wide variety of flowers including milkweed, clover, thistles, and phlox. Males patrol open areas and hilltops looking for females. Mating occurs, females lay eggs on host plants, and the cycle begins again.
In the warmest parts of the range, two to three generations can complete in a single season. In cooler northern regions, there may be only one or two. The black swallowtail is one of many related species with similar but distinct life histories. The guide to swallowtail species puts the black swallowtail in context alongside eastern tiger, spicebush, and other common North American species.
Full Timeline From Egg to Adult
Here’s how the timing breaks down under typical summer conditions in the middle of the range, where temperatures are warm and consistent:
| Stage | Duration | Notes |
|---|---|---|
| Egg | 3-5 days | Pale yellow, laid singly on host plant foliage |
| Instar 1-2 | 4-6 days | Dark brown with white saddle, bird-dropping camouflage |
| Instar 3-4 | 6-8 days | Transition to green and black banded pattern |
| Instar 5 | 5-7 days | Full-sized, bold yellow-spotted pattern, heavy feeding |
| Chrysalis (summer) | 10-14 days | Brown or green, attached to stem with silk girdle |
| Chrysalis (overwintering) | Up to 9 months | Diapause triggered by short days and cool temperatures |
| Adult | 7-14 days | Active nectar feeding, mating, egg-laying |
The total from egg hatch to adult emergence runs about 28 to 40 days in summer. Add the five days of the egg stage and the full cycle from laying to adult is right around 30 to 45 days. In cooler conditions, each stage slows down. A caterpillar developing during a cool, cloudy stretch in early June may take longer than one developing in the heat of July.
Temperature is the single biggest variable across all stages. Development essentially pauses below about 50°F (10°C) and accelerates significantly above 80°F (27°C). This is why the species can squeeze in multiple generations in the South while northern populations may only complete two per year.
For researchers and citizen scientists tracking butterfly phenology, the Butterflies and Moths of North America database maintains occurrence records for the black swallowtail across its range, which gives a useful picture of how timing varies by latitude and year. The North American Butterfly Association also provides range maps and identification notes that help put local observations in a larger context.
Frequently Asked Questions
How long does the black swallowtail life cycle take?
The full cycle from egg to adult takes about 30 to 45 days during warm summer months. The egg stage runs 3 to 5 days, the caterpillar stage takes 3 to 4 weeks across five instars, and the chrysalis stage lasts 10 to 14 days. Adults live roughly one to two weeks. Chrysalises produced in late summer enter diapause and overwinter, making that generation’s pupal stage last several months instead of days.
What do black swallowtail caterpillars eat?
Black swallowtail caterpillars feed exclusively on plants in the carrot family (Apiaceae). Garden herbs including dill, parsley, fennel, and cilantro are among the most common host plants in cultivated settings. In wild areas, they rely on Queen Anne’s lace, golden Alexanders, and other native members of the carrot family. They won’t feed on plants outside this family, so the presence of caterpillars on your herb garden is a sign you’re providing the right habitat.
Why does the black swallowtail caterpillar look different at each stage?
The color changes between instars reflect shifts in defensive strategy as the caterpillar grows. Small early-instar caterpillars mimic bird droppings, which makes them effectively invisible to predators scanning for food. As the caterpillar gets larger, that disguise becomes less convincing, so later instars switch to bold warning coloration. The bright green, black, and yellow pattern signals that the caterpillar may be unpleasant to eat, which deters some predators even if the caterpillar itself is not actually toxic.
What is the orange thing the caterpillar sticks out when disturbed?
That’s the osmeterium, a forked gland located just behind the head. It’s everted when the caterpillar feels threatened and releases a pungent chemical smell meant to repel predators. The smell is often described as similar to rotting citrus or a sharp, acrid odor. It retracts quickly once the threat passes and causes no harm to people handling the caterpillar. Every instar has one, from the tiny first-instar through the full-grown fifth-instar larva.
Why is the black swallowtail chrysalis sometimes brown and sometimes green?
The chrysalis color is influenced by environmental cues during the first hours of pupation. Light level, surface texture, and background color around the pupation site all appear to play a role. Caterpillars forming their chrysalis against woody brown stems tend to produce brown pupae. Those against green vegetation tend to produce green ones. Both forms provide camouflage suited to the surface they’re attached to. The color is locked in within the first day and won’t change after that point.
How many generations of black swallowtails are there per year?
The number of annual generations, called broods, varies by location. In the deep south, three broods per year are common, with butterflies on the wing from early spring through late fall. In the middle of the range, such as the mid-Atlantic and Midwest, two broods are typical. In the far north, there may be only one complete brood, with the chrysalis overwintering and producing a single flight in late spring or early summer. The length of the warm season and the availability of host plants both factor into how many generations can complete before winter.
