Spicebush Swallowtail (Papilio troilus)

In this guide

1. Description and identification
2. Life cycle: egg, caterpillar, chrysalis and adult
3. Where the spicebush swallowtail lives
4. Host plants and diet
5. Behavior: mimicry, flight and courtship
6. Similar butterflies and how to tell them apart
7. Quick facts
8. Did you know?
9. Pictures
10. Frequently asked questions

Description and identification

The spicebush swallowtail is one of the dark Papilio species of eastern North America, a member of the swallowtail family that is named for its main larval food plant. Its older name, green-clouded butterfly, describes the adult better: a large black butterfly with a soft cloud of blue-green scales spread across the middle of each hindwing. It is an edible species that survives by resembling the poisonous pipevine swallowtail.

Adult butterfly

• Wingspan: 3.0–4.0 in (75–100 mm), placing it among the larger butterflies of the eastern woods.
• Upperside: black to brownish-black. A line of ivory, cream or greenish-white spots runs along the outer margin of the forewing, and a second, larger set edges the hindwing. The middle of each hindwing carries the cloud of iridescent scales that gives the species its name, and the tails are long and spoon-tipped.
• Underside: the hindwing shows two rows of orange spots separated by a wash of blue-green, and pale green crescents along the margin. The two rows are the key separation from the pipevine swallowtail, which has only one.
• Sexes: alike in pattern but not in shade. The male's hindwing cloud is blue-green; the female's is deeper, more extensive and more uniformly blue. Females also average slightly larger.
• Body: black, with pale dots along the sides of the abdomen and black, club-tipped antennae.

How to identify a spicebush swallowtail in the field

Look for a large black swallowtail flying low along a woodland edge, a wooded roadside or a swamp margin, dropping onto flowers with its wings still beating. The pale marginal spots read as ivory or greenish-white, never the bright lemon-yellow of the eastern black swallowtail, and there is no band of yellow spots across the wing. If it settles with the wings closed, count the rows of orange spots on the hindwing underside: two rows means spicebush swallowtail, one row means the pipevine swallowtail it copies.

Life cycle: egg, caterpillar, chrysalis and adult

There are two broods in the north and three in the Deep South, with adults flying from April to October. Egg to adult takes about a month in summer, and the last generation of the year passes the winter as a chrysalis.

Eggs

The female lays her eggs singly on the underside of a young leaf, choosing small saplings and shaded shoots rather than mature trees. Each egg is rounded, smooth and greenish-white to cream, about 1 mm across, and it darkens before hatching. Incubation takes four to ten days. Because the eggs are placed one to a leaf, a single spicebush may carry several caterpillars of different ages without any of them meeting.

Caterpillar

This is the stage the species is famous for. The first instars are dark brown with a white saddle and pass convincingly for bird droppings. From the fourth instar the caterpillar turns smooth green with a swollen thorax and develops two enormous false eyespots — black, ringed in yellow-orange, each with a white highlight — plus a second, smaller pair behind them. The effect is a small green snake, and the illusion is completed by the orange osmeterium, a forked gland the larva flicks out like a tongue when it is touched. Just before pupation the whole caterpillar turns yellow or orange.

The larva is also an architect. It spins a pad of silk on a leaf; as the silk dries it contracts and folds the leaf into a tube, and the caterpillar rests inside this shelter by day and comes out to feed at night. A folded, half-eaten spicebush or sassafras leaf is the surest sign the species is present.

Chrysalis

The full-grown larva leaves the shrub and pupates on a nearby stem or twig, upright in a silk girdle. The chrysalis is green with yellow markings in the summer generation and brown, angular and bark-like in the generation that will overwinter. Summer pupae open after about ten days to two weeks; winter pupae wait from autumn until the following spring.

Adult butterfly

Adults live around two weeks. They are day-flying, feed heavily on nectar, and are most numerous in midsummer when two broods overlap along the same woodland edges.

Where the spicebush swallowtail lives

The spicebush swallowtail is an eastern North American butterfly. It occurs from southern Ontario, New England and the southern Great Lakes states south to Florida and the Gulf coast, and west to eastern Kansas, Oklahoma and central Texas, with strays appearing well north and west of that line. It is commonest in the southeastern states, where the laurel-family shrubs it depends on are most abundant.

Habitat is the sunlit boundary between woodland and open ground: the edges of deciduous forest, wooded swamps and bottomland, pine barrens, second growth, shaded stream banks, parks, gardens and country roadsides. The adults need open flowers in the sun while the caterpillars need spicebush or sassafras growing in the shade below, so the species thrives exactly where the two meet and is scarce in both dense forest interior and treeless farmland.

Host plants and diet

Caterpillar host plants. The larvae feed on trees and shrubs of the laurel family, Lauraceae. Spicebush (Lindera benzoin) and sassafras (Sassafras albidum) are the main hosts across most of the range; in the southeast the butterfly also uses redbay and swampbay (Persea), sweetbay magnolia is used occasionally, and the introduced camphor tree is accepted in the Gulf states. Older accounts that add tulip tree and prickly ash confuse this species with its neighbours: tulip tree is a host of the eastern tiger swallowtail and prickly ash of the giant swallowtail, not of Papilio troilus. Our guide to butterfly host plants explains how these plant families divide up the swallowtails.

The reliance on laurels has become a conservation issue in the southeast. Laurel wilt, a fungal disease spread by an introduced ambrosia beetle, has killed large numbers of redbay trees along the Atlantic and Gulf coasts, removing a host that both this butterfly and the palamedes swallowtail use. The spicebush swallowtail is buffered by its wide choice of other plants; the palamedes, which depends almost entirely on redbay, is not.

Adult diet. Adults take nectar from Japanese honeysuckle, jewelweed, thistles, milkweeds, joe-pye weed, azalea, sweet pepperbush, lantana and garden zinnias, and they are strong pollinators of woodland-edge flowers. Males also gather at damp sand, gravel and puddle margins to take up sodium.

Behavior: mimicry, flight and courtship

Mimicry at every stage. Few butterflies bluff as consistently as this one. The young caterpillar imitates a bird dropping, the older caterpillar imitates a snake, the chrysalis imitates a dead leaf or a piece of bark, and the adult imitates the toxic pipevine swallowtail. The adult disguise is Batesian mimicry — the butterfly itself is perfectly edible — and it works because birds that have been sickened by the model avoid anything with dark wings and a blue hindwing. The same protection is used by the dark females of the eastern tiger swallowtail and of the eastern black swallowtail.

Flight. The flight is low and relaxed, close to the ground or just above the shrub layer, with deep, flopping wingbeats. Like its model, the spicebush swallowtail keeps fluttering its wings while it drinks from a flower instead of holding them still, and this restless feeding posture is a good field mark in itself.

Courtship. Males patrol likely habitat rather than defending a perch, flying steadily along woodland edges and past stands of spicebush in search of freshly emerged females. Courtship involves a hovering flight in which the male releases scent from his hindwings; mating follows on a leaf or twig and can last well over an hour. Our guide to how butterflies mate and reproduce describes the process in detail.

Daily routine and defence. Adults bask flat-winged in the early morning, feed through the middle of the day and roost under leaves at night. The caterpillar spends the daylight hours sealed inside its folded leaf; when a bird or a wasp finally pulls the shelter open it rears up, swelling the false eyes and everting the orange osmeterium, and the display is enough to make many predators let go.

Similar butterflies and how to tell them apart

Quick facts

Did you know?

• The caterpillar's "eyes" are painted on the thorax, not on the head. The real head is small and tucked underneath, so the snake stares at a predator from a part of the body that could not see anything at all.
• The false eyespots even have a white highlight in the corner, the reflection a real eye would show — a detail that makes the illusion work at close range.
• The larva builds its own house. Silk laid on the leaf surface contracts as it dries and rolls the leaf into a tube, and the caterpillar enlarges the shelter at each moult.
• Adults flutter continuously while feeding. It is a wasteful way to drink nectar, but it copies the behaviour of the poisonous pipevine swallowtail and reinforces the disguise.
• Only the female's blue hindwing cloud is really extensive; the male keeps a greener, smaller patch, which is one of the few reliable ways to sex a swallowtail in flight.
• The species is easy to attract. A spicebush or a young sassafras in a shaded corner, plus nectar flowers in the sun, is usually enough — and because each adult lives only about two weeks, the visitors change from week to week through the summer.

In summary

The spicebush swallowtail, Papilio troilus, is a large black swallowtail of eastern North American woodland edges and swamps, recognised by ivory marginal spots, a blue-green cloud across the hindwing — blue-green in males, bluer in females — and two rows of orange spots on the hindwing underside. It lays single pale eggs on spicebush, sassafras and other laurels; its caterpillar folds a leaf into a shelter and mimics first a bird dropping and then a snake, complete with false eyespots; and it overwinters as a brown, bark-like chrysalis. The adult is harmless but borrows the reputation of the toxic pipevine swallowtail, which is why a black butterfly with a blue hindwing is so often left alone.

Spicebush Swallowtail butterfly pictures

Frequently asked questions about the spicebush swallowtail butterfly

What does a spicebush swallowtail butterfly look like?

It is a large black swallowtail with a wingspan of three to four inches, a row of ivory or greenish-white spots along the wing margins, long spoon-tipped tails, and a cloud of iridescent scales across the middle of each hindwing — blue-green in males and a deeper, more uniform blue in females. The hindwing underside carries two rows of orange spots.

How can you identify a spicebush swallowtail caterpillar?

Young larvae are dark brown with a white saddle and look like bird droppings. From the fourth stage the caterpillar is smooth green with a swollen front end and two huge false eyespots ringed in yellow-orange, each with a white highlight, so that it resembles a small snake. It usually sits inside a leaf folded into a tube.

What do spicebush swallowtail caterpillars eat?

They eat trees and shrubs in the laurel family. Spicebush and sassafras are the main foods across the range, with redbay and swampbay important in the southeast and camphor tree used along the Gulf. They do not feed on tulip tree or prickly ash, which are the host plants of the eastern tiger swallowtail and the giant swallowtail.

Why are spicebush swallowtail chrysalises different colours?

The colour matches the season the pupa must survive. Chrysalides formed in spring and summer are green and blend with living foliage, while those formed in late summer and autumn, which will spend five or six months attached to a stem, are brown and angular and look like a piece of bark or a dead leaf.

How can you tell a male from a female spicebush swallowtail?

Look at the cloud of iridescent scales in the middle of the hindwing. In males it is blue-green and relatively restricted; in females it is a deeper, more extensive and more uniform blue. Females also average slightly larger. Both sexes share the same ivory marginal spots and the same two rows of orange spots underneath.

What is the wingspan of a spicebush swallowtail?

The wingspan is about 3 to 4 inches (75 to 100 mm), which makes it one of the larger butterflies of eastern North America. Individuals from the summer brood are generally bigger than those that emerge in spring from overwintering chrysalides, and females average marginally larger than males.

Why is it called the spicebush swallowtail?

It is named for spicebush, Lindera benzoin, the shrub on which females most often lay and the caterpillars most often feed across the eastern United States. The butterfly's older name, green-clouded butterfly, refers instead to the adult, and specifically to the cloud of blue-green scales spread across the middle of each hindwing.

Where do spicebush swallowtails live?

They live in eastern North America, from southern Ontario and New England south to Florida and the Gulf coast, and west to Kansas, Oklahoma and central Texas. Typical habitat is the sunny boundary between woodland and open ground: forest edges, wooded swamps, pine barrens, shaded stream banks, roadsides, parks and gardens.

When are spicebush swallowtails most active?

Adults fly from about April to October, with two broods in the north and three in the south, and numbers peak in midsummer when broods overlap. They are strictly day-flying, basking flat-winged in the early morning, feeding through the warm middle hours and roosting under foliage at night.

Is the spicebush swallowtail poisonous?

No. The butterfly stores no toxins and is edible to birds; it survives by resembling the genuinely poisonous pipevine swallowtail, a strategy called Batesian mimicry. The caterpillar is equally harmless — its snake face is a bluff and the orange osmeterium it flicks out only produces a bad smell, so neither stage can hurt a person.