Connections: The Pawpaw Tree and the Zebra Swallowtail Butterfly

Asimina triloba and Eurytides marcellus

By Laura Seale

Editors: Devin Floyd, Clyde Kessler and Scott Jackson-Ricketts

A pair of survivors
The Zebra Swallowtail butterfly and the Common Pawpaw tree have been through a lot together. If you see one of these distinctive long-tailed butterflies glide past, it's likely there's a pawpaw patch nearby. The Zebra Swallowtail, Eurytides marcellus, is the only species of the genus Eurytides (the kite swallowtails) that makes its home in the temperate zones of North America. Other species from this genus live in subtropical zones. The sole source of food for the Zebra Swallowtail's caterpillars is the foliage, particularly the young leaves, of trees in the genus Asimina, the pawpaws. It happens that the Common Pawpaw, Asimina triloba, from the same family (Annonaceae) as the tropical custard apple and cherimoya, is also the only of the Asimina genus that is able to grow in this region of temperate North America, suggesting that the Zebra Swallowtails and the pawpaws have been co-evolving here since this region was very different than it is today.

A first good look
The first time I saw a Zebra Swallowtail, it was clinging with wings broadly spread, to a piece of tall grass at the edge of a cluster of pawpaw trees. Wind gusts from an approaching storm were keeping it grounded, which gave me ample time to admire and study its bold coloring. Its wings are more triangular in shape (with a span ranging from 2.5 to 4 inches) and its “tails” are longer and more slender than those of our other native swallowtails. Along with the zebra-stripes of black and greenish-white, the Zebra Swallowtail has two blue spots at the base of the upper surface of the hindwing, another bright red spot close to the body, and a red median stripe on the underside of the hindwing. The summer zebras are darker, with thicker black wing-stripes, and have longer tails than the ones that hatch early in spring.

Caterpillars' diet for self-defense
The Zebra Swallowtail female chooses young terminal leaves of the pawpaw on which to lay her eggs. The eggs are laid singly, one per leaf, because if the caterpillars encounter each other, they become cannibalistic. Those young paw paw leaves and their stem bark are especially rich in acetogenins, chemicals that make the foliage unpalatable to browsers, like deer and rabbits, and the leaf-munching caterpillars distasteful to potential predators. The Zebra Swallowtail mamas might get a little help from a pyralid moth called the Asimina webworm moth, Omphalocera munroei, that eats older leaves of the pawpaw to encourage growth of new acetogenin-rich terminal leaves.

Life as a zebra swallowtail
The Zebra Swallowtail larvae are hairless and appear somewhat humpbacked. When young, the caterpillar is dark with with many narrow transverse bands of black, yellow, and white. When older, they still have the stripey rings, but are greener, and often have one thicker black band across the top of the first abdominal segment. Like some other swallowtail caterpillars, the Zebra Swallowtail caterpillar can fend off predators by pushing out its osmeterium, which is a yellow, forked organ that emits foul-smelling odors. These odors are synthesized from essential oils, called terpenes, in the host plant. When ready, the caterpillars pupate under pawpaw leaves. Their pupae are green or brown and patterned to look like a curled leaf. After hatching, the females stay near the pawpaw patch, while the males can be seen patrolling for females, or puddling, sipping mineral rich water from moist soils. The adults nectar at a variety of flowers, including common milkweed, dogbane, redbud, and blackberry. It is notable that this butterfly only occasionally strays to elevations above 3000 ft. since it generally stays in the lower elevations where the pawpaws are.

Pawpaws' ploy for pollination
Of no interest to the Zebra Swallowtail butterfly are the pawpaw's flowers, as the butterflies do not pollinate the flowers or nectar on them. In fact, there is relatively sparse pollination of the early spring flowers, resulting in few fruits compared with the number of blooms. The flowers of Asimina triloba are reddish-brown, looking and smelling like rotting meat to attract carrion flies and beetles (and occasional Polygonia species butterflies). As pawpaws are not self-fertilizing, they require pollinators to travel from patch to patch. Local flies and beetles are not especially effective with this pollination, begging the question: “Was there once a more efficient pollinator, perhaps gone the way of the long-extinct mega-herbivores that roamed this continent in the past?”

Seeds can't move themselves
In addition to lacking ideal pollinators, pawpaws also lack an ideal method of seed dispersal. I have personally slipped (a la banana peels) down a hillside slick with fallen pawpaws, coating my seat with fermented pulp, wondering as I fell why there were so many of these sweet fruits rotting underfoot. It's possibly because our native Common Pawpaw is an anachronism, missing its original seed dispersing agents. The pawpaws may have evolved with the elephant-like mastodon and other extinct mega-fauna like the American camel and the giant ground sloth. The seeds are too large to be ingested, transported, and dropped in the dung of even our largest existing native herbivores, with the possible exception of the black bear. Deer show little interest in the leaves or the fruit, probably due to the plant’s chemical defenses. Since the extinction of mega-fauna, humans have likely been responsible for most of the redistribution of pawpaw seeds.

Patches of clones

One reason why pawpaws have continued to exist in this region, despite the spotty pollination and the lack of wide seed dispersal is their habit of cloning. The long-lived roots send up genetically identical suckers. The young trees are able to grow in the shade of the older ones, which eventually form the clonal “pawpaw patch.” Though the individual trees themselves tend to be short-lived, no one knows how old the root systems in pawpaw patches can get.
"Connections". Can you think of ways that this sketch says, "pawpaw"? The sketch includes a variety of indirect references to the pawpaw tree. (See bottom of article for some answers). Print the image above as a coloring sheet (PDF)

Native food staple

The first historical mention of pawpaws was in a 1541 report from an expedition of Spanish explorer, Hernando de Soto. His party observed Native Americans cultivating and eating pawpaws. John Lawson, an English explorer, wrote about encountering Native Americans using pawpaws in his 1709 book,
A New Voyage to Carolina: “ The Papau is not a large Tree. I think, I never saw one a Foot through; but has the broadest Leaf of any Tree in the Woods, and bears an Apple about the Bigness of a Hen's Egg, yellow, soft, and as sweet, as any thing can well be. They make rare Puddings of this Fruit.”

Small but chemically significant
Common Pawpaw trees often grow in the understory of a forest. They don't get very tall, their maximum height being about 40 feet. The bark is brown and relatively smooth, though covered with wart-like pores. The leaves are large, 5-11 inches long, and obovate in shape, which means they broaden towards the leaf tip. If you crush a leaf with your fingers, you'll find that it has an unpleasant smell (unless you like the smell of fresh asphalt). Chemical compounds from the pawpaw stem bark and ground seeds have been used medicinally for their insecticidal and anti-parasitic properties, and their annonaceous acetogenins are now being studied as a potential cancer treatment, as they seem to inhibit tumor cell growth.

A local fruit worth trying
Pawpaw fruits start out green, then ripen through yellow to a purplish-brown. They often grow in small clusters, which have a way of hiding under the large leaves. The fruits of the Pawpaw have custardy pulp, and 10-14 tough seeds that look like big brown beans. Some liken their flavor to banana or pineapple, though the ones I have eaten taste like a very mild soft mango. Around Central Virginia, the fruits ripen in mid to late August. People make them into puddings and pies. I have eaten pawpaws fresh, in ice cream, and in paw paw bread (recipe similar to banana bread). Cooking with them is more labor-intensive than using thick-skinned bananas or fruit with a convenient single pit, but since they are the closest thing to a locally grown, tropical-tasting fruit, and since I have a free source of pawpaws, they are worth the effort. Considered delectable by many, pawpaw fruits are not widely distributed in supermarkets due to the easily bruised flesh and short shelf-life of the ripe fruit. Large-scale commercial pawpaw cultivation schemes have not been successful so far, due to the difficulties with pollination. Maybe someday pawpaws will be available in grocery stores of every region, but for now they're an exclusive treat for to those lucky enough to find a pawpaw patch. Just follow the Zebra Swallowtails.



Some answers to the sketch "connections" question above:
  • The human in the sketch is a Native American child. Native Americans utilized the pawpaw tree as a food source.
  • The plant in the middle ground is Apocynum cannabinum (Indian hemp, hemp dogbane) and was a primary fiber material. It was used to make string, rope, bags, sacks, and many other things. It is not out of the question that a collection bag or basket constructed of Apocynum cannabinum fiber would have been used to collect pawpaw fruit.
  • The zebra swallowtail frequents Apocynum cannabinum for its nectar.
  • The pawpaw tree is the zebra swallowtail's only host plant. It's caterpillar can't survive without it.
  • Those are wild turkey feathers in her hair, and the wild turkey loves pawpaws fruit.
  • Her necklace might contain pawpaw seed beads. The seeds are beautiful and hard.
The sketch illustrates, in a simplifed way, the very circular nature of connections between things. A pawpaw tree can be communicated without actually showing it.