By Laura Seale
Editors: Devin Floyd, Clyde Kessler and Scott Jackson-Ricketts
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.
Seeds can't move themselves
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.
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
A local fruit worth trying
- http://bugguide.net/node/view/3101 www.ncbi.nlm.nih.gov/pubmed/9917274
- Barlow, Connie. The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms.
- 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.