~ Profiles of Pollinators and Backyard Wildlife ~
The zebra swallowtail (Protographium marcellus, formerly known as Eurytides marcellus) is native to the central and southern regions of the eastern U.S. It is more common in the southern parts of its range and less common the further north you go. Tennessee gives the zebra swallowtail the special honor of being its state butterfly. Like its larval host plant, the zebra swallowtail is the only species of its kind in Kentucky and surrounding states.
Once you’ve seen a zebra swallowtail, it’s pretty easy to understand how they got their common name. It’s our only black and white swallowtail and the black and white stripes play essentially the same role for the butterfly as they do for its mammalian namesake. Zebra swallowtails are smaller than the bright yellow tiger swallowtails that many of us may be more familiar with, but are still relatively large butterflies. They have a wingspan of 2.5 – 4 inches wide, or approximately half the length of a dollar bill.
Zebra swallowtails are most commonly found in or near wet or moist woods. Adults can also be found on flowers growing in open shrubby or scrubby areas, often semi-near waterways or wet / moist woods. Zebra swallowtails, especially the males, will also gather at mud puddles or on damp ground to suck minerals and nutrients out of the soil. Rotted fruit or animal dung can also serve as substitutes for mud puddles or damp ground.
Adult zebra swallowtails will drink nectar from a variety of flowers that have more open access to their nectaries. A zebra swallowtail’s tongue, or proboscis, is shorter than that of many other swallowtail species meaning that zebra swallowtails can’t reach nectar that is located at the end of a long, narrow flower tube. A few species of flowers that adult zebra swallowtails frequently use include: blueberry, blackberry, lilac, redbud, verbena, dogbane, and common milkweed.
Like monarch caterpillars, zebra swallowtail caterpillars are extremely picky eaters. Zebra swallowtail caterpillars can only eat pawpaws, and there is only one species of pawpaw – the common pawpaw (Asimina triloba) – that grows in Kentucky and most of the butterfly’s range. In some ways, zebra swallowtail caterpillars can be considered even pickier than monarch caterpillars because zebra swallowtail caterpillars have a distinct preference for young vegetation, in addition to only eating one genus of plants.
Female zebra swallowtails lay their eggs individually near the tip of a young pawpaw leaf. The little, round, bb-shaped egg starts out green, but turns tan as it ages. When the caterpillar emerges, it is almost black. After its second molt, it is a charcoal black color. After its fourth molt, and final molt as a caterpillar, it can be either green or black.
At all stages of the caterpillar’s life, it has a forked scent gland located behind its head. Most of the time, it keeps the scent gland hidden away. However, if it is threatened, the caterpillar can unfurl the scent gland which looks like two yellowish antennae and gives off an unpleasant odor. The caterpillar may also regurgitate its last meal as an additional defense mechanism. These defenses are apparently pretty effective against ants and spiders, but potentially less effective at driving away parasitoid wasps.
When they get ready to pupate or turn into a chrysalis, zebra swallowtail caterpillars attach themselves to a pawpaw stem or leaf. The chrysalis looks very similar to a folded leaf and can be either green or brown, partly depending on whether it is attached to a live leaf or a dead leaf and partly depending on the season. If the day length is growing (spring / early summer), then the chrysalis is more likely to be green. If the day length is getting shorter (late summer / fall), then the chrysalis is more likely to be brown.
In much of the zebra swallowtail’s range, there will be two generations of butterflies each year. (Florida and the far southern portions of the zebra swallowtail’s range may have three generations.) The adult butterflies of the first (spring) generation are smaller and brighter than the second (summer) generation which are larger and darker. The caterpillars produced by the second generation butterflies are the ones most likely to form the brown chrysalis. They are among our many butterfly species that will pause their development and spend the winter as a chrysalis.
Zebra swallowtails can be harder to attract to your property than many other butterfly species, unless you live near a grove of pawpaw trees. However, I think they are still a really cool butterfly and the fact that they are the only non-tropical species of kite-swallowtail (the subgroup, or tribe, of swallowtails that they belong to) only makes them more interesting to learn about. Plus, pawpaws can be found growing in many of the mature woods of the central and southern regions of the eastern U.S. So, if you’re hiking near a river or stream surrounded by bottomland woods, then this is one of the butterflies that you might encounter. If so, it means there are pawpaws somewhere in the surrounding woods, because this butterfly rarely ventures far from its larval host plant.
Shannon Trimboli is a beekeeper, farmer, wildlife biologist, and author. She owns Grassy Roads Farm and Busy Bee Nursery & Consulting. Busy Bee Nursery & Consulting specializes in plants and habitat consulting services for honey bees, native pollinators, and wildlife conservation. In 2018, her first book, Plants Honey Bees Use in the Ohio and Tennessee Valleys, was published. Shannon also writes a weekly blog called Kentucky Pollinators and Backyard Wildlife. The blog features profiles of pollinators and wildlife, tips for attracting pollinators and wildlife, highlights of different plants for pollinators and wildlife, and life on the farm and nursery. You can sign up to have her blog emailed to you.