Wild Ideas: Here be giants

When a giant swallowtail folds its wings, it displays its bright-yellow underside, with black outlines.By Flemming Christiansen via Wikimedia
When a giant swallowtail folds its wings, it displays its bright-yellow underside, with black outlines.
Rappahannock County resident Robin Williams made a startling discovery last month in her pollinator garden: a giant was circling a tree there . . . a giant swallowtail butterfly, that is.

A huge nature fan, Robin has served on the Old Rag Master Naturalists board of directors and currently serves as vice president of the Piedmont Chapter of the Virginia Native Plant Society board. Wanting to help pollinators and encourage them to come to her and her husband’s place in Slate Mills, several years ago she planted a garden filled with native plants, including the hop tree the butterfly had been circling.

The top side of a giant swallowtail butterfly, seen here feeding on the nectar of ironweed blooms, is mostly black.By Andy Reago and Chrissy McClarren via Wikimedia
The top side of a giant swallowtail butterfly, seen here feeding on the nectar of ironweed blooms, is mostly black.
Although Robin often sees many species of butterflies, along with a lot of other pollinators, in her garden, she said she was surprised to see the giant swallowtail (Papilio cresphontes), which is rare in our area. Although Butterflies and Moths of North America (BAMONA) has maps showing a few sightings of the giant swallowtail not far from Rappahannock, no giant swallowtails have been recorded in the five years of our local butterfly count.

With a wingspan of 4 to 6 1/4 inches, the giant swallowtail is not quite the largest butterfly in North America. According to “Butterflies of the East Coast: An Observer’s Guide,” by Rick Cech and Guy Tudor, the endangered Papilio homerus, in Jamaica, is larger, as are some eastern tiger swallowtail (ETS) females, both in the same genus. Although the giant’s size is impressive, Robin says, what really caught her eye was the its bold coloring, which flashed yellow and black as it flapped its wings.

Both genders have similar markings: on the top, mostly black with two lines of yellow spots (one running horizontally across its forewings and thorax, and one running diagonally across the forewings and hindwings), and mostly bright yellow, with black outlining, underneath. See a video of a giant swallowtail feeding on minerals at YouTube.

With plants in the citrus (rue) family (Rutacaea) as its main hosts, the giant swallowtail is more often seen in the Deep South, especially the citrus-growing areas of Florida. However, some wild citrus plants that serve as hosts for the giants are also native to our area, and even further north, including the hop trees (Ptelea trifoliata) Robin planted in her pollinator garden and elsewhere on the property, Hercules club (Zanthoxylum clava-herculis), northern prickly ash (Zanthoxylum americanum) and common rue (Ruta graveolens).

Some giants colonize areas with these host plants, whether occurring naturally or planted. As Robin says, “Put it out there, and they will come.”

More gross than scary, in an early instar the giant swallowtail butterfly mimics bird lime, an effective camouflage against predators.
More gross than scary, in an early instar the giant swallowtail butterfly mimics bird lime, an effective camouflage against predators.

Unfortunately, the giant swallowtail on the hop tree in Robin’s garden came and went before she could get a photo of it, but it left something special behind. Robin had observed the butterfly circling the tree, which is about 8 feet tall, occasionally dipping its abdomen toward the leaves, she says, and she figured it was laying eggs. Days later she found five little brown-and-white blobs that looked like birdlime (bird poop) on the leaves of the tree. At first Robin thought they were the larvae of the giant, but one blob was larger than the others.

After some research, Robin discovered that, in the early instars (larva developmental stages), giant and eastern tiger swallowtail larvae look quite similar, mimicking birdlime to discourage predators. As the giant is rarely seen here, she thought the caterpillars were more likely those of the ETS. While she says she was happy to have any butterfly larvae taking advantage of the host plants in her garden, she was still “disappointed” that they might not be the larvae of the giant swallowtail she saw.

However, as the larvae moved through the next instar stages, the large one turned green and white, indicating it was an ETS, while the others retained their brown and white coloring, and so were giants. The greater size of the ETS indicated it had hatched from an egg that were laid before the giant’s.

In a later instar, the front end of the giant swallowtail caterpillar morphs into what looks like a snake’s head, also as protection against predators.
In a later instar, the front end of the giant swallowtail caterpillar morphs into what looks like a snake’s head, also as protection against predators.

The caterpillars of both swallowtails shared one feature: as they matured, their front ends changed to mimic an animal likely to scare off most predators — a snake, with markings resembling large eyes. The markings also resemble the face of a dog, so, in citrus-growing areas particularly, the larvae are also known as “orange dogs,” for their looks and for eating the leaves of orange trees. While Robin’s giants retained their fierce look through their final instar, the ETS caterpillars became bright green and less snakelike, but both retained the eyelike spots. Another weapon in the giant swallowtail caterpillar’s mimicry arsenal is its orange, retractable osmeteria, which look like horns.

Although the ETS caterpillar eventually disappeared, Robin says, all four giants remained. To protect them from predators, which could have been responsible for the disappearance of the ETS, she put mesh bags purchased from a paint store over the tree limbs the giant swallowtail caterpillars were on.

By early September, the caterpillars had stopped eating and were trying to escape the bags (with one succeeding after the tie used to close it came loose), a sign that they were ready to go onto their next life stage, as pupae. During the pupa stage, the caterpillar’s tissue breaks down and the adult butterfly forms, all happening inside a hard casing. Many butterfly species overwinter in this stage, and the giant continues its use of protective mimicry — at this stage camouflaged to look like the tree stem to which it attaches itself with strong silk threads.

Robin has decided to overwinter the giant swallowtail pupae. Giant swallowtails are known to colonize areas with their hosts, and she says she hopes adults will emerge from the pupae next spring and colonize the hop trees she had planted specifically to attract this illusive but impressive butterfly.

© 2015 Pam Owen

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