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  • Writer's picturePam Owen

Awaiting a lepidopteran metamorphosis

Updated: Jan 23, 2022

Early last September, I had a surprise that led me to raising two Eastern Black Swallowtail butterfly caterpillars. This spring, I waited in anticipation for the cats, which pupated last fall, to emerge from their chrysalises as adult butterflies and start the next, and last, stage of their lives.

Butterfly life-cycle basics

While some butterfly species, such as the Monarch, migrate every year to spend the winter in warmer climes, other species overwinter in their breeding grounds. Most of those that overwinter go into the chrysalis (pupa) stage of their development, a transition between the caterpillar (larva) and adult stages. Swallowtail butterflies can have up to three generations here in the Blue Ridge, with the last one overwintering.

When caterpillars pupate, they form a hard case around them, the pupa or chrysalis and, inside, they turn into DNA soup. For overwintering cats, as the temperature drops, the transformation process goes into diapause (dormancy), resuming in the spring. These overwintering species tend be cold hardy and, with their early emergence, can start reproducing before migrating butterflies return.

Swallowtail species

Where I live, on a forested mountain bordering Shenandoah National Park, I usually only see swallowtail species whose host plants are trees and shrubs. The most numerous of these is the Eastern Tiger Swallowtail, which lays its eggs on several trees and shrubs. Next is the Spicebush Swallowtail, whose hosts are Northern Spicebush and related species. Zebra Swallowtails also show up every year, but a few, because not a lot of its host plants, in the genus Asimina (pawpaw), appear to grow on the property.

I had never seen a Giant Swallowtail in the 10 years I've lived here. Its hosts are mainly the citrus family, including Wafer Ash. I also haven't had the pleasure of seeing the Pipevine Swallowtail here, and have never found the host plants of either of these swallowtail species on the property.

I never got around to learning which plants host another swallowtail that is fairly common in our area, the Eastern Black Swallowtail. I'd never seen this species in any form here. I had photographed a caterpillar earlier that summer, in a friend’s naturalized garden, but had forgotten about it.

The Pipevine, Spicebush, Eastern Black, and a dark form of the Eastern Tiger female are all predominantly black, with somewhat similar markings, so can be hard to tell apart. Why is this? They are thought to have evolved to look like the Pipevine Swallowtail, named for its host plant, a lush, woody vine in the genus Aristolochia. Its members are commonly known as Pipevine, Dutchman's Pipe, or Birthwort. The genus has a lethal toxin, ristolochic acid. While the caterpillars ingest the toxin when they feed on the plant, they are immune to it, but predators aren’t and usually avoid the cats and the adults, which also carry the toxin.

Spicebush Swallowtail (left) and dark-morph female Eastern Tiger Swallowtail


Not remembering photographing the caterpillar in June, I was confused and surprised in September when I found the three caterpillars of the same species on my deck, feeding on a pot of parsley I had put out with some other herbs. When my friend with the native garden reminded me about photographing a cat of the Eastern Black Swallowtail reminded me I'd done that, I dug up the photo and saw the cat was also on parsley. With some quick research, I found that most of this species hosts are in the carrot family, and mostly nonnative, including not only parsley, which doesn't grow wild, but other sun-loving plants, such as Queen Anne's Lace, a common plant in open spaces.

Two of the cats were in the earliest instar (developmental stage), in which they look like bird poop—a evolutionary mimicry trick that discourages predators. Another, in a later instar, looked more like a caterpillar, with white stripes, and black stripes with orange and yellow markings on them, giving them a wave effect.

Early instar of Eastern Black Swallowtail caterpillar
Early instar
Later instar of Eastern Black Swallowtail caterpillar
Later instar
Final instar of Eastern Black Swallowtail caterpillar
Final instar

I decided to keep an eye on the cats, hoping they wouldn’t be picked off by avian predators hunting in the copse near the deck. While one of the early instars didn’t survive, the other two cats did. They ate their way through the parsley plant, so I bummed the parsley plant my friend had planted in her garden, now that the Eastern Black Swallowtail caterpillars that had been feeding on it had moved on.

The next stage

Within two weeks, one of the cats stopped eating and started to move off the parsley plant, signaling it was about to pupate. I found it on the wall of my house, near where I’d placed the parsley pot. It was hanging upside down, in the shape of a J, as caterpillars do at this point in their transition.

I found a small Y-shaped branch of a nearby tree, a shape the swallowtails prefer for attaching their chrysalis, which they do by a silk thread. Once the chrysalis forms, it changes from the green of the caterpillar to brown, looking like just another part of the branch. I put the branch in my Port-a-Bug mesh cage, along with the cat, and hung the cage from a light above my deck. By the next day, the chrysalis was formed and turned brown.

First caterpillar pupates

A few days later, the other cat also showed signs of pupating, so I put it into another small-animal transport cage that was larger and made of canvas, with a mesh top. I stood that up on its end on a small table under the other cage, so the mesh side was facing out. While these cats would experience most of the winter weather in this spot, the deep eave above and the house itself would shelter them from the worst of the it.

The long wait this spring

As spring approached this year, I anxiously watched the two chrysalises for signs of the adults emerging. We’d had a cool spring, but Tiger Swallowtails had been flying on warm(ish) days since early March. April arrived, with no change.

By May, I’d pretty much given up hope of seeing adults emerge from either chrysalis and thought about just adding them to my nature-artifact collection. Then, on May 5, just before dark, as I was doing my usual last check on the chrysalises, an adult male was clinging to the mesh of the larger critter carrier, calmly fluttering his wings. Once again, I had missed the show but was glad the butterfly had survived. He didn’t look desperate to get out, and with light fading, I decided it would be better for him to spend the night in the cage. That would also give me the chance to photograph him before I released him the following morning.

The first adult, a male, emerges from his chrysalis on May 5

The next morning was still too chilly for the butterfly to take flight, so I placed him on a pot of pansies, the only plant on the deck offering cover from predators. I had checked the weather, and temps were supposed to fall to below freezing that night and over the next few nights, so I wasn’t sure what to do. Although swallowtails are hardy and adept at finding shelter during cold snaps, I thought I’d at least keep this little guy until the next day, which was forecast to be warmer. I brought the pot with the butterfly into the house, putting it into a room I don’t heat, so the butterfly would not get too active and potentially injure himself and burn off energy he'd need when he was released.

The next morning, I put the pot and butterfly back on a deck and took some photos as the butterfly crawled around on the pansies. It was still a bit chilly, so I loosely covered the plant with a mesh dryer bag to protect the butterfly from predators and from too much sun before it was ready to take off. I checked on it a few times during the day. Finally, when I took the mesh bag off the pot, the butterfly had disappeared. I hoped it had made it to the shelter of the copse nearby and would survive the coming freeze. The other pupa was still showing no sign of life.

The waiting ends

Eleven days after the emergence of the first adult, spring seemed to have finally arrived, with a little breeze and just enough humidity to produce that gentle Virginia spring day that I'd yearned for all winter. After checking on the remaining chrysalis once more, I decided to take advantage of the fine day and go birding for a couple of hours, trying to document some migratory birds that were finally arriving.

When I returned home, I went out on the deck to review and file my list to eBird, taking a cup of coffee with me. As I was sitting down, out of the corner of my eye, I caught some movement in the remaining butterfly cage. Going over to it, I could see the swallowtail—another male—had finally emerged from his chrysalis. I put him on the pot of pansies, wistfully thinking that my record of missing this event was still unbroken.

The second adult, also a male, emerges from his chyrsalis on May 16

I took some photos and sat down to drink the rest of my coffee, when the butterfly clumsily took off, veering out of sight below the deck. I went down to the yard to try to find it, with no luck. I hoped it, too, had made it to safety. (For more photos, see slideshow below.)

Thoughts of an empty nester

It's so sad when kids grow up and leave home, or so parents tell me. While I used to pay little attention to black swallowtails flying around my yard, assuming they were just Eastern Tiger or Spicebush swallowtails, I now look more carefully. The smaller size of the Eastern Black Swallowtail should make it easier to spot, but the swallowtails are really active right now, looking for mates and places to lay their eggs, or for nectar on the few flowers that are blooming this early in spring. I could have missed one of my guys passing by.

I thought I might get a better chance to spot one of my guys once the Meadow Phlox in one of my gardens starts to bloom, which will be soon. The Eastern Tiger and Spicebush swallowtails show up by the dozens to nectar on the blossoms, and perhaps Eastern Black Swallowtails will join them this year.

Adult swallowtails have only one purpose in the two months before they perish—find a mate and reproduce. I had overwintered the parsley plants that had hosted the caterpillars and put them back outside. I keep checking them, hoping to find what could be the offspring of my beautiful Eastern Black Swallowtails.

© 2020 Pam Owen



Adventures in Nature

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