I’ve been pondering why the pupae of spicebush swallowtail butterfly are dimorphic in color, some green and others brown.
Last week I spotted a caterpillar that had the markings of a spicebush caterpillar, but the few of these I’ve seen, all in their last instars (larval stages), have been bright green, and this one was orangish brown. But it has the two sets of eyespots, an evolutionary adaptation to make it look like a snake to scare off birds, their chief predators.
I thought of the eastern tiger caterpillar I had written about pupating in my July 11 column. This species is also normally bright green in its final instar, but this one was turning brown and became a darker-brown pupa. I figured the spicebush cat was about to do the same thing. Within an hour or so, it had made it almost to the eaves of the house and was, indeed, starting to pupate, turning darker brown as it did.
In previous years, I had seen two green pupae of this species, although not as bright a green as the caterpillars are. I discovered the first Sept. 6, 2015, on the same side of the house as the one last week but lower — attached to the concrete foundation, which is gray. The other pupa was from a caterpillar I had found on a spicebush on July 12, 2016.
To try to learn more about the coloration of spicebush swallowtail pupa, I went to the website of the University of Florida entomology department. It’s one of the best sites for information on insects — not surprising, considering how many insects inhabit the state. An article there offered some information that didn’t quite clear the air for me but did confirm that spicebush swallowtail pupae can be either brown or green, depending on when a caterpillar develops.
This species has two overlapping broods (generations) in Virginia each year, from April to September, according to “Northern Virginia Butterflies and Skippers: A Field Guide.” Some of the pupae from the later brood overwinter, going into diapause (shutting down internal processes) and emerging as adults in early spring to breed.
Basically, the University of Florida entomologists researching this pupa dimorphism found that cats that develop during the end of the breeding period, when daylight is short, and are heading into diapause turn brown as they pupate. The pupae from cats that develop during long photoperiods (closer to the summer solstice) may be either green or brown. Citing other research, the article also explains that the caterpillars developing during longer photoperiods can match the color of their pupa to the substrate (such as a green leaf stem or brown branch) to which it will be attached by detecting the substrate’s color through its eyes. The brown coloration of the cats heading into diapause over winter is an evolutionary adaptation to help them remain camouflaged on bare branches until spring.
But why was the pupa I found in September on my gray foundation green? With the timing, I would have thought it would be from the second brood, meaning it would have developed under a short photoperiod and should be brown. If it came from a late-hatching first-brood cat, perhaps it wasn’t geared to the gray, human-made structure?
Either way, it was sad to think that the caterpillar would only have had to crawl another foot or so to the siding of the house. At that time, it was pumpkin colored and would have done a much better job of camouflaging the pupa. Instead, the pupa soon disappeared, likely down the throat of one of the many birds that glean insects off my house. The new pupa will likely also be out of luck, since the siding was painted since 2015 — to a minty shade of green, so not good camouflage for a brown pupa.
The other green pupa, from July 2016, did survive, but it had pupated under unusual conditions. I had found the cat that created it on a spicebush before it pupated. To watch the cat transition, I put a mesh bag over the end of the branch where it was feeding on leaves. Not being able to leave its host plant, as it normally would to avoid predators, it ended up attaching its chrysalis to a green stem on the shrub. A few weeks later, an adult emerged, which I released.
I was glad to see my first brown spicebush swallowtail cat and pupa but need to keep digging into the issue of pupal dimorphism to truly understand its complexities.
Update (Aug. 22): Two days after filing this column, the pupa on the side of the house had disappeared. It was not on the ground, so a predator probably got it.
© 2019 Pam Owen