Plant spicebush, sit back, wait for spicebush swallowtails.

I have, more or less, written this post before. Last year it was about Monarchs and the milkweed I planted for them. I believe I could write a thousand versions of this without the thrill wearing off. There is something that speaks to life’s mysteries in species that have host specific relationships, meaning they need a specific other species to live or reproduce. There is absolute poetry in finding those two species together, especially if you planted one of them yourself.

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The small, bedraggled, nibbled-upon spicebush, basically fading into the background but for the 8 caterpillars on it.

Last fall I planted two spicebushes (Lindera benzoin). I planted them mostly so that someday, far in the future, spicebush swallowtail butterflies (Papilio troilus) would come to my yard. Well, spicebush swallowtails have come. This is incredible. Spicebush swallowtails rely on spicebush to reproduce. They are capable of reproducing on a few different plant species but they overwhelmingly favor and experience the best success on spicebush and, to a lesser extent, sassafrass. The spicebush swallowtail caterpillars digest the leaves from these plants more efficiently than those from other species. If there are no spicebush nearby you are unlikely to see spicebush swallowtails. If you do have spicebush you are still unlikely to see spicebush swallowtails unless there are other spicebushes or sassafrass somewhere close enough by for the butterflies to “discover” your new bushes. This, at least, was what I figured would be true.

There are no spicebushes or sassafrass trees that I know of anywhere near my house. The ones I planted? They are about 12 inches tall. From a landscape perspective they are essentially invisible. I have almost mowed over them twice. They cannot be seen from the back porch of our house. They have about 40 individual leaves between them. And they are crawling with spicebush swallowtail caterpillars. Nine of them, to be exact.

With or without the butterflies the spicebush is an excellent native shrub of the eastern United States. Growing to 10 or so feet tall and wide, it prefers partial shade where it exhibits a wonderful open, layered growing pattern. Its numerous small yellow flowers show up in the very early spring, often before the leaves appear for a striking, though subtle display. The two in my yard I planted on a small embankment below which I have placed a swing passed on to me by my parents. While digging the small holes for these very young bushes I envisioned swinging on a brisk spring afternoon 10 years or so from now with yellow spicebush blooms and the cocoons of spicebush swallowtails behind me. Those bushes were investments for old age.

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And “eye” looms from the fold.

About a month ago the larger spicebush lost a few leaves. I thought nothing of it. We’ve hardly had rain since June and several plants in the yard have experienced defoliation. The other afternoon, however, while walking by the spicebushes I realized the leaves hadn’t really fallen off; their bare skeleton veins remained. Not only that, many of the remaining leaves were oddly pinched and folded over. I bent down, gingerly grabbed a leaf and, holding it as steady as I could, peered down the middle of its fold. I laughed out loud. Of course it was a spicebush swallowtail caterpillar. It was the first I’d ever seen in person. It was smaller than I expected but so shockingly real. I carefully lowered that leaf back into place and moved to inspect the next folded over leaf. Same looming eyes. Spicebush swallowtail caterpillar number two. And so with the third and the fourth. I counted eight caterpillars on the first plant, one on the second. The entire first plant had maybe 30 leaves! Are three or four spicebush leaves enough to sustain the development of a single larvae through several instar phases, growth to full-size caterpillar, cocoon construction, molting, cocoon exiting, and post-molting scleritization? Was the tiny spicebush bravely churning out new foliage to sustain them? How is this possible? Somebody do the energy calculations!

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The spicebush swallowtail is difficult to differentiate from the other swallowtails on the wing but the  caterpillars, with their large false eyes (the Palamedes swallowtail, Papilio palamedes, looks similar but without the yellow spot behind the false eye and anyways exists farther south along the gulf coast), presumably there to startle and dupe predators, are unmistakable. It is almost impossible to not personify them. To me they are comely, a kind of relatable larvae, the wide eyes speaking of trustworthy innocence. My wife thought they looked creepy. In reality the “face” and “eyes” are purely superficial. The real eyes, located on the pale fleshy head ahead of the colorful markings, are tiny dots best viewed with a microscope.

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The actual head is visible as the darker fleshy protrusion below the front yellow marking.

The spicebush swallowtail caterpillars hatch from eggs that the adult butterfly lays on the underside of leaves. Again, usually spicebush leaves. They eat the leaves but they also use the leaves for cover, weaving a silken-like web over the leaf that, upon drying, folds the leaf in half. The caterpillars hang out in the fold during the day. From what I read they come out to eat at night. Maybe night is really early evening because I went out at 3:00 am and only found a bunch of small caterpillars hiding in the same small leaf creases. It is strange to see juvenile insects doing anything other than eating. Some caterpillars construct their cocoons and emerge the same fall while others overwinter. Supposedly those which emerge in summer create green cocoons resembling living leaves while those that build cocoons late in fall create brown, crumpled dead leaf-looking cocoons.

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One caterpillar was far away from any protected folded leaf. Looking for a site to begin cocoon construction?

The takeaway here is a predictable one. I valued my spicebushes, most definitely. But I valued them for what they would become. They provided me no shade, produced for me no flowers, afforded me no privacy and, I was sure, attracted me no wildlife. I cared for them with the future in mind. Protectively, I should say. Almost everything in my yard I have planted because it provides a food source for something else and that includes the infant trees and shrubs. But I tend to see those three and four-foot tall trees as fragile, unrealized investments. My gut reaction upon finding them nibbled on is annoyance. Last year rabbits ate half of a fothergilla and a buttonbush just before winter. I wanted to sit them down for a lecture on how much more food their kids could get in two years if they could refrain, for the love of everything holy, from rashly chomping all the best plants to the ground now. I found that cicadas had laid eggs down the trunk of a three year-old chestnut oak. “It’s young and delicate!” I yelled at a dried carapace, “Let it grow!”

Then I found eight small caterpillars surviving on 30 spicebush leaves. Spicebush leaves that, as a result of evolutionary specialization, are the caterpillars’ far and away most digestible food for acres, maybe miles, around. There are some things that put delayed gratification and investing for the future in perspective, and provide helpful reminders that what defines value is different to different individuals and species. Not every organism on this earth has the luxury of waiting 10 years to cash in on invested resources.

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The silken webbing used to create the leaf shelter is visible in the flashlight glare. This 5th-instar spicebush swallowtail is almost to the end of the caterpillar portion of its life.

It is amazing to me that bees can distinguish between different flower species in a field. It is staggering that monarch butterflies can locate milkweed plants on the ground during a transcontinental migration. I cannot fathom how a spicebush swallowtail mother located and identified the 40-odd edible-to-her-young spicebush leaves in my backyard. What sacrifices did she make, what energy did she expend to reach them? Defoliation is a dirty word in the landscaping industry. The next time you see a defoliated plant think of the mother spicebush swallowtail who made it to my yard, who could make it to yours, and the offspring carrying on her lineage. Eat on young caterpillars.

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Useful information on Spicebush swallowtails:

  1. Butterfly Gardening and Conservation: Spicebush Swallowtail Page. Excellent pictures of different instars, cocoons, and butterflies.
  2. Wikipedia Spicebush Swallowtail page. – Actually a pretty good description of the species and its variants.
  3. Dallas Butterflies: Butterfly species association list. Not about spicebush swallowtails, but an enormous and fascinating list I came across listing butterfly species and their required host species.

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