I didn’t even know he was a silk moth when I first met him. It was October, a smoky afternoon pierced by the song of the tree crickets. His cocoon was hanging from a goldenrod stem along the stream, carefully woven and as long as a plum. I had never seen a silk moth cocoon before, and I didn’t recognize it. Several banded garden spiders (Argiope trifasciata) had spun their webs nearby; was this a spider egg sac?
I posted images to BugGuide, a delightful website that often proves helpful for identifying mystery insects and spiders. It was not a spider egg sac, I soon learned. There’s something pretty special about finding one of these cocoons. Silk moths (Saturniidae) are the insect world’s equivalent of owls. They’re visually pleasing creatures, mysterious, active at night, and easy to love. Even people who wouldn’t ordinarily care about insects will get excited about silk moths.
Earlier in the summer, I had met my first-ever silk moth, an incredibly marked, furry, maroon and chocolate female. Perching on a fireweed stem in the mountains, she let us approach within inches. She was as large as my palm, docile and quiet. And what species was this beautiful moth? Well, she wasn’t, actually. It turns out that, for silk moths of the genus Hyalophora, western Montana is home to a population that blurs the lines between species. To the east is Hyalophora columbia; to the west, Hyalophora euryalus. And in between, this silk moth and others like her form a stable hybrid population, constantly influenced by cross-breeding with both adjacent species. Informally, this population is known as Hyalophora “kasloensis.” Formally, this moth doesn’t belong to a species; she is a blend of two.
Into the world of the silk moths
This October cocoon hanging from the goldenrod was an invitation to dive deeper into the world of the silk moths. I began reading more about them. Montana has just a handful of species, and of those even fewer spend the winter hanging from a branch in a cocoon. And while the Hyalophora moths, with their mind-boggling hybrid populations, overwinter in a pointy-ended cocoon, this gently-rounded one had to be something else. According to the books, the only possibility that fit was the Polyphemus moth, Antheraea polyphemus. Like most silk moths, the adults live for just a matter of days.
I stopped to check on the moth periodically during the winter. Heavy in his cocoon, silent and apparently unchanged, he weathered snowstorms and periods of -20°F cold.
In February, I found another silk moth cocoon, on a chokecherry stem along the same stream. This one was a stripy silvery-gray, with pointed ends: a Hyalophora moth. I thought that in the spring I would bring both cocoons into a terrarium, where I could watch them complete their development. When they emerged, I would take photos and then release the moths to live their brief, adult lives in the wild.
But when I returned in March to check on them, the Hyalophora cocoon was gone. The chokecherry stem was bare now except for a few tiny wisps of silk. I searched the leaves all around, but the cocoon was nowhere to be seen. In this hungry season, I could only assume that a magpie or a deer mouse had found it. A silk moth pupa would be a good find for a starving scavenger.
Into the terrarium
The Antheraea moth was still resting, his cocoon undiscovered where I had found him five months before. But now I was worried: what if a predator found him, too? So I gently clipped the goldenrod stem and carried him home. I put him in a terrarium with a screen top, still hanging from the goldenrod stem. I left him outside in the shade of the garage, the screen top weighted down with a piece of firewood so that predators couldn’t get in. Hot and cold, night and day: I hoped that the fluctuations of temperature and light here would be similar to those along the stream. Temperature is a critical cue that guides the emergence of adult silk moths.
There he sat for months. When the cottonwoods leafed out in May, and the sandbar willows followed, I checked him almost every morning. By now, the leaves were out on the willows, maples, and birches, our most common local food plants for Polyphemus moth caterpillars. Shouldn’t he be emerging to mate?
I began to worry that I had messed something up by moving him. But when I emptied out the spring rains as they accumulated in the bottom of the terrarium, his hanging cocoon still felt reassuringly heavy.
Now it is June 25th. This has always been a special day for me: it’s my dad’s birthday and my brother Jeff’s birthday. This year it is bittersweet, though: my dad passed away last fall.
I check the cocoon in the morning and nothing has changed. But when I go back in the afternoon, something is different. A dark, furry brown triangle is resting there!
First I notice his broad, feathery antennae, intricately patterned. His furry chocolate legs are surprisingly stout. I reach towards him with my hand and he grips my finger. At the same time, he lets out a thick, whitish fluid. This is meconium, his accumulated waste from nine months in a cocoon. Ugh! This rather indelicate behavior may be a defense against predators.
His wings are incredible. The complex shadings of charcoal and brown are set off against the eyespots, translucent windows ringed in yellow and blue. I transfer him to a soft-walled butterfly cage and give him a branch to cling to. He hangs there through the evening, motionless and upside down.
Scents on the night breeze
The sun sets. Gradually, the prolonged summer twilight is fading. I know he doesn’t have much time now: adult silk moths are short-lived. Carrying fats they have stored since their caterpillar days, they don’t feed at all. They have just one priority now: find a mate and bring forth the next generation.
This male’s antennae are so feathery for a reason. They are finely tuned instruments, far more sensitive than any human nose: this male must find a mate by smell. The night after a female emerges, she will release a pheromone plume, a mix of chemicals specific to the Polyphemus moth. Using his delicate antennae, this male will search the breeze for her perfume. He will fly miles – perhaps as many as 20 – following her scent on the air currents.
After they mate, she will lay her eggs, a few on each host plant she chooses. Willows, maples, and birches are the usual hosts. With this long-distance signaling, Polyphemus moths can stay rare and still survive. This way, a solitary cocoon is able to overwinter near the willows along a stream, alone but connected to mates by messages on the summer night breeze. These low-density populations may manage to avoid predators and parasitoids that can plague more-common moths.
Lights stabbing the night
He’s still hanging quietly from his branch in the butterfly cage, now on the dining room table. The indoor light is still on; maybe he’s waiting for the dark. I gently pick up his cage and carry him out into the night. The trees are dark shadows around us, but the night itself is not dark. A crescent of yellow lights rings the edge of Hauser Lake. And up on the hill, it looks as if a spaceship is about to take off: an otherworldly beacon of blue and white pierces the night, while music echoes over the water. What if he’s attracted to the lights? Nine months in a cocoon, surviving all of the hazards, only to die futilely, circling the lights of a fatal party? I couldn’t bear that. I can’t release him here.
Instead, I tuck the cage into my car and drive him to the Helena Regulating Reservoir. Here, we’re away from the bright lights. It’s a cool, still night, the constellation Cassiopeia looking down from overhead. The air is damp. Water gushes over the spillway into the reservoir with a soothing roar. The shadows of the cottonwoods surround us. Farther away, the mountainous horizon is a dark silhouette. The only lights are the stars and a few distant pinpricks from the ranch houses.
On furry wings
In the car he had stayed still, hanging upside down from his branch. But now as I carry the soft cage, he seems eager, fluttering his wings against the top.
I walk towards the shadows of the trees. We move away from the roaring of the water, just in case his flight is feeble at first. I unzip the cage.
He launches without saying goodbye. A flutter of soft, furry wings brushes me and then he’s gone. I spot him once more, silhouetted against the sky, a quiet shadow in the night. Or perhaps it was actually a bat, or my imagination.
I drive home again, towards the lights stabbing the night over the lake and the occasional pop of fireworks. He’s out there, somewhere. Fuzzy body filled with fat for the marathon ahead. Chocolate wings beating through the night. Feathery antennae tuned for a smell that only a Polyphemus moth can catch.
On the wings of a silk moth
Bitter freezes, snowstorms, battering winds: he’s made it through so much. He’s escaped deer mice, magpies, and downy woodpeckers. He’s avoided parasitoids and survived my novice care. And now he’s traveling through the night, trying to accomplish the single task that all of his larval feeding, all of his pupal waiting and transforming has prepared him for. With luck, he’ll mate and help bring forth the next generation. With luck, there will be Polyphemus moth cocoons this fall, too, hanging from the goldenrods near the willows, for some other person to wonder at and fall in love with.
A part of me is up there with him, hoping he makes it. Hoping he doesn’t get trapped by a light, doomed to waste his energy circling an LED, or lured to his death by a headlight. I’m hoping that there are female Polyphemus moths out here tonight, that they’re thriving, that these moths aren’t among the insects that are declining steeply in many areas.
Happy birthday, Dad. I think you would have liked to see this. If you’re awake up there tonight, maybe you can come down for a little while and ride on the wingbeats of a Polyphemus moth. Guide him toward a female and a nice stand of willows. And please keep him away from the lights.
Boyes, D.H., Evans, D.M., Fox, R., Parsons, M.S., & Pocock, M.J.O. (2020). Is light pollution driving moth population declines? A review of causal mechanisms across the life cycle. Insect Conservation and Diversity 14(2):167-187. https://resjournals.onlinelibrary.wiley.com/doi/full/10.1111/icad.12447
Tuskes, P.M., Tuttle, J.P., & Collins, M.M. (1996). The wild silk moths of North America: a natural history of the Saturniidae of the United States and Canada. Ithaca, NY: Cornell University Press.