College Park Wild: Sleeping in silk sheets

By Rick Borchelt

Come a warm, still late afternoon in May, a male cecropia moth — Hyalophora cecropia, the largest native moth in North America — begins to shiver on the shaded perch where he’s been napping the day away. Shivering not with cold, but with anticipation.  

Somewhere in a nearby forest, a female cecropia is beginning to force her way out of the cocoon that has been her home since October. Even before she is fully clear of the tough brown outer silk, she begins releasing potent airborne chemicals, pheromones, to scent the air.

For up to a mile away, every male cecropia picks up the heady scent. Male cecropias have wider, more plumose antennae than females do, antennae that bear a precision array of thousands of chemical sensors, each exquisitely tuned to pick up even a few molecules of a female’s potent sex lure.  

That male on the shaded perch has been waiting for this cue for days, and the scent sets his whole body to vibrating. His anticipation building, he soon flutters off in search of her. Meantime, she pumps excess fluid from her plump abdomen into wing pads that expand to five inches or more wide before they harden in the air while she waits for her suitors. All the while, she releases more pheromones from a special gland she extrudes from her abdomen, just in case males out there missed the news.

The male follows the trail of her scent molecules, zigzagging over the landscape in what seems a random pattern to anyone watching him fly. In reality, he’s detecting slight differences in pheromone concentrations on the breeze and follows wherever the trail is smelliest, a path that may take him through dense trees, across fields and streams, over hills and highways, or through suburbia. 

All sorts of hazards await him on his nuptial journey. Bats are fond of these large flying burritos, and screech owls also find them a worthy snack. The female of at least one species of spider, the bolas spider, secretes a mimic pheromone that lures male moths close enough for her to fling a sticky ball of silk to ensnare the hapless moth. We have four species of bolas spiders lying in wait for moths in Maryland; the most common is the toadlike bolas spider, Mestophora phrynosoma.

And our male moth might also get distracted by bright lights. In addition to following pheromone trails, silk moths normally navigate their environment by moonlight and starlight.  There is truth to the old saying, “like a moth to a flame.” Moths — and many other nocturnal insects — find the illumination of street lamps, porch lights and neon signs irresistible and, once enraptured by the bright light, remain enthralled until the light is turned off, dawn arrives. 

It’s likely he is not traveling alone. Other male cecropias may also be winging toward the source of the scent, and it’s a race to see who finds the female first and locks her into a breeding embrace that shuts down her pheromone factory. They stay locked together for up to 24 hours, at which point she will knock him loose, and they go their respective ways — he to perhaps find another female and she to find a suitable plant on which to lay her eggs.  

Mating accomplished, neither are long for this world. Like other members of the giant silk moth family, cecropias have no mouth or gut — they have to live on whatever stored fats they brought into the cocoon with them the previous fall. A two-week-old cecropia is practically a moth Methuselah.  

So the female cecropia can waste no time; she has more than a hundred creamy white eggs to lay. Here in Maryland, young caterpillars typically feed on maple, willow, cherry and birch; the female may visit dozens of these trees, depositing small clumps of eggs on leaves each time. When she’s done, she dies.  

The eggs hatch in about two weeks. Life is very uncertain for these young cecropias — they’re on everyone’s menu. If they survive, they’ll molt four times, from rice-grain-sized, spiky black larvae to 2-inch-long, bright green behemoths. As they reach this final stage, they wander off to find a suitable spot to spin a cocoon. They molt for the last time inside the cocoon, leaving a hard, brown bullet-shaped pupa snuggled in silk.  

The cocoon itself is a woven marvel of three separate insulating silk layers, the outer one tough and water repellent, often wrapped in leaves. But even these tough sheaths are not proof against predators. Mice, chickadees and downy woodpeckers have all figured out there is a tasty meal inside the lumpy cocoon. Cecropias that survive the winter in their silk chambers will wiggle, squirm and muscle their way out of the cocoon, using a special escape hatch they spun into the weave. 

Populations of giant silk moths of all kinds are declining dramatically in eastern North America, mostly due to human-induced causes. The caterpillars are very susceptible to insecticides, especially mosquito sprays like Mosquito Joe. Forest spraying for pest caterpillars like spongy moth, aka gypsy moth (Lymantria dispar), takes a huge toll where it is done.  

But probably the worst culprit is a parasitic fly, Compsilura concinnata, that was introduced from Europe with the best intentions of being a biological control agent for spongy moth (also a European import). Unfortunately, the fly proved to be only mediocre as a control agent for the pest; it much prefers larger targets like giant silk moths. Most of the silk moth cocoons you find in the winter in our region have been parasitized by the fly, and the pupa inside is dead. 

Winter, with its bare branches, is the time to look for cocoons on trees and shrubs. If you do find a viable giant silk moth cocoon on a tree branch in the winter, resist the temptation to bring it indoors. If you do, it’ll hatch too early in the warmth of your home, and there will be no other moths around to create or follow their airborne scent trails.  

Have questions for Rick about the world of nature in and around the city, or suggestions for future College Park Wild columns? Drop him a note at rborchelt@gmail.com.