They are the real-world equivalents of nocturnal wood sprites, gossamer-winged fairies flitting in and out of the shadows and into the night. Although many people may have never seen them, a single glimpse of one of these beauties will take your breath away and not soon be forgotten. They are the imperial moths or giant silk moths, known scientifically as the Saturniidae. While numerous species of these large and showy moths exist in in North America, we can usually rely on seeing only a handful here in northern Illinois. Many a curious child has discovered giant silk moths after discovering an oval leaf-wrapped cocoon in autumn and keeping their find in a jar until the cocoon reveals its mysterious inhabitant. Just as often, if the jar is kept indoors over winter, the beautiful silk moth emerges well before the weather has warmed outdoors, dooming the moth to a short un-mated existence. Better to keep these silken treasures outdoors or in the garage, where they are exposed to normal winter and spring temperatures and will therefore hatch when warm weather arrives.
The most common giant silk moths to be found in our area are the Cecropia moth and the Polyphemus moth. These are their Latin genus names, and while I have never come across a common name for them, I feel none is needed. They are distinctive enough that Latin seems to fit. Both moths are stunning in appearance, with large eye-like spots on their wings, rich colors, furry legs and bodies, and wing spans between 4 ½ and 6 inches. Their larvae are equally impressive, growing into chubby green caterpillars often over 3 inches in length. Cecropia moth caterpillars are also festooned with colorful, spiny bumps along their back, which offer some defense from predators. While the caterpillars are prodigious eaters, adult moths do not eat or drink during their short lives. The adults typically live for a week to 12 days, living for one purpose only, and that is to find a mate. The large feathery antennae of the males are chemo-receptors that detect any hint of female pheromones on the breeze and track down the source of the ‘scent.’ The antennae of the females are scant in comparison.
Another common silk moth is the truly ethereal Luna moth, sometimes called the moon moth. These lime green beauties with 4-5 inch wingspans are graced with long tassels which hang from their hind wings. When the moths fly, these tails twist against each other in a rolling motion, adding to the enchantment of seeing one of them flying. The fourth common silk moth in our area is the Imperial moth. These mostly yellow, brown, and sometimes, magenta-colored moths more readily come to lights than the other silk moths. They are therefore more commonly seen at night.
Silk moths may appear in our area anytime between May and August, depending on the species. Both Luna moths and Polyphemus moths have two generations each year. Overwintering pupae emerge in May or early June, and the progeny from these moths mature and emerge in late July or August. Cecropia moths occur as a single generation each year; however only a small percent emerge in late spring. The majority do not emerge from cocoons until late June or early July. Imperial moths occur as a single generation which emerges anytime between June and August. The caterpillars of all silk moths feed on a wide variety of tree leaves, growing successively larger in stages called instars. The caterpillar must shed its exoskeleton (skin) as it grows. After the fifth instar, the now sausage-sized green caterpillar begins to spin a cocoon, often weaving leaves into the structure. The finished silken shelter for the pupa is a tough, dense fabric which is not easily broken into. Mice, though will chew into the cocoons to eat the pupa during winter.
Although still relatively common, giant silk moths have been declining in recent years due to habitat destruction and pesticide use. Entomologists who study them have found that they often increase in number in areas with smaller trees and open habitat. It is guessed that as trees mature and habitat becomes denser, birds become more numerous and increase predation on the moths. For this reason, new suburban areas with plantings of new young trees have potential to help boost numbers of silk moths.
Giant silk moths are sometimes discovered, by chance clinging to the side of a building during daylight hours. If found in the morning AND the wings appear shrunken, observe them quietly for a few hours but leave them be. These moths have just emerged from their cocoon and must expand and dry their new wings before flying away in the evening. A moth discovered later in the day may have begun to lay eggs. If so, placing the moth temporarily inside a large jar or box will result in eggs being laid on the surface of the container. The tiny caterpillars that emerge several days later can be raised on a steady supply of fresh leaves until ready to build a cocoon.
One of the best ways to see them is to search bushes and leafy areas in late autumn for cocoons. By patiently storing the cocoon in a garage or unheated room until springtime, you can observe the newly emerged adults up close on that magical warm day in late spring or early summer. Knowing now how short a time they have to fulfill their life’s purpose, you can then ceremoniously release them into the night, ensuring that these real-life fairies grace summer evenings-to-come.
Notes From the Valley 