Now is a perfect time to look for cocoons, with leaves off deciduous shrubs and trees. Giant silk moth cocoons are especially evident, due to their large size (often 2″-4″). The Promethea Moth caterpillar (Callosamia promethea), one of several giant silk moths in the Northeast, hatches in the summer and reaches its full size by fall. It then chooses a leaf and reinforces the leaf’s stem, or petiole, with silk so as to make it less likely that the leaf will detach from the branch it is growing on. The caterpillar then spins its silken cocoon inside the curled leaf, and spends the winter pupating inside the cocoon. Look for their well-camouflaged cocoons on low-hanging branches.
Come May or June, the moth will emerge, and if it’s a female, will produce pheromones that may attract males from as far as several miles away.
At this time of year, most deciduous trees have lost their leaves. Bare branches make looking for cocoons and finding them much easier. A single leaf dangling from an otherwise barren branch of a tree might very well turn out to be the winter domicile of a Promethea Moth (Callosamia promethea) pupa.
These giant silk moths construct their protective two-inch-long silken cocoons while still in their larval/caterpillar stage. First silk is spun around the stem of the leaf (petiole) as well as where the petiole attaches to the branch, in order to reinforce the attachment of the leaf to the tree. The cocoon is then spun, with the leaf serving as its outer covering. The result is a perfectly disguised shelter that looks like a dead leaf hanging from a branch.
The caterpillar pupates after completing the cocoon. After spending the winter in the pupal stage, the adult moth will emerge in early summer through a valve-like opening at the upper end of the cocoon.
Although a lack of snow makes tracks difficult to find, there are other, more permanent, animal signs such as bird nests and cocoons that are visible this time of year. Among the more obvious is the cocoon of the Promethea Moth – a giant silk moth. When the time for pupating arrives the Promethea caterpillar selects a leaf and strengthens its attachment to the tree by spinning silk around the petiole of the leaf as well as the branch it grows on (to assure that it doesn’t fall off the tree). With more silk it rolls the leaf up into a tube and then proceeds to spin its cocoon inside the rolled-up leaf, leaving a valve-like structure at the top of the cocoon through which the adult moth exits in the spring.
Unfortunately for silk moths, many are parasitized by flies and wasps (there are nearly 100 natural parasites that affect the 24 species of silk moths east of the Mississippi River). Frequently flies or wasps lay their eggs in silk moth caterpillars and then develop inside them. Eventually the fly or wasp larva secretes a substance that causes the caterpillar to pupate, at which time the fly or wasp also pupates and then exits the moth pupa and cocoon (see exit hole in smaller photo), causing the death of the moth pupa. Silk moth populations are decreasing, in part as a result of these parasitoids. Among others, a non-native parasitic tachinid fly, Compsilura concinnata, is wreaking havoc on silk moths.
When a Promethea Moth caterpillar, one of our giant silk moths, is ready to pupate at the end of the summer, it strengthens the stem, or petiole, of a leaf on its host plant with silk and then attaches the silk to a nearby branch, assuring that the leaf will remain attached to the tree. (Imagine having the instinctive foresight in your youth that this caterpillar had!) The caterpillar then curls the leaf around itself and spins its cocoon inside the curled leaf. The cocoon dangles from the host plant throughout the winter and in early summer the moth emerges. Now is the perfect time for finding a Promethea Moth cocoon, as last year’s leaves are gone on most trees, and this year’s buds have yet to open. Look for a tree or shrub that has just one dead leaf hanging from one of its branches. (Cecropia caterpillars favor black cherry, poplar, ash, maple, oak and willows trees among others.)
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