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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Silkmoth Season, Part 1: Imperial Moth, Eacles imperialis


Fully the length of a pointer finger, an Imperial moth caterpillar is one of the largest larvae to be found.


Last week while hiking along a trail in western Maryland, I was greeted by an astounding caterpillar ascending the trunk of a large maple tree. This giant of the insect world, an Imperial moth larva, is a relative of the orangestriped oakworm we met in last week’s episode. Larvae and adults of this magnificent moth can be found from Canada to Argentina, although populations of Imperial moths in parts of New England appear to be declining. Some believe this decline is linked to insecticide applications to control nefarious gypsy moth caterpillars and the release of exotic parasitoid flies that attack and kill not only gypsy moth caterpillars, but also caterpillars of several members of the silkmoth clan.

Another factor believed to imperil Imperial moths and their relatives are high intensity street lamps that are very attractive to night-flying moths. These artificial illuminators may disrupt the normal mating rituals of many insects, including silkmoths. Unlike the orangestriped oakworm, which has two generations each year in Maryland, Imperial moths are univoltine, meaning they have but one. Imperial moths spend winter and spring as a pupae in a subterranean burrow. Sometime in June or July, adult moths complete development and eclose from their pupal cases. At night, females fly to the canopy of trees where they release sex pheromones to attract a mate. Following a romantic interlude, females deposit eggs singly or in groups of 2 to 5 on leaves.

This giant of the moth world can't decide which way to turn.

 Caterpillars hatch from the eggs in about two weeks and feed for several more on leaves of a wide variety of woody and evergreen trees. Dr. Douglas Ferguson, an expert on silkmoths, lists “oak, hickory, walnut, sycamore, basswood, maple, honey locust, chokecherry, sumac, sweet gum, sassafras, elm, beech, hornbeam, birch, alder, pine, spruce, hemlock, cedar, cypress, and juniper” as food sources of Imperial moth caterpillars. The prodigious appetites of Imperial moth caterpillars ensure that larvae acquire sufficient nutrients to sustain them both in their youth and also as adults. The mouthparts of Imperial moths are vestigial and they do not feed.

Gorgeous Imperial moths can sometimes be spotted on tree trunks, where they await their mate. 

 Despite the decline of Imperial moth in parts of New England, this remarkable insect thrives throughout much of its range in North, Central, and South America.  The image of the adult Imperial moth in this episode was taken on a maple tree at a child care center in Columbia, Maryland. Although rare or absent in much of New England, on the isle of Martha’s Vineyard a sturdy, pine-eating race of Imperial moths seems to have escaped the perils of habitat destruction, pesticides, and imported parasitoids. For fortunate Labor Day vacationers heading to that picturesque island, the Imperial moth serves as a spectacular reminder of a less human-muddled natural world.          


We thank Dr. Shrewsbury, who wrangled the Imperial moth caterpillar that served as the inspiration for this episode. Two excellent references, “Life history of the Imperial Moth Eacles imperialis (Drury) (Saturniidae: Ceratocampinae) in New England, U.S.A.: distribution, decline, and nutritional ecology of a relictual islandic population” by Paul Goldstein, and “The moths of North America, Fascicle 20.2A Bombycoidea: Saturniidae (Part)” by Douglas Ferguson provided information for this episode.

To learn more about the Imperial moth, please visit the following web site: