The Imperial Moth

Sep 22, 2011

by Lorraine Julien

Photos: Imperial Moth caterpilar (l) and the ault moth (r).One day this past August, we discovered a very large, scary-looking caterpillar near the base of some pine trees at my brother’s cottage. It was about 4 inches long and dark blackish brown with what appeared to be white spots and fine hairs all over its fat sausage-like body. On closer inspection, the spots turned out to be white barbs.

After some research, we discovered that it was the caterpillar form of the Imperial Silkworm Moth (Eacles imperialis - a member of the family Saturniidae). This species is known to have two basic colour forms: a blackish/orange form and a green/yellow form but either one looks pretty ugly. In fact, up close it looked like something out of a horror movie. Although this caterpillar may look fierce and dangerous, it is harmless and does not sting or stab humans. We didn’t take any chances though and had no intention of touching it!

Before settling down to enjoy a few meals, these caterpillars like to wander around a bit.

The first stage (instar) lasts only a few days but, interestingly, at the end of each stage, a small amount of silk is spun on the main vein of a leaf. The caterpillar then latches onto the silk and begins to molt. It gets a bit bigger and harder as it goes through each stage, and may eat the old outer skeleton for protein and nutrition.

Since they mature in early fall, our caterpillar was probably in its fourth or fifth instar of development. During these stages, individual Imperial Moth caterpillars can consume a lot of foliage but are seldom present in large enough numbers to do really serious damage. They tend to prefer pine although they do feed on a variety of deciduous and coniferous trees. At breeding time, they are particularly vulnerable to predators, particularly foraging raccoons. By late summer, they come down from the trees and wrap themselves in leaf litter or bury themselves for the winter wherever there is a patch of soft soil. At this stage they weave themselves into a silk cocoon.

When the warm days of spring arrive, they emerge as adult moths ready to lay eggs and begin the cycle all over again. The attractive adult moth is very large with a wingspan as much as 5.5 inches. Generally, the colour is a soft yellowish brown marked with lilac patches, spots, lines and flecks. More northerly moths like the ones found here would usually show more yellow. As with all of the moths in the Saturniidae family, the adults do not feed - their mouthparts are vestigial which means they are reduced or non-existent. Consequently, the adult moth lifespan is only about a week.

In northern areas, the Imperial Moth has only one brood per year. In warmer climates, it could have two broods. Imperial Moths range from Mexico to Canada and from the Rocky Mountains to the Atlantic Coast. In modern times, its range has receded northward but, according to the Butterflies and Moths of North America website, it still occurs in southern Ontario. Its numbers in the eastern and southern U.S., while still common, have been declining. Reasons for this are unclear, though habitat degradation could be a factor.

Both the caterpillar and moth stages of the Imperial Silkworm Moth are very striking. Keep an eye out for them next spring and summer.

Please feel free to report any observations to Lorraine Julien at This email address is being protected from spambots. You need JavaScript enabled to view it.  or Steve Blight at This email address is being protected from spambots. You need JavaScript enabled to view it.