NC State Extension Publications

Description and Biology

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Imperial moths, Eacles imperialis, are large, mostly yellow moths with spots, lines and splotches of light to dark brown. They are classified in a section of the giant silkworm family called the royal moths. In late spring, Imperial moths emerge from the soil, mate, and females lay hundreds of eggs on a wide variety of trees. The eggs are flattened spheres almost 1/8 inch across. From the eggs hatch orange caterpillars almost half inch long with conspicuous black spines. Caterpillars eventually develop into large, fat, green to brown to almost black worms three to 5 inches long! Some brown caterpillars may have conspicuous orange spots. Mature green caterpillars have yellow knobs on their thorax and yellow spots down each side (the spiracles through which they breath). The other color forms have dark knobs. Mature caterpillars are also covered with long, fine hair. Fully grown caterpillars crawl to the soil and dig in. They molt into a pupal stage in the soil. The pupae are reddish brown, cylindrical, and taper slightly toward the rear. There is a small, forked structure at the tip end. The pupae evidently use that structure to help dig up to the soil surface. It is generally thought that a second generation of moths emerges from the pupal stage sometime later in the summer. These females mate and lay eggs for the next generation of caterpillars. When fully grown, they dig into soil to pupate and spend the winter in soil. Moths emerge from the overwintering pupae the following spring. However, there is some uncertainty whether we have two generations per year in North Carolina or one. Some overwintering pupae may delay development until well into the summer giving the appearance of a second generation of moths. Certainly further north only one generation occurs.

Imperial moth

Imperial moths are large, showy insects usually flying at night.

Male imperial moth

Male imperial moths have more feathery antennae.

Imperial moths lay egg

Imperial moths lay eggs on the foliage of host plants.

Imperial moth caterpillar

Imperial moth caterpillars have be reported to grow to 5.5 inches!

Imperial moth caterpillars

Imperial moth caterpillars can be quite variable in appearance but always large.

Imperial moth pupa

Imperial moth pupae are dark brown to almost black.

Host Plants

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Imperial moth caterpillars can feed on dozens of kinds of trees but seem to prefer pines, oaks, maples, sassafras, and sweetgum. Cedar, elm, persimmon, hickory, beech, honeylocust and cypress are other less common hosts as well as a slew of other plants.

Residential Recommendations

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Because imperial moth caterpillars are usually rare, the only noticeable sign of an infestation may be pellets of frass that fall to the pavement under an infested trees. Each female moth lays hundreds of eggs. A logical question would be, why are the moths not more common? Evidently the caterpillars are delicious to birds. Caterpillars that feed in an exposed position on foliage are ready targets for predators. In addition, many kinds of parasites and diseases plague caterpillars. So a mother moth is lucky, indeed, if two daughters of the hundreds of her offspring survive to produce grandchildren. Because imperial moth caterpillars are usually rare, it seems a pity to use an insecticide for their management. However, they are susceptible to most of the insecticides labeled for home landscape use, should the need arise.

Pellets of frass

Pellets of frass often drop beneath infested trees.

References

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For assistance with a specific problem, contact your local Cooperative Extension Center.

This Factsheet has not been peer reviewed.

Author

Professor Emeritus
Entomology and Plant Pathology

Publication date: Feb. 9, 2017
Revised: Sept. 24, 2019

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