NC State Extension Publications

Description and Biology

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Spiny elm caterpillars, Nymphalis antiopa, the immatures of the mourning cloak butterfly, are relatively large, spiny caterpillars. Even though they have a common name, the caterpillars are rarely reported in North Carolina. These caterpillars usually feed in groups defoliating one branch before moving to the next. They feed for about 5 to 6 weeks and grow to 2 inches long. The velvety black body is covered with tiny white speckles. A row of eight conspicuous red spots runs down the back between conspicuous spines. Because the caterpillars are covered with spines, people often mistakenly assume that they sting (they don't). When the caterpillars have finished feeding, they scatter to hang from a branch and transform into a chrysalis from which an adult mourning cloak butterfly emerges about 3 weeks later. The chrysalis more or less resembles a mottled, brown, dead leaf. We have two broods each year from May to June and again in July and August. Mourning cloak butterflies overwinter in tree cavities, wood piles, hidden behind bark flaps, and in other protected sites. Some few overwinter as a chrysalides. During warm days in late winter mourning cloak butterflies sometimes emerge and fly. Shortly after the first new growth emerges on elms, females lay eggs in a ring around small twigs and branches. Mourning cloak butterflies are beautiful although somewhat somber. These butterflies have rich brown wings with yellow borders and rows of blue spots. The mourning cloak butterfly was named for the white or yellow band around the outside edge of the cloak worn during periods of mourning in medieval Germany and Scandinavia.

mourning cloak butterflies

The yellow markings of mourning cloak butterflies often fade to almost white.

spiny elm caterpillar

The red spots, white speckles, and spines define the spiny elm caterpillar.

eggs in a mass, spiny elm caterpillars

Because mourning cloak butterflies lay their eggs in a mass, spiny elm caterpillars often occur in bunches.

Spiny elm caterpillars hang down just befor

Spiny elm caterpillars hang down just before pupating into a chrysalis.

spiny elm caterpillar chrysalis resembles a dead leaf

This spiny elm caterpillar chrysalis resembles a dead leaf.

Host Plants

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As the name implies, spiny elm caterpillars feed on the foliage of elm trees, but they also feed on willow, birch, cottonwood and hackberry.

Residential Recommendations

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Because of our long growing season, our elm and other shade trees are normally tolerant of some defoliation, especially early in the season. Control efforts are unnecessary if only a few larval caterpillars are present. Established trees can tolerate the rare defoliation caused by these caterpillars. In the rare event of very high spiny elm caterpillar populations, it may feel necessary to control the worms. Most insecticides labeled for residential landscapes should give more than adequate control. Treat small caterpillars as soon as they are noticed for best results. Consider removing the caterpillars by handpicking or pruning if the tree is small enough to do so without having to use a ladder.

Other Resources

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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 & Plant Pathology

Publication date: July 9, 2020
Revised: July 9, 2020

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