Spiny Elm Caterpillar
The spiny elm caterpillars are striking in appearance. They grow to 2 inches in length and have a black body covered with tiny white dots. A row of conspicuous red dots runs down the middle of the back, between the branched spines for which the caterpillar is named.
The mourningcloak butterfly (named for the yellow band around the outside edge of the dark wings, which resembles the coloration of the robe or cloak worn during periods of mourning in Germany and Scandinavia) is a medium-sized butterfly that spends the winter in the adult stage and therefore, is one of the first butterflies to be seen in the spring, often by early to mid-April. The female butterflies lay eggs on the host plants named below, and the gregarious caterpillars feed in clusters, defoliating one branch before moving to the next. Caterpillars feed for about 5 to 6 weeks.
First generation caterpillars turn to butterflies by mid-June to early-July and a second generation of caterpillars will feed on tree foliage in July and August. These caterpillars transform to the adult stage which spends the winter and repeats the cycles the following year.
The caterpillar of the mourningcloak butterfly is called the spiny elm caterpillar. As the name implies it feeds on the foliage of elm trees, but also foliage of willow, birch, cottonwood and hackberry. This is a moderately common insect in Iowa and significant damage to elm and willow trees is unusual.
Spiny elm caterpillars are rarely present in high enough numbers to seriously injure otherwise healthy trees and control is seldom warranted. Well-established tree can tolerate the rare defoliation caused by these caterpillars. In cases of unusually large numbers or severe attack on small or newly-transplanted trees control may be justified to prevent stress to the tree. Treat small caterpillars as soon as they are noticed for best results. Handpick or prune to remove caterpillars if possible.