Spiny Elm Caterpillar

Spiny elm caterpillars are common but usually do not develop high enough numbers to be considered a pest. It is possible that they can become very abundant on a local level and severely attack a small number of trees.

The spiny elm caterpillar, Nymphalis antiopa, is present throughout northern Illinois. The adult stage is commonly referred to as the mourningcloak butterfly. The larvae (caterpillar) feed on a wide variety of trees including elm, poplar, willow, hackberry, birch, and linden. However, elm and willow are the preferred hosts.

This caterpillar is the immature form of the familiar mourningcloak butterfly. This moderate sized butterfly has a wingspan of two to three inches. The wings are dark brown with a row of blue spots and a creamy yellow band along the edge of them. This is one of the few butterflies that overwinters as an adult so it is one of the first you will see in the early spring. This year, there were a number of sightings of mourningcloak butterflies during early April.

Life cycle: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.

Damage: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.These caterpillars attack a recently transplanted tree (within the last few years) or a tree that is already stressed or unhealthy, severe feeding could injure the tree. If it’s necessary to protect your trees from spiny elm caterpillars, it’s best to treat them as soon as caterpillars are noticed. They smaller the caterpillars are when you treat them, the more effective the insecticide application will be while also reducing the feeding damage.

Control: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.

When the spiny elm caterpillar occurs in abundant numbers, it can be a serious pest. The larvae can be controlled with sprays of the bacterium Bacillus thuringiensis var. kurstaki (Dipel, Thuricide, Javelin). Control is best obtained when the larvae are young and before they form a chrysalis. Larvae killed by the bacterium will hang upside down from tree branches. Spiny elm caterpillar is highly susceptible to free-living natural enemies such as parasitoids (stingless wasps) and birds.

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