The spiny elm caterpillar is the larval (immature) form of the mourning cloak butterfly. Lepidopteran caterpillars that can sometimes cause damage to (ex. defoliate) trees and shrubs are most often moths as adults, however, as in the case of the spiny elm caterpillar, some of them mature into butterflies. This beautiful butterfly is native to the United States and Canada. It is a very rare occurrence that spiny elm caterpillars need to be managed on their host plants and professionals are encouraged to communicate with property owners and encourage tolerance of their presence whenever possible. In fact, management of this insect is not necessary unless in the very rare possibility a prolonged, repeated defoliation event is caused by a very large population increase. Even so, it is very likely that such an increase will not last. The spiny elm caterpillar may have up to two generations per year. The adult butterfly is the overwintering life stage, making it one of the first butterflies to be seen in the spring. Adult mourning cloak butterflies will find sheltered areas in which they spend the winter. (In rare cases, pupae have been reported overwintering in their chrysalis.) Females lay their eggs in masses of 300-450 on the twigs of host plants where buds are newly opening. The eggs are yellow in color and ribbed when first laid. As they mature, the eggs turn black in color. As eggs hatch, the larvae (spiny elm caterpillars) feed together in groups (are gregarious). Leaves are typically eaten one branch at a time. Mature caterpillars may be up to 2 inches in length with intimidating looking but harmless spines. Spiny elm caterpillars possess red abdominal legs and eight red spots in a single row on the dorsal side. Black areas of the caterpillar appear speckled with smaller white spots. Once the caterpillars are fully mature, they find a safe place to prepare for pupation. Often, this is done by hanging from a twig, where the chrysalis is formed. This stage lasts for approximately a week. By approximately August, the second generation begins (Johnson and Lyon, 1991).
The spiny elm caterpillar preferentially feeds on the leaves of elm and willow. Caterpillars tend to be gregarious (feed in groups) and may strip their host plants of leaves, one branch at a time. However, significant damage to host plants is very rare. Often, no damage to the overall health of the tree or shrub is caused by this insect, and it can be tolerated and encouraged in managed landscapes. In fact, many land managers and property owners appreciate certain butterfly and moth larvae (caterpillars) for the biodiversity they bring to the landscape, as well as the important role they play as food for other wildlife.
If a local outbreak of the spiny elm caterpillar is occuring, monitor for caterpillars on susceptible hosts for a period of approximately 5-6 weeks in both May/June and again in July/August. Again, this is a rare event.
Encourage and enjoy sharing a managed landscape with spiny elm caterpillars (mourning cloak butterflies). These insects rarely, if ever, require management. In the rare event that significant defoliation is occuring, caterpillars can be hand picked from susceptible hosts or removed by pruning out impacted branches, if safe to do so and without disfiguing the host plant.
Many fly and wasp parasites as well as bird predators are known of the spiny elm caterpillar (mourning cloak butterfly). Some examples include but are not limited to: the Tachinidae (Compsilura concinnata, Chetogena spp., and Lespesia spp.) and the Braconidae (ex. Apanteles atalantae), and the Ichneumonidae (ex. Probocampe confusa) (O'Hara, 2013; Krombein et al., 1979). These natural enemies typically keep their populations below damaging levels. Thus, management of this insect is very rarely, if ever, warranted. Bacillus thuringiensis, a bacterial pathogen, also readily kills spiny elm caterpillars.
Abamectin (NL)
Acephate (NL)
Acetamiprid (L)
Azadirachtin (NL)
Bifenthrin (NL)
Bifenthrin+Imidacloprid (L)
Carbaryl (L)
Chlorantraniliprole (NL)
Chlorpyrifos (larvae) (N)
Cyantraniliprole (NL)
Cyfluthrin (NL)
Flonicamid+cyclaniliprole (N)
Tau-fluvalinate (NL)
Gamma-cyhalothrin (L)
Horticultural oil (L)
Indoxacarb (L)
Insecticidal soap (NL)
Isaria (paecilomyces) fumosoroseus (NL)
Lambda-cyhalothrin (L)
Malathion (L)
Neem oil (NL)
Pyrethrin+sulfur (NL)
Spinetoram+sulfoxaflor (N)
Spinosad (NL)
Tebufenozide (NL)
Chemical management of the spiny elm caterpillar is rarely, if ever, necessary.
Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), azadirachtin (injection, soil drench), chlorantraniliprole (soil drench), cyantraniliprole (soil drench, soil injection), and neem oil (soil drench).
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
