How to Manage Pests

Spiny elm caterpillar—Nymphalis antiopa

Larvae (caterpillars) of this butterfly (Nymphalidae) feed on elm, poplar, and willow and less often on other trees.

Identification

The larvae are mostly black with a row of orange to brown spots down the back and rows of tiny, white dots on each segment. Larvae grow up to 2 inches long, have numerous branched spines, and usually feed in a group.

The pupae are black, brown, or gray and typically occur on bark. The adult, known as the mourningcloak butterfly, is mostly brownish-black to purple, with a wingspan of about 2 inches. Adult wing margins are often ragged and have a cream-colored or yellowish outer margin bordered inwardly by blue spots.

Life cycle

The tiny, almost cylindrical eggs are laid in masses of several dozen on leaves, limbs, or twigs. The emerging larvae develop through five increasingly larger instars (growth stages) before pupating into adult butterflies.

Spiny elm caterpillar has about two generations per year. In southern California, both adults and larvae may be observed during almost any month.

Damage

The group-feeding larvae cause ragged, chewed leaves, often on a single branch, which may be entirely defoliated. Otherwise healthy trees tolerate this loss of leaves, and spiny elm caterpillars generally do not harm the tree. The caterpillars mature into attractive mourningcloak butterflies.

Solutions

Usually no control is warranted to protect tree health. Handpick the larvae or clip and dispose of the infested branch if trees are small or unhealthy or the caterpillars cannot be tolerated.

Wear chemical-resistant gloves if handling the caterpillars and avoid inhaling insect parts. The distinctive row of black spines around each larval segment can irritate human skin.

Spiny elm caterpillar (larva)

Caterpillars feed in a group

Mourningcloak butterfly