Tree Pests: Caterpillars to Watch Out For

Butterflies and moths are in the insect order Lepidoptera. These insects have complete development. They have egg, larval, pupal, and adult life stages. The larval stage is important when it comes to tree health. Lepidopteran larvae are caterpillars that feed on foliage or bore into the wood. Thus, this life stage may have to be targeted for control to protect a tree's appearance and health. There are thousands of native and non-native caterpillars in North America that feed on trees. Many of them go unnoticed as they feed on leaves. Others cause noticeable damage yet are not an overall concern for tree health. However, some species can cause enough damage to be a health concern. Let us look at a few.

Spongy moth (Lymantria dispar)

The spongy moth overwinters as an egg in a mass stuck to tree bark or any other convenient surface. The egg mass is covered in hairs that the female scraped off during the egg-laying process. The white-colored adult females do not fly, so they will lay eggs on whatever tree they developed on. Eggs hatch in the spring when new foliage is emerging. Caterpillars go through developmental stages called instars, each larger than the previous. Early instars of spongy moths have black dots down the back. As they progress, they develop five blue pairs of spots starting behind the head, followed by six pairs of red spots. The caterpillars can be extreme defoliators. They will eat an entire leaf, leaving only the midvein. Healthy trees may flush more leaves, but this requires the use of stored carbohydrates. Sequential years of heavy defoliation can kill a tree. Spongy moths use hundreds of plant species as hosts. Oak, apple, basswood, birch, poplar, sweet gum, and willow are favorites. Spongy moth has defoliated significant acres in the northern tier of Pennsylvania over the past four years (2021-2024).

Eastern (Malacosoma americanum) and forest tent caterpillar (Malacosoma disstria)

Unlike the spongy moth, the tent caterpillars are native species. They have evolved with our native trees. However, this does not mean they cannot be problematic, especially the forest tent caterpillar. These caterpillars overwinter as eggs embedded in a "Styrofoam-like" egg mass attached to small twigs and branches. Eggs hatch in the spring. The caterpillars of both species produce silk to make a web. Eastern tent caterpillars are constructed in the forks of branches or where a branch meets the trunk. Forest tent caterpillars make theirs along the trunk or on a large branch. Caterpillars of both species are colorful, with blue and orange areas. The distinguishing characteristic is the white area down the back. Eastern tent caterpillars have a white stripe. Forest tent caterpillars have white, key-hole-shaped marks down the back. Large-stature trees can tolerate heavy feeding damage if it is a single event. Smaller trees will be stressed by defoliation. Eastern tent caterpillars like to feed on apple, cherry, and crabapple. Wild black cherry is a favorite host. Forest tent caterpillars prefer sugar maple, oaks, birch, and black gum.

Spring (Paleacrita vernata) and fall cankerworm (Alsophila pometaria)

These two native species get their common names from when the adults are active. Spring cankerworm adults are active in March and April. Fall cankerworm adults are active in November and December. Eggs for both are laid on the tree, which is the overwintering stage. Caterpillars of both move by anchoring the prolegs at the end of the abdomen and stretching forward. This movement gives them the common name inchworm, looper, or measuring worm. Color patterns vary for the caterpillars. They can be light green with pale stripes, green with dark brown or black stripes, and almost completely black. Early feeding consists of small holes chewed between the leaf veins but then progresses to defoliation. Fall webworm is the most problematic of the two. Host trees include ash, basswood, beech, black cherry, red maple, sugar maple, red oak, and white oak.

Management

The first option is to do nothing. This is dictated by who owns or manages the tree and that person's tolerance for damage. A large tree with feeding damage from the eastern tent caterpillar will be fine. However, a smaller-stature ornamental crabapple or cherry with a similar infestation will be stressed from heavy defoliation. A second non-chemical option is to hand-remove caterpillars or nests. Obviously, this is only a viable option when there is either a small number of caterpillars/nests and they are within reach. A third non-chemical option is to remove egg masses before spring. Spongy moths and eastern and forest tent caterpillars all overwinter as noticeable egg masses attached to trees. Spongy moth eggs can be scraped away and destroyed, while tent caterpillar eggs can be pruned off small twigs. Note that this is not a practical or effective management option when large trees are involved. There are naturally occurring predators, parasites, and pathogens that can play a significant role in causing populations to crash. It may take a few years for their populations to build in order to provide an acceptable level of control. Finally, treat the tree(s) with an insecticide. In Pennsylvania, a Category 6 or 23 pesticide applicator license issued by the Pennsylvania Department of Agriculture is required to apply a pesticide to a tree. Insecticide active ingredients effective against leaf-feeding caterpillars include Bacillus thuringiensis kurstaki (often abbreviated Bt), chloraltraniliprole, and spinosad. There are others, but they have a greater negative impact on beneficial insects.