The cankerworm is a native North American pest known for its ability to defoliate a wide variety of hardwood trees. While they are generalist feeders, they show a strong preference for certain species, posing a threat to both forest and ornamental trees. Understanding their specific feeding preferences is the first step in managing their presence, as the larval stage is responsible for nearly all observed damage.
The Destructive Life Stage: Larval Feeding
The cankerworm’s life cycle ensures that only the larval stage, also known as inchworms or loopers, consumes plant material; adult moths do not feed on foliage. Larvae emerge from eggs in early spring, precisely timed to coincide with the opening of tree buds, providing them with tender, newly emerging leaves.
There are two primary species: the Fall Cankerworm (Alsophila pometaria) and the Spring Cankerworm (Paleacrita vernata). Despite the names referencing adult activity, the larvae of both species feed simultaneously in the spring and early summer. This feeding period is brief but intense, lasting approximately four to six weeks before the caterpillars drop to the ground to pupate. This synchronized, short-duration feeding causes the rapid and noticeable defoliation observed in infested areas.
Primary Host Trees and Preferred Foliage
Cankerworms primarily consume the leaves of a broad range of deciduous hardwood species. Although they are generalists, they exhibit a distinct preference for certain common shade trees, such as American Elm and Apple trees, which often sustain the most severe feeding damage.
A wide array of other hardwood trees are also readily attacked, especially during outbreaks. The larvae focus on the soft, succulent foliage of the trees they infest. Common hosts include:
- Oak (red and white species)
- Maple
- Basswood (Linden)
- Ash
- Cherry
- Birch
- Hickory
- Hackberry
- Hawthorn
Specific Feeding Habits and Resulting Damage
The mechanism of consumption changes as the cankerworm larvae grow, leading to different visual signs of damage. When the larvae are very small, they feed on the leaf tissue between the small veins. This initial feeding results in a characteristic “shot-hole” or “tattered” appearance on the leaves, where small, irregular holes are visible.
As the caterpillars mature, their feeding becomes much more destructive. They consume larger portions of the leaf, often eating all the soft green tissue and leaving only the midrib and major veins intact—a process known as skeletonizing. High populations can lead to complete defoliation of the tree crown in a matter of weeks. Targeting newly emerging buds and leaves in the spring is particularly damaging, as it forces the tree to expend stored energy for a second leaf-out, which can weaken the tree over multiple years.