Cankerworms are common defoliating insects that impact trees and plants. Understanding their life cycle and the damage they cause is important for managing their presence in landscapes. These insects are a recurring concern due to periodic outbreaks.
Cankerworm Identification and Life Cycle
Cankerworms are caterpillars belonging to the moth family Geometridae, recognized by their unique “looping” or “inchworm” movement. This movement occurs because they lack several pairs of fleshy legs, called prolegs, along their abdomen. Mature cankerworm larvae measure about 1 inch in length and can vary in color from light green to dark green, brown, or black, and may display white or dark stripes.
Two primary cankerworm species in North America are the spring cankerworm (Paleacrita vernata) and the fall cankerworm (Alsophila pometaria). While their larvae appear similar and feed at the same time, their adult stages and egg-laying habits differ. Adult males are dull gray-brown moths with a wingspan of about 1 to 1.5 inches. Adult females of both species are wingless, with a spider-like or dull gray/brown appearance, measuring 5/16 to 1/2 inch long.
The cankerworm life cycle involves four distinct stages: egg, larva, pupa, and adult. Fall cankerworm adults emerge from the soil in late autumn, typically after the first hard freeze. Wingless females crawl up tree trunks to lay eggs in neat, compact masses or rows on twigs and branches. Spring cankerworm adults emerge earlier, in late winter or early spring. Their wingless females also climb trees to lay eggs, but in less organized clusters within bark crevices.
For both species, eggs hatch in early spring, around the time tree buds open, beginning the larval feeding stage. Newly hatched larvae are small and can spin silk threads, allowing wind to carry them to new trees, a process known as “ballooning.” These caterpillars feed for approximately three to six weeks, growing through several instars.
Once fully grown, the larvae drop to the ground, often by silken threads, and burrow into the soil to pupate. Fall cankerworm pupae remain in the soil until the fall, while spring cankerworm pupae overwinter in the soil and emerge as adults the following spring. Each species has only one generation per year.
Damage to Trees and Plants
Cankerworms primarily damage trees and plants through defoliation. The larval stage is responsible for this feeding. When young cankerworm larvae first hatch, they create small, “BB-sized” holes or “shotholes” in newly expanding leaves, often feeding between the veins. As the caterpillars mature, their feeding becomes more extensive, consuming most of the leaf tissue and leaving behind only the larger veins and midribs.
This feeding can result in a “skeletonized” appearance of the foliage or, in severe infestations, complete defoliation of the entire tree. While a healthy, mature tree can often withstand one or two seasons of complete defoliation by refoliating, repeated defoliation for two or more consecutive years can severely weaken the tree. Such stress can lead to reduced growth, branch dieback, increased susceptibility to other pests and diseases, and even tree mortality, especially for young, newly transplanted, or already weakened trees.
Cankerworms feed on a wide variety of deciduous trees and shrubs, including common species such as elm, oak, maple, apple, ash, hickory, linden, and cherry. Beyond the direct damage to foliage, large populations of cankerworms can also become a nuisance. They produce frass (feces) that accumulates on surfaces below infested trees, and larvae also drop from trees on silk threads, landing on people and property.