Cankerworms, often called inchworms or loopers due to their distinctive movement, are caterpillars known for feeding on the foliage of various deciduous trees. These insects are the larval stage of specific moth species, primarily the fall and spring cankerworms. They undergo a complete biological transformation.
The Journey of Transformation
Cankerworms undergo a four-stage life cycle: egg, larva, pupa, and adult. The timing of these stages varies slightly between the two main species. Fall cankerworm moths emerge in late fall or early winter, with females laying barrel-shaped eggs in neat clusters on tree twigs and branches that then overwinter. Spring cankerworm moths emerge in early spring, and females deposit oval-shaped eggs in less organized clusters within bark crevices.
Eggs from both species typically hatch at a similar time in the spring, coinciding with the emergence of new tree leaves, usually from late April to mid-May. Once hatched, the larvae begin to feed on the tender, expanding foliage. These caterpillars are slender, growing up to an inch long, and display varied colorations from light green to dark brown or black, often with longitudinal stripes.
A distinguishing feature of these larvae is their “inching” movement, a result of having fewer prolegs (fleshy, leg-like appendages) on their abdomen compared to other caterpillars. Fall cankerworm larvae have three pairs of prolegs, while spring cankerworm larvae have only two. After feeding for three to six weeks, mature larvae descend to the ground, often by spinning silken threads and “ballooning” on the wind, before burrowing into the soil to pupate. Fall cankerworm pupae remain in the soil until adults emerge in autumn, while spring cankerworm pupae overwinter underground, with adults appearing the following spring.
Adult Cankerworm Identification
Cankerworm larvae transform into adult moths, which are generally brownish-gray. A key characteristic is the pronounced sexual dimorphism in both fall and spring cankerworm species. Male moths possess functional wings, typically spanning about one inch. Male fall cankerworms often exhibit glossy brownish-gray wings with mottled patterns or jagged white bands, while spring cankerworm males tend to have light brown or drab gray wings, sometimes marked with three indistinct darker diagonal lines.
In contrast, female moths of both cankerworm species are wingless. These flightless females are smaller than males, measuring approximately 5/16 to 1/2 inch long. Their appearance can sometimes be mistaken for spiders due to their lack of wings and prominent legs. Without wings, females must crawl up tree trunks to reach branches for egg-laying. Adult female cankerworms also have non-functional mouthparts, meaning they do not feed and typically die shortly after completing egg-laying.
Why Their Life Cycle Matters
Understanding the cankerworm life cycle helps explain their impact on trees. Cankerworms are known defoliators, feeding on the leaves of numerous deciduous trees, including elm, oak, maple, and apple. The damage, ranging from small holes to complete defoliation, occurs during their larval stage in the spring.
While healthy trees can often recover and produce new leaves after a single season of defoliation, repeated severe defoliation over several consecutive years can significantly weaken trees. This stress may reduce growth, cause branch dieback, and potentially lead to tree mortality, especially if trees are already compromised by factors like disease or drought. Knowing the timings of their life stages, such as when larvae hatch or when wingless females climb trees to lay eggs, can help anticipate periods of potential tree stress and damage.