Cankerworms are a common concern for tree health in urban and suburban environments. These insects, also known as inchworms or loopers, are defoliating pests that can cause significant damage to a variety of deciduous trees. Understanding their characteristics and behaviors is important for protecting trees from their feeding activity.
Identifying Cankerworms
Cankerworms are primarily recognized in their larval (caterpillar) stage, as this is when they consume tree foliage. These larvae are slender and typically grow to about one inch in length. Their color can vary, ranging from light green to brownish or even black, sometimes with pale stripes along their sides. A distinctive feature of cankerworm larvae is their “looping” movement, which gives them the nickname “inchworm.” This movement occurs because they have fewer prolegs (fleshy, stubby legs on the abdomen) compared to most caterpillars, with spring cankerworms typically having two pairs and fall cankerworms possessing three.
Adult cankerworms are moths, with the males and females appearing quite different. Male moths are generally brownish-gray and have wings with a span of about one to 1.25 inches. In contrast, the adult female moths are wingless and grayish-brown, resembling spiders as they crawl. These wingless females are approximately 5/16 of an inch long.
The Cankerworm Life Cycle
Cankerworms undergo a complete metamorphosis, progressing through egg, larval, pupal, and adult stages, with one generation occurring each year. The timing of adult emergence and egg-laying distinguishes the two main species: spring cankerworms and fall cankerworms. Fall cankerworm moths typically emerge from the soil in late autumn, often after the first hard freeze. These wingless females then crawl up tree trunks to lay their barrel-shaped eggs in compact masses on twigs and branches, where the eggs overwinter.
Spring cankerworm moths, on the other hand, emerge earlier in the year, usually in late winter or early spring, shortly after the snow melts. They also have wingless females that ascend tree trunks to lay eggs, but their eggs are typically laid in loose clusters within bark cracks and under bark scales. For both species, eggs generally hatch in early spring, around the time tree buds begin to open, from late April to mid-May, depending on local weather conditions. The larvae feed for approximately three to six weeks, then drop to the ground to pupate in the soil, where they remain until their adult emergence.
Impact on Trees and Plants
Cankerworm larvae primarily cause defoliation, meaning they consume the leaves of host trees and plants. They begin by chewing small holes in newly expanding leaves, which may initially resemble “shot-holes.” As the larvae grow, their feeding intensifies, leading to larger, irregular holes until only the major leaf veins and midribs remain, giving the foliage a lacy or skeletonized appearance. Common host trees susceptible to cankerworm feeding include elm, oak, maple, apple, hackberry, basswood, ash, and linden.
The consequences of defoliation vary depending on the severity and frequency of infestations. A healthy, mature tree can often withstand one or two years of complete defoliation by producing a new set of leaves. However, repeated severe defoliation over three or more consecutive years can weaken trees, reduce their growth, and lead to branch dieback. Young, newly planted, or already stressed trees are more vulnerable to injury and may experience increased susceptibility to other pests or diseases, potentially leading to mortality.
Managing Cankerworm Infestations
Managing cankerworm populations involves several approaches, often combining physical barriers and biological controls. A common and non-chemical method involves applying sticky bands around tree trunks. These bands, typically made of a flexible material wrapped around the trunk and coated with a sticky substance, prevent the wingless female moths from crawling up the tree to lay eggs in the fall or spring. For effectiveness, these bands should be tied tightly to prevent females from finding crevices to bypass the barrier, and the sticky material should be applied to the band itself, not directly to the bark.
Another effective control option, particularly when larvae are small, is the application of Bacillus thuringiensis (Bt). Bt is a naturally occurring bacterium that, when ingested by cankerworm larvae, disrupts their digestive system, leading to their demise. It is generally considered safe for pets, beneficial insects, and the environment. For optimal results, Bt should be applied when larvae are less than half an inch long, usually around the second week of feeding, as they must consume it while actively feeding on leaves.
Horticultural oils can also be used during the dormant season to control overwintering cankerworm eggs on bark. Additionally, natural predators play a role in regulating cankerworm populations. Birds, ground beetles, and ants feed on the larvae, while certain parasitic wasps target cankerworm eggs. Maintaining overall tree health through proper watering and care can also enhance a tree’s natural resilience to pest pressure.