Cankerworms are the larval stage of moths that pose a significant threat to deciduous trees across North America. These pests are notorious for their rapid and severe defoliation, particularly of hardwood species like oak, elm, maple, and ash. Repeated defoliation can weaken a tree, making it vulnerable to other pests, diseases, or even death. Effective control relies on a multi-step approach that targets the cankerworm at its most vulnerable life stages.
Understanding the Cankerworm Life Cycle
Controlling cankerworms requires understanding their annual cycle, which involves two distinct species: the fall cankerworm and the spring cankerworm. Both species spend their feeding stage as caterpillars in the spring, but they are named for the season when their adult moths emerge to reproduce. Fall cankerworm moths emerge after the first hard frost, while spring cankerworm moths emerge later, during the early spring thaw.
A key factor enabling control is that the female moths of both species are wingless. They must crawl up the tree trunk to lay their eggs in the canopy. Fall cankerworms lay eggs on twigs that overwinter until spring. Spring cankerworms lay eggs in bark crevices in late winter or early spring.
Both sets of eggs hatch around the time of bud break, creating the tiny larvae that begin feeding on the new foliage. The larvae feed for approximately six weeks, and this caterpillar stage is the primary target for most intervention methods.
Physical Removal Methods
Physical controls offer a highly effective, non-chemical way to manage cankerworm populations by creating a simple barrier against the wingless female moths. The most common method is tree banding, which involves wrapping the trunk with a sticky material to trap the ascending females before they can lay eggs.
To apply a band, first wrap a non-porous material, such as foil-faced insulation or heavy paper, around the tree trunk four to five feet from the ground. This protective wrap prevents the sticky substance from contacting and damaging the bark. Next, apply a thick, even layer of a sticky insect barrier product, like Tree Tanglefoot, directly onto the wrap, creating a band four to six inches wide around the circumference.
Timing is important for maximum effectiveness. Bands should be installed in late October or early November to capture the fall cankerworm moths emerging after the first frost. They must be maintained throughout the winter and into the spring, often until mid-May, to intercept the later-emerging spring cankerworm moths. Check the band regularly and clear any debris, such as leaves or trapped insects, as these can create a bridge allowing the moths to cross.
Manual Removal
For trees that are small enough to reach, manual removal of caterpillars can also be done by knocking them off the branches with a strong jet of water from a hose.
Utilizing Biological Controls
For widespread infestations or tall trees, biological controls provide a targeted, low-impact solution. The primary agent used against cankerworms is a naturally occurring soil bacterium known as Bacillus thuringiensis, abbreviated as Bt. This bacterium is highly specific and is considered harmless to humans, pets, birds, and most beneficial insects.
Bt works as a stomach poison, requiring the cankerworm larvae to ingest the treated foliage to be effective. Once consumed, the Bt protein destroys the caterpillar’s gut lining, causing it to stop feeding within hours and die within a few days. The timing of the application is critical because Bt is only effective on young, actively feeding larvae, generally those less than one-half inch in length.
Since the product degrades quickly when exposed to ultraviolet light, application should occur in the evening or on a cloudy day to maximize its persistence. A thorough coating of the leaf surface is required to ensure the caterpillars consume the bacterium. This application window usually occurs in the spring when the leaves are just beginning to expand, around the time of bud break. Depending on the weather, a second application may be necessary one week later to treat newly hatched larvae.
When to Consider Chemical Treatments
When physical and biological methods are not feasible, chemical treatments may be considered as a last resort, such as in cases of severe defoliation across large, inaccessible trees. These options involve broad-spectrum insecticides, like those containing carbaryl or pyrethroids, which act quickly and provide immediate relief from heavy larval feeding.
These broad-spectrum products are less selective than Bt and pose risks to non-target organisms, including natural predators and pollinators. Their use should be minimized and timed carefully to avoid peak bloom periods when pollinators are most active. Caution must also be used to prevent runoff into nearby water sources, as many of these chemicals are toxic to aquatic life.
For large trees, applying these treatments requires specialized high-pressure spray equipment, making it best to hire a certified arborist or pest control professional. If applying a product yourself, strictly follow all label directions regarding mixing, application rates, and required personal protective equipment. Chemical control should only be pursued when the survival of the tree is threatened and less-invasive measures have been exhausted.