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Commonly known as the “inchworm”, there are actually two species of cankerworms that attack fruit and ornamental trees: the “fall” and “spring” cankerworms. Their names refer to the season during which their eggs are laid, although the eggs of both species will hatch about the time buds on trees begin to open in spring. As a result, both species occur together in mixed populations and can often be found in the same tree.
Both species’ young larvae feed on the buds and unfolding new leaves of host trees in spring, devouring all but the midrib of the leaf and often defoliating the entire tree. Trees suffering a heavy infestation will usually produce a second crop of leaves after the initial spring defoliation, but its overall vitality may be diminished. Therefore, it is advisable to fertilize affected trees and accompany the fertilization with sufficient watering to restore the tree to health.
The cankerworm larvae are often seen dropping by silken threads from host trees before complete defoliation is noticed. Other infestation signs to watch for are their droppings on cars, sidewalks, and driveways. Favored trees are elm and apple, but several species of oak, cherry, hickory, honey locust, maple, ash, beech, linden, birch, pecan, and hawthorn are also affected.
Fall Cankerworm (Alsophila pometaria)
Growing to about one inch in length, the adult moth is brownish-gray with a green or black head and 6 pairs of legs. While the female is wingless, the wings of the adult male are marked with two whitish bands running diagonally. Larvae are light to dark green with dark stripes on the back, 3 narrow white stripes on the side, and a yellowish one below. The three pairs of prolegs (small suction cups) are on the fifth abdominal segment.
The adults emerge from their pupal stage in the ground after fall frosts to mate and lay their eggs. The wingless adult female must crawl up the trunk of host trees to deposit her eggs, which are brownish, shaped like tiny flowerpots, and laid in neat clusters of about 100 on twigs or branches. The fall cankerworm overwinters in the egg stage. Eggs hatch in the spring, producing larvae that feed on tender emerging buds and leaves of the host plant. Reaching their peak population in June, the mature larvae will then drop by silken threads from the trees to the ground below, where they spin cocoons and pupate.
Spring Cankerworm (Paleacrita vernata)
Smaller than the fall cankerworm at about 3/4 inch long when fully grown, the adult male moth can be identified by his silky forewings, which have loosely attached brownish scales and three diagonal dark lines, pale gray hindwings with a dusky distal spot, and a dirty white or mottled brown head. The female is wingless but may be characterized by a dark mid-dorsal stripe and a double row of reddish spines across the top of the abdominal segments. As larvae, the spring cankerworm can range in color from light green to brown or black, with whitish lines running down its back.
Unlike the fall cankerworm, which hibernates in the egg stage, the spring cankerworm overwinters as larvae in cells in the ground. Pupation takes place in late winter, with adults emerging about the time frost leaves the ground in early spring. As with the fall cankerworm, the wingless female must crawl up the trunk of a host tree to lay her eggs, which are white, oval-shaped clusters of about 100 deposited in bark crevices along the trunk and larger limbs. Eggs will hatch in the spring at the same time as the fall cankerworm, making identification of their larvae confusing.
There are a variety of options available to manage infestations of cankerworms. The cankerworm’s natural enemies include flies, wasps, and birds. It is also possible to trap and destroy the wingless female moth before she deposits her eggs by applying a trap of a commercially available sticky compound, such as Tanglefoot™, to the tree trunk. It is important to remember that the two species are named because of the season in which their eggs are laid, so traps are most effective in the late winter (February) and fall (mid-October).
Cankerworm larvae feed on leaves and buds in early spring, so this is when they are the most vulnerable to chemical control. Should cankerworms suddenly appear in very large numbers, they can be quickly controlled with an application of Bacillus thuringiensis var kurstaki (Btk). Btk is a naturally occurring, spore-forming bacterium that causes a fatal disease in cankerworms. Another option is to spray the infested tree with a residual contact insecticide soon after the larvae first become active and after the leaves have expanded. Note that by the time defoliation has occurred, chemical measures will be largely ineffective. For pesticide recommendations refer to University of Illinois Extension publication "Pest Management for the Home Landscape". Use pesticides safely and wisely; read and follow label directions. The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.