Insect Profiles: Fall Cankerworm (Alsophila pometria)
Alsophila pometria, also referred to as fall cankerworm, is an insect native to North America. It is a member of the family of moths called Geometridae. Large populations of fall cankerworm larvae are capable of stripping the leaves from trees, causing extensive defoliation.
Distribution & Habitat
In the United States, fall cankerworm ranges from New England to Georgia, and west to Colorado, Missouri, Montana, New Mexico, Utah, and California. In Canada, fall cankerworm can be found from the Maritime Provinces to Alberta.
Fall cankerworm consumes the leaves of apple, ash, basswood, beech, birch, black cherry, boxelder, dogwood, elm, hickory, red maple, sugar maple, red oak, and white oak. Oak is the insect’s preferred host. Extensive outbreaks have occurred in forests comprised primarily of oaks.
Fall cankerworm passes through four stages during its life cycle: an egg stage, a larval stage, a pupal stage, and an adult stage.
The Egg Stage
Fall cankerworm produces one generation of eggs per year. Adult females deposit around 100 eggs in compact rows on the twigs and branches of their hosts. Eggs are dark gray or brown, with a distinct black spot on the upper surface. They are shaped like barrels, and measure less than 0.04 inches in diameter.
The Larval Stage
Eggs hatch from late March to early May, a period that coincides with bud expansion on host plants. Larvae, often referred to as inchworms, loopers, or measuringworms are less than 1/16 of an inch in diameter when they first appear. Larvae vary between light green and dark brownish-green in color. Light green larvae feature a longitudinal white line that extends along the length of their backs. Dark brownish-green larvae feature a longitudinal black stripe that runs along their abdomens. When the darker colored larvae are present, it is often indicative of heavy infestation. Fall cankerworm larvae are unique in that they form three sets of prolegs. The third set, located on the fifth abdominal segment, is reduced in size, and non-functional.
Shortly after emerging, larvae spin silken threads, which they cling to. During windy periods, they are disseminated into tree canopies, whereupon they begin to feed on the buds and expanding leaves. As they feed, they create small holes in the leaves, consuming all of the leaf tissue, with the exception of the major veins and midribs. Within four to six weeks, larvae reach maturity. Once they are fully developed, larvae weave a silken thread, and descend to the ground, where they wriggle into the soil. Within the soil, larvae envelop themselves in a cocoon composed of silk and soil particles. They pupate within the cocoon until late October.
The Pupal Stage
Initially, pupae are yellow to yellow-green. Later, their color deepens, turning dark red-brown. Pupae are found just beneath the surface of the ground, enshrouded in a silken cocoon. Larvae pupate until late October, when they emerge as adult moths.
The Adult Stage
Adult moths emerge from their cocoons in fall, appearing from late October to November. They may continue to emerge through January, with some individuals appearing in spring. Periods of emergence often occur immediately following bouts of cold weather. Males emerge a few days before the females. Males have glossy, brown forewings that are mottled with dark brown and light gray scales. The forewings of some individuals are crossed with irregular white bands. The hind wings are gray-brown. This coloring enables the males to blend with the stems of host trees.
Females are wingless, and measure around ½ an inch in diameter. They are densely covered with gray-brown scales. Once the females have emerged, the males navigate to them, and begin the mating process. Upon mating, females deposit around 100 eggs in a single layer on the twigs and branches of host plants. Adults die shortly thereafter. The eggs overwinter, and hatch the following spring.
Effects on Trees
Severe infestations can cause extensive defoliation in trees. Successive defoliation can result in significant branch dieback. It may also contribute to a general decline in tree vigor. Trees that are defoliated for three or more consecutive years may be weakened, and become prone to failure. Young, newly transplanted, or weakened trees are susceptible to injury stemming from defoliation.
- Begin monitoring for fall cankerworm in early May, when the larvae first emerge.
- Numerous insecticides can be applied to reduce larvae populations. Bacillus thuringiensis, carbaryl, cyfluthrin, esfenvalerate, malathion, and permethrin are the most effective insecticides available for commercial use. These insecticides will not harm beneficial insects, wildlife, or humans. For the best results, applications should be made when larvae are less than ½ an inch in diameter. Larvae become more resistant to insecticides as they develop.
- Cool, wet spring weather has an adverse effect on populations.
- Late spring frosts can kill newly emerging foliage on host trees, eradicating food sources for early stage larvae, and causing a collapse in larvae populations.
- Several insects assist in limiting cankerworm populations. A tiny wasp called Telonomus alsophilae parasitizes the eggs. Calasoma frigidum, a ground beetle, preys on the larvae.
- On isolated shade trees, bands composed of a sticky substance can be placed around the trunk in late fall. This will deter females from ascending the tree, and depositing their eggs. This method is not as effective in forested settings where there is an abundance of trees, and infestations are more difficult to manage.
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