Fall cankerworm

Side view of a dark-coloured caterpillar giving a clearer view of the third pair of prolegs Thérèse Arcand

French common name: Arpenteuse d'automne
Scientific name: Alsophila pometaria [Harris]
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Family: Geometridae

Distribution

Western Canada

Both are native North American species, widely distributed throughout the southern half of the three Prairie Provinces

Damage, symptoms and biology

The fall cankerworm is a closely related defoliator of many deciduous trees and shrubs in various types of rural and urban tree stands and plantings. Infestations occur often and affect both large and local areas.

Damage by the fall cankerworm usually begins in late May when young larvae chew small holes (shot-holes) in the developing leaves. As feeding continues, these holes gradually enlarge until only the larger leaf veins and midribs remain. When cankerworm populations are large, starving larvae in search of food may drop on silken threads and become a nuisance around homes or in well-used areas. During outbreaks lasting from 1 to 4 years, trees may be completely defoliated; however, most trees usually refoliate in July, 3–5 weeks after the first attack.

Three or more consecutive years of severe defoliation may cause many of the upper branches to die and affect tree appearance. Severe defoliation may also contribute to tree mortality.

The fall cankerworm species has a 1-year life cycle. Their larvae are slender and move with a looping motion. Fall cankerworm eggs overwinter on host trees and hatch into larvae in late May, when they begin feeding as the new leaves unfold. When fully mature at the end of June, they are 25 mm long and vary in colour from light green to dark brownish green, with longitudinal stripes. The mature larvae drop to the ground, spin cocoons in the soil, and pupate. They emerge as adults in October. Adult females are wingless, grayish brown, and about 12 mm long, while the grayish brown, adult male moths have wings with a span of about 30 mm. The adult females climb host trees and shrubs, and mate; each female then lays about 100 eggs on the upper twigs and branches.

Other information

A number of natural agents may control cankerworm populations by affecting the abundance of two species. Parasitic insects attack the egg, larval, and pupal stages of cankerworm life cycles, while other predators (insects, spiders, birds, and small rodents) may attack all the stages. Cold winter temperatures, late spring frosts, starvation, or disease may also cause the collapse of larval populations.

Canadian Forest Service Publications

Fall cankerworm

Information on host(s)

The preferred hosts of the fall cankerworm are Manitoba maple and American elm, but it also attacks ash, basswood, bur oak, Siberian elm (improperly called Chinese elm), aspen, white birch, and various fruit trees.