Spring cankerworm

Distribution

Western Canada

This is a native North American species, widely distributed throughout the southern half of the three Prairie Provinces.

Micro-habitat(s)

Leaf, Trunk, Soil

Damage, symptoms and biology

The spring cankerworm is a defoliator of many deciduous trees and shrubs in various types of rural and urban tree stands and plantings.

Damage usually begins in late May when young larvae chew small 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 spring cankerworm has a 1-year life cycle. The larvae are slender and move with a looping motion. The mature larvae overwinter in the soil and pupate in the very early spring. Adult wingless females and winged males soon emerge to mate, and the females climb host trees and shrubs, where they lay eggs in clusters on the stems and lower branches. Mature spring cankerworm larvae are about 20–30 mm long, and they vary in colour from yellowish green to almost black, with a mottled appearance.

Other information

A number of natural agents may control cankerworm populations. 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.

Main host(s)

American mountain-ash, ashes, basswood, black ash, blue ash, bur oak, common prickly-ash, European ash, European mountain-ash, fruit trees, green ash, mountain-ash, northern red ash, Oregon ash, pumpkin ash, siberian elm, Sitka mountain-ash, trembling aspen, white ash, white birch