Cankerworms

Fall cankerworm larva. Fall cankerworm larva.

Spring cankerworm larva. Spring cankerworm larva.

Fall cankerworm female moth laying eggs. Fall cankerworm female moth laying eggs.

TREE HOSTS

  • Apple
  • Ash
  • Cherry
  • Elm
  • Honeylocust
  • Linden
  • Maple
  • Boxelder
  • Red and White oak
  • And many more deciduous trees and shrubs

DESCRIPTION

Cankerworms, also known as inchworms, loopers, or spanworms, are native, green caterpillars that feed on the foliage of many deciduous trees in forested or urban settings. There are two species that occur in Utah, the spring cankerworm (Paleacrita vernata) and the fall cankerworm (Alsophila pometaria). These pests cause defoliation during periodic outbreaks. Both species have similar habits and are named based on the season when they lay their eggs. Trees can tolerate complete defoliation in spring as they will push out a new set of leaves. However, if trees have been defoliated multiple years in a row or lack vigor, death is more likely.

BIOLOGY

Cankerworm larvae are only present in spring for 4 to 6 weeks. Cankerworm adults are moths that are brownish-gray in color. Females are wingless, while males have wings. Because females lack wings, they crawl upward into trees to deposit their eggs. Spring cankerworms lay loose clusters of eggs in spring, while fall cankerworms lay compact masses of barrel-shaped eggs in the fall.

For both species, eggs hatch toward the end of May. There is only one generation per year. After feeding on leaves and buds for 4 to 6 weeks, the mature larvae crawl down the trunk or drop to the ground by silk threads. In the soil, they transform into pupae. Fall cankerworm moths emerge from the soil in early fall to lay eggs while spring cankerworm pupae remain in the soil all winter, and emerge a few days after spring snowmelt.

SYMPTOMS

  • Small, random holes in leaves
  • Larger holes between leaf veins
  • Complete defoleation

GENERAL MANAGEMENT

Typically in Utah, management is not needed for cankerworms. Every 7 to 10 years, there can be localized outbreaks that may require treatment. To prevent egg-laying, create sticky barriers (similar to those used for gypsy moths) around tree trunks. Cotton batting can be placed under the sticky band if the bark has deep grooves. Avoid placing sticky material directly on the bark to prevent girdling.

Caterpillars can also be managed by a foliar (organic) insecticide spray of a product containing spinosad or Bacillus thuringiensis (Bt). There are many brands that contain either of these products. Apply when larvae are small (less than ½”) as the caterpillar must consume the product to be effective.


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