Cankerworms
Click for larger image Cankerworms (Lepidoptera) feeding on apple foliage (Malus)

The two most destructive species of cankerworms (a kind of inchworm) in the St. Louis area are the fall cankerworm and the spring cankerworm, both of which feed in the spring. They attack many different species of trees and shrubs.

Symptoms and Diagnosis

Leaves are chewed. Complete defoliation and crop loss may occur; or after repeated damage, less and less fruit is produced until little, if any, fruit matures. Damage is first evident in the early spring or summer after trees have fully leafed out. This is about the time that larvae of both species emerge and begin to feed.

Life Cycle

Cankerworms are small caterpillars, only getting up to an inch long. Adult males are small, gray moths with a wingspan reaching only 1 inch long. The females of both species are wingless.

FALL CANKERWORM: The adult fall cankerworm, Alsophila pometaria, emerges from the ground in late November or early December. The wingless adult females climb up the trunk of the host tree and lay barrel-shaped eggs in masses on twigs and branches. The eggs hatch and larvae begin feeding about the same time leaves begin to emerge. The larvae drop down and dangle on silken threads until swept away by a breeze. The ballooning larvae may travel from plant to plant until a suitable host plant is found. The larvae feed for about a month before they fall to the ground to pupate. The worms have brown backs with white stripes running lengthwise down their backs. The fall cankerworm has three sets of prolegs with the foremost proleg severely stunted. This distinguishes it from the spring cankerworm that has only two pairs of prolegs.

SPRING CANKERWORM: The adult spring cankerworm, Paleacrita vernata, emerges in late February or early March. The wingless adult females climb up the trunk of the tree and lay barrel-shaped eggs in cracks in the bark and underneath the bark and bud scales. Yellow-striped green worms hatch at the same time as fall cankerworm larvae and follow the same general feeding pattern. They too drop to the ground where the larvae overwinter in a protective cell. Spring cankerworm larvae have only two pairs of prolegs.

Integrated Pest Management Strategies

1. Sticky barriers may be banded around tree trunks. Apply a band of a sticky material, such as Tanglefoot, around the trunks of trees in October. The sticky material will trap the wingless females of the fall cankerworm moths as they crawl up the tree trunk to lay eggs in late November or early December. Renew the traps in February to trap the emerging female adults of the spring cankerworm that climb the trunk to lay eggs in late February or early March.

2. Prevent larvae from emerging. In the spring apply a horticultural oil spray on the tree before the larvae hatch, about the time trees just begin to leaf out in the spring.

3. Spray Bt (Bacillus thuringiensis) or Sevin after leaves are fully expanded and flowering is finished. Repeat this application two more times, at 14-day intervals. Sevin is very toxic to bees, hence spraying should be delayed until after flowering to limit damage to these beneficial insects.

Organic Strategies

Strategy 1 is a strictly organic approach. For an organic approach to Strategy 3, consult the Organic Materials Review Institute (OMRI™) for appropriate Bacillus thuringiensis (Bt) products.

More images

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Cankerworm (Lepidoptera) stretched out on apple foliage (Malus)
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Cankerworm (Lepidoptera)
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Cankerworm (Lepidoptera)
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Apple foliage (Malus) damaged by cankerworms (Lepidoptera)