Identification

• Cankerworms are 1/4 to 1 inch long. The two types are the spring cankerworm and the fall cankerworm. Cankerworms are frequently called inchworms because of the distinctive way they crawl. They stretch their body, then pull the back end toward the front creating an arch. Spring cankerworms are yellow or light green and fall cankerworms have a distinctive dark brown stripe down the back. Before they pupate, the mature worms lower themselves to the ground on a thread of silk. They are easily seen as they dangle from the tree in early summer. In severe infestations, the fracas of hundreds of worms is visible below the tree.
  
  

Life Cycle

• Adult fall cankerworms emerge from the ground on a warm day in late fall or early winter. Spring cankerworm adults emerge in early spring. The wingless female climbs up the trunk of a tree and mates with a winged male. Once mating occurs, the female lays clusters of eggs under the bark of the tree. As the eggs hatch in late spring, the larvae feed on the foliage of the tree. If the worms fall from the tree, they can be seen feeding on shrubs and grasses below the tree. Once the larvae mature, they use a silk thread to lower themselves to the ground. They burrow into the ground to a depth of up to 4 inches where they pupate. The adults emerge once again in fall or early spring, depending on the type of cankerworm, and start the cycle anew.
  
  

Significance

• Cankerworms are voracious eaters and skelotonize new and mature leaves of oaks and other trees and shrubs in the landscape. As the worms mature and their appetite increases, an entire tree, landscape or a large section of forest may be affected. A heavily infested tree spends considerable resources refoliating and may die if the infestation is followed by another stressful event, such as drought. Cankerworm infestations may be severe for one year or several years in a row, then hardly noticeable for years afterward.
  
  

Control

• Because cankerworm infestations are unpredictable, they are difficult to control, especially in a large landscape. If an infestation is under way, spray the infested tree and area below the tree with Bacillus thuringiensis (Bt) or spinosad. They are both biological controls designed to stop worms from feeding on foliage. Spinosad remains active longer than Bt. Horticultural oil sprayed on the limbs of the dormant trees in early spring will smother the eggs of the cankerworm. Sticky pest traps applied to the trunk of the tree will trap the females as they leave the ground to mate.