Cankerworm Diagnostic Guide
Paleacrita vernata (Spring Cankerworm)
Alsophila pometaria (Fall Cankerworm)
Spring cankerworm, Paleacrita vernata, and fall cankerworm Alsophila pometaria, often called inchworms because of the way they crawl, feed on buds and expanding leaves in the spring. The larvae’s droppings can also be a nuisance. Low populations of cankerworms do not cause extensive damage, but high populations can defoliate trees by late spring. Cankerworm populations build up over several years and then decline due to environmental causes. The feeding habits and damage are very similar for the spring and fall cankerworm, but the life cycles are different.
Trees at Risk
Cankerworms have a wide host range, but prefer elm, apple, dogwood, cherry, variety of oaks including Willow oak, linden, ash, hickory, hackberry, maple, and beech.
Signs of Damage
- Small holes on leaves in spring are caused by young larvae.
- Complete defoliation, except for the leaf veins, may occur by late spring by older larvae.
- The spring and fall cankerworm are very similar in appearance.
- They are often called inchworms because of the way they crawl.
- They arch their back into a loop and extend the front to move forward.
- Fall cankerworm adults emerge beginning in late winter and lay eggs in the crown.
- Spring cankerworm adults emerge in early spring and lay eggs in the crown.
- Eggs hatch in mid-spring at about the same time as the opening of elm buds.
- Larvae feed on leaves for about four weeks before dropping to the ground to pupate in the soil where they will emerge the next spring as adults.
Healthy trees can withstand defoliation since cankerworm damage is done early in the spring and trees have the chance to leaf out again. Control is recommended for new transplants, high valued specimen trees, or fruit and nut bearing trees. Trees that have a previous history of stress such as defoliation, drought, or disease should also be treated. Trees should be confirmed with larvae and damage before treatment is necessary.
Chemical treatment is most effective in the second week of feeding after egg hatch. Damage is minimal and caterpillars are still small at this time, so close inspection is needed. During the third and fourth weeks after egg hatch, cankerworm damage becomes extensive and very noticeable. Treatment is not effective at this time because the damage is already done.
Other Treatment Practices
- Promote health and vigor with proper irrigation, mulching, proper pruning and prescription based fertilization practices.
- Horticultural oil sprays can be effective against cankerworms eggs.
- Soil injections of Lepitect™ will begin efficacy within 3-10 days for most pests. Control on larger trees will not occur as quickly (2-3 weeks). Lepitect™ soil applications last for 30 days after treatment. Repeat applications may be necessary if pest activity persists for longer than 30 days.
- Conserve™ has a short residual (4-7 days) but is softer on beneficial insects, whereas Up Star Gold™ will provide slightly longer residual, but is broad spectrum.
- Use Lepitect™ infusible on trees that cannot be sprayed or treated with soil applications.
A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org