Cankerworms are green caterpillars that dangle from trees in spring and illicit everything from curiosity to disgust from homeowners. They pose no threat to people, but they can defoliate landscape trees. In 2012, my home state of North Carolina and other Southeastern and Mid-Atlantic states had more cankerworms and defoliation than usual. In this article you will learn the basic biology of these critters and why management has to occur now – in the dead of winter.
Photos courtesy of S.D. Frank
Spring cankerworm (Paleacrita vernata) and fall cankerworm (Alsophila pometaria) are different species but have very similar characteristics. They both feed in spring on the leaves of deciduous trees such as oak, elm and maple. Cankerworms can completely defoliate trees, but 10 percent to 50 percent defoliation is more common. A year or two of defoliation will not generally affect the long-term health of large trees. However, they will be ugly the year they are defoliated, and it causes extraordinary concern among the public.
Spring and fall cankerworm eggs hatch in early spring, and caterpillars feed for four to six weeks. They spend the rest of the summer pupating in mulch and leaf litter beneath trees. In November or December, fall cankerworm adults emerge from pupae and climb up the trunk of nearby trees. They lay clusters of eggs on twigs then die. Spring cankerworms climb up trees to lay eggs in late winter and spring.
The female moths of both species climb because they do not have wings. You can use sticky bands to capture them on the way up and prevent them from laying eggs. No eggs means no caterpillars and no defoliation. Sticky bands are made by wrapping duct tape or similar product around trees and covering it with sticky Tangle Foot™.
A wingless cankerworm adult is stuck to Tangle FootT bands on a willow oak.
Bands need to be installed before fall cankerworms begin climbing up trees in late fall, but we do not have a good way to predict this. One strategy is to band a tree outside of your office or anywhere that you will see it everyday. When you see the first moth, go out and band all the trees you intend to. Once bands are installed, they can trap a lot of moths; I have found nearly a thousand per week on some trees. Eventually moths can walk across the bodies and up the tree. Therefore, maintenance of the bands by cleaning them, replacing them or reapplying Tangle Foot is required periodically. It is also best to install the bands after all the leaves are down so they do not get covered in leaves.
Most people don’t even notice cankerworm caterpillars until they become large and defoliation is severe. By this time though they are about done feeding and treatment will not make much difference. The only time to really affect cankerworm abundance and damage is in winter. So go stick it to them!
Steve Frank is Assistant Professor and Extension Specialist in the Department of Entomology at North Carolina State University. You can receive pest alerts and updates by following @OrnaPests on Twitter and checking his blog at http://ecoipm.com. He can be reached at firstname.lastname@example.org.