Fighting Defoliators One Worm at a Time

Fighting Defoliators One Worm at a Time

May 25, 2007

The new leaves are a welcome harbinger of spring. Green cankerworms eating the leaves are just as seasonal but not nearly as welcome. The process of controlling cankerworms starts in the spring and will result in decreased activity next year. You are not alone in your efforts. A number of parasitic insects feed on the eggs, larvae and pupas of these insects. Spiders, birds and small rodents also feast on the cankerworms at various times of the year. While all this eating is beneficial it does little to curb an epidemic population of worms. That is usually controlled by weather and soil conditions. Favorable weather conditions, like cold winters and late season frosts may control populations naturally. All of these factors combine to create a cyclical population boom and bust. Rather than waiting for the weather, or just talking about the weather, there are steps that you can take on your property to reduce the extent of the infestation next year. The first step is to arm your self with information on the life cycle of the two cankerworm species. The primary culprit that turns your elm, ash, or maple into a communal salad bar is the fall cankerworm, (Alsophila pometaria).

The life cycle of the fall cankerworm begins with the eggs over wintering in the upper branches of the host tree. The eggs hatch in late May as the new leaves unfold. Warm weather in spring will speed up the emergence of the leaves and the larvae. It takes approximately three weeks for them to mature and then the larvae spin a thread and drop to the ground like super heroes. When they reach the ground they spin a protective cocoon and change into pupae. Pupas are a stage in the development of insects where they change into moths. They emerge in September or October, as male and female moths. The males have wings and the females do not. At this point they have only to mate and lay eggs to start the process again. While the males are off being distracted by lights and flames, the females begin the long crawl to the top of the host tree to prepare to mate and lay eggs.

The spring cankerworm (Paleacrita vernata) has a similar life cycle with the larvae over wintering in the soil and pupating in spring. Both forms have wingless females. The female moth’s wingless crawl presents an opportunity to control both spring and fall forms of this insect. The application of sticky bands around the trunk of the tree will trap the females and prevent the deposition of eggs in the upper crown.

Start in mid summer, before the September long weekend, by lightly brushing off loose bark in a 6 inch band at 5 to six feet up the trunk of the tree. Do not girdle the tree by removing all the bark. You can then wrap glass wool, backed with stiff paper or foil in a four inch band around the tree. It can be secured with natural fiber like jute twine at the top and bottom of the strip. It is important to secure the band tightly to avoid the moths crawling under and not to tight as to injure the tree. Once you have the band in place paste sticky tangle foot, available from nurseries, evenly on the band. This will trap the females, so be sure to check it often to make sure there is no bridging over the trapped females. This would allow late arriving females to pass over the band. Reapply the tangle foot as necessary in the early spring to catch the spring cankerworm. Remove the bands in summer and reinstall in late summer. This will allow the tree to grow, and not damage the bark during the growing season. With single isolated trees this can significantly reduce the population of worms the following year. In a group, all trees will have to be banded.

When out breaks are severe, spraying with various products, as directed on the label, will give additional control. Btk, a bacterial insecticide works well when the insects are actively feeding.