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Protecting Trees from Cankerworms

Source: NDSU Extension Service Horticulturist

When cankerworms get busy in the spring and early summer, they can chew up many of the leaves on certain shade trees in just a few days. Elm and apple trees seem to be their favorites, although cankerworms will not hesitate to feed on ash, oak and several other shade trees.

Spraying cankerworm-infested trees when the tiny greenish or brown measuring worms first appear in mid-May usually will provide control when approved insecticides such as malathion, Orthene, Sevin or BT Bactericides are used according to label directions. However, large trees may be too large to spray with small hand-operated spray equipment. If spraying is not a practical control measure, then the inexpensive "banding" method of control can be considered.

Why band trees? The female cankerworms are wingless moths. They emerge from the soil in the fall (mid- to late-September) if they are the fall cankerworms or in early spring (early- to mid- April) if they are the spring cankerworms. Both species are present in North Dakota. Soon after emerging from the soil, the wingless females crawl up the tree trunk into the branches and twigs where the eggs are deposited. The eggs of both species hatch in May. By placing a sticky tanglefoot bank around the tree trunk, the female cankerworms cannot crawl up into the trees. The result is no cankerworm larvae to feed on trees during the spring and early summer.

The sticky non-drying compound commonly called "tanglefoot" is available at most local hardware stores and nurseries in paste or aerosol form. Apply it in a continuous band around the tree trunk. A band 4 to 6 inches wide, 6 to 7 feet up from ground level, will do the job.

Apply tanglefoot directly to the bark. However, when the paste form is used, you must smooth the outer surface of the bark with a wood chisel, draw knife or other sharp tool. This method is probably not as desirable as the use of the aerosol form of tanglefoot since the tree will be somewhat disfigured. If the bark is smoother, keep in mind that it is only necessary to remove the outer ridges. Do not girdle the trees by removing inner bark down to the sapwood.

Banding should be done in the spring around early April and in the fall about mid-September.

A putty knife or old paint brush will work to apply the paste form of tanglefoot. The aerosol formulation of tanglefoot can be sprayed on tree trunks without smoothing the bark.

For further information, contact your county office of the NDSU Extension Service.

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