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Borers of Woody Plants

Almost all woody plants are subject to attack by insects that tunnel or bore into the stem, branches and shoots. Most borer problems occur in plants under stress. This stress may be a poor planting site, drought injury, construction damage or lawn mower wounds. Healthy trees attract fewer insects and can withstand some borer activity. The healthy tree can also flush out borers with a heavy sap flow.

The first indication of borer attack is usually some dieback or thinning of the tree. If you see the sawdust or holes in the side of the tree, this is also an indication that insects have invaded the woody portions of the plant. You can control some borer problems by pruning out the infested plant parts. Before you can begin any chemical treatment you should know what type of insect is involved.

There are only short times during the year when borers are susceptible to sprays. These times vary for each species. For many borer problems, spraying will not be the best answer for control. Insects that have already invaded the tree cannot be killed with chemicals. You will usually use chemicals to prevent further attack. Check with your local nursery person, arborist, garden store clerk or county extension agent for help in diagnosing the problem.

Some borers are secondary pests. They attack trees that are dying or dead from other causes and are not the ultimate cause of death. Controlling these secondary pests will do little to solve the primary problem with the tree. You should consider anything that will strengthen the health of the tree. Actions you can take include fertilizing, watering and pruning.

North Dakota birches, Scotch pines, honeylocust, ash and dying poplars are the most common woody plants attacked by borers.

For future reference, you may want to obtain a copy of Extension bulletin E-296, "Insect Pests of Trees and Shrubs," which is available at your county office of the NDSU Extension Service.

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