Bark Beetles and Borers

Description
Most borers that tunnel in wood or soft stems are the larvae of beetles or moths. Their presence is usually indicated by the sap or sawdust that surrounds holes in the tissue, and by wilting and dying foliage on the affected stems or branches. Hundreds of different kinds of borers infest trees, shrubs, and herbaceous plants. Borers that infest trees and shrubs usually favor weak, wounded, or newly planted specimens. However, borers commonly attack healthy flowers and vegetables as well. Vigorous trees and shrubs are often able to resist borer infestations. When the larvae try to burrow into such plants, they are overcome by oozing plant sap. Weak plants, such as those that have damaged roots or are suffering from drought, have reduced sap flow, making it easier for borers to enter them and develop inside. There are hundreds of different species of beetle borers that infest plants. They fall into two general categories: bark beetles and flatheaded or roundheaded borers.

Bark beetles: These beetles mainly attack and kill weak conifer trees, but are also capable of attacking and killing a healthy tree when present in large numbers. The larvae are white or cream and from 1/16 to ¼ inch long. Adult bark beetles are the same size and usually black, brown, or dark red. The adults tunnel into and under the bark and lay eggs along the tunnels. The larvae hatch and generally bore away from the parent tunnel, creating characteristic tunnel designs that can be used by experts to identify the kind of bark beetle involved.

Roundheaded or flatheaded borers: These larger insects attack severely weakened trees or freshly killed trees. The larvae, usually white or cream colored, are approximately ¾ inch in length at maturity. Some have heads that are flattened and triangular; others have rounder heads. Adults are from ¼ to 1½ inches long and are often metallic or brightly colored. The larvae bore tunnels that travel in a meandering, random path, mostly in the wood but also just underneath the bark.

Control
The best control for borers in many herbaceous plants is to cut out and destroy infested stems and remove and destroy dying plants. To control with chemicals, sprays must be applied before the insects burrow into the plant. The time varies with different species. For timing of sprays, look up your plant in the index, or contact your local County Extension Agent (see County Extension Agents). Many woody plants can be sprayed with an insecticide containing lindane. Spray herbaceous plants with an insecticide containing carbaryl. Make sure your plant is listed on the product label. Keep plants in a vigorous growing condition by fertilizing and watering regularly, and by controlling diseases and other insects. Avoid wounding plants. When pruning woody plants, do not leave stubs that can attract or harbor borers.