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Bark Beetles

By Bailey Walten 1992 Newsletter

Bark beetles represent more than six hundred species of beetles, some of which are terribly destructive to deciduous as well as coniferous trees. Some varieties such as the European Elm Bark Beetle can also be important disease vectors (Dutch Elm Disease). This past Summer we saw heavy outbreaks of certain species of bark beetles in several varieties of trees; in particular, red oaks and white pines. Under severe drought stress, weakened trees send out resinous signals that can attract these beetles which bore into trees and lay eggs along the main tunnel under the bark. These eggs hatch and the larvae bore their own galleries radiating off the main tunnel. These larval tunnels get larger as the insect grows and form characteristic patterns under the bark according to the species involved. After the larvae mature, they will pupate and emerge as adults from the tree leaving small exit holes resembling damage that might be left by a shotgun blast. It is this feature that gives rise to the common name, “shothole borers”, referring to these beetles.

The tiny cylindrical beetles are usually brown or black and less than 1/4″ long. One beetle can cause little damage to a tree but by virtue of sheer numbers they can overwhelm even a healthy host by girdling and cutting off sap flow because attacking beetles will emit pheromones that attract others to the site. Some species produce six or more generations per year. Once beetles have successfully bored into trees, control is very difficult because the larva and adults are safely shielded under the bark from pesticide sprays. The best controls are obtained by keeping trees healthy with regular fertilization and watering during dry periods along with prompt removal of infested hosts thus reducing their spread to neighboring trees. The best control at this time is preventative sprays using “Astro”, an insecticide which penetrates the wood to prevent many new generation adults from initially penetrating the bark of the target trees. In this case an ounce of prevention IS worth a pound of cure!

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