Bark beetles are the most destructive insects in the coniferous forests of the Southwest. There are many bark beetle genera, of which the most important with respect to forest damage are Dendroctanus, Ips, and Scolytus. Adult bark beetles bore through the outer bark to the inner cambial layer, where they channel out galleries in which to lay eggs. Larvae hatch in these galleries and may excavate additional channels as they feed. As bark beetles carve out galleries, they introduce blue-stain fungi. This fungi grows in the wood, interfering with the tree’s water transport system. Tree deterioration and eventual mortality result from two factors: tree girdling caused by gallery excavation, and spread of blue-stain fungi. Several species of bark beetles may attack in concert, partitioning the tree by elevation. Roundheaded pine beetle, western pine beetle, mountain pine beetle, and several species of Ips may all be found on severely infested trees. Infested trees may be recognized at a distance by fading foliage high in the tree, initially a light green, changing to a light straw color in a few weeks, and eventually to yellowish-brown. Close inspection may show a fine reddish-brown boring dust in bark cervices and at the base of the tree. Small pitch tubes, or globules of pitch may be seen on the tree trunk. Cream to dark red pitch tubes, resin mixed with boring dust, ¼” to ½” in diameter, are an indication of a successful bark beetle attack. In some cases where the number of attacking bark beetles is not high, the tree may have sufficient resin available to eject the attacking bark beetles by extruding resin at the attack site (“pitching out”). Pitch tubes of whitish resin ¾” or more in diameter are evidence of an attack successfully resisted. Other evidence of bark beetle infestation includes galleries discovered under the bark, sapwood discolored by blue-stain fungi, woodpecker feeding holes and bark removal by woodpeckers. Below are some links for information about the region’s bark beetle problem and some valuable material.
Thank you to the UofA and Patrick Rappold, Area Assistant Agent – Forest Health for the information above.
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