The Big Sandy Mountaineer -

Bear Paw and Highwood Mountain Land Owners Should Scout for Dying Fir Trees

In October of 2017, the Bear Paw Mountains were buried with three feet of snow. Along with the snow came severe damage to many Chouteau County conifers. After the damage was completed, the Douglas-Fir Beetle (Dendroctonus pseudotsugae) moved into damaged tree locations where they reproduced and rapidly built up their numbers. In the spring of 2018, D. Pseudotsugae adults moved from the damaged trees to neighboring large trees. First the needles turned yellow, and then reddish brown. The end result was a small population of dead fir trees. Land owners need to take some time this fall to scout for and manage dying fir trees. If infested and not managed this fall, fir tree mortality will increase dramatically in 2019.

Similar to the Mountain Pine Beetle (Dendroctonus ponderosae), Douglas-Fir Beetles (DFB) feed in the phloem layer of inner bark girdling the tree. Tree girdling usually leads to mortality. In addition, various fungi introduced by the beetles also contribute to mortality of infested fir trees. DFB often kills groups of adjacent trees in successive years. Susceptible trees include trees greater than 12 inches in diameter or older trees that are in poor health with little remaining foliage.

Evidence of DFB damage is red-orange boring dust in bark crevices or on the ground around the fir tree. Pitch streamers that look like spaghetti are occasionally observed high in the tree. Most broods overwinter as adults in the outer bark. The primary attack period is from Mid-April to early June.

Beetles that overwinter as larva attack in mid-summer. Adults are about ¼ inch long and are black with red-brown wing covers.

In some cases, it is feasible to reduce the threat to live trees by killing beetles within infested trees before they exit. Invaded trees need to be cut down this fall, cut into one-two foot lengths, split or chipped and removed from the area. Harvested lumber can be used as firewood. If trees are still green, the fire wood needs to be moved at least a mile or more from susceptible fir trees.

Information for this article came from the Field Guide to Diseases and Insect Pests of Northern and Central Rocky Mountain Conifers, written by Susan K, Hagle and others. One other valuable resource used for this article is the Forest Insect and Disease Leaflet 5 written by Richard F. Schmitz and Kenneth E. Gibson from the USDA. For further information or verification of DFB infestations, please contact Tyler Lane at the Chouteau County Extension office at 622-3751. You are also welcome to stop in for a visit. The Extension office is located in the green building next to the Chouteau County Courthouse.

Montana State University U.S. Department of Agriculture and Montana Counties Cooperating. MSU Extension is an equal opportunity/affirmative action provider of educational outreach.