When MPB attacks a tree and lays eggs, the parent adult and larvae carve out galleries in the inner bark (phloem) and other sapwood of the tree. The phloem of a tree carries the food produced by the needles throughout the tree and into the roots. The galleries that are produced interfere with this movement of needed food through the tree. The sapwood is the outer rings of wood that carry water from the roots up the tree to the needles. The galleries that cut into the outer sapwood do not have much impact on water movement in the tree. However, MPB carry fungi with them in their bodies that they inoculate into the tree. These blue stain fungi rapidly grow throughout the sapwood and phloem. They interrupt the flow of water in the tree which reduces the amount of pitch that is produced. Pitch is the main defense of a tree against the bark beetle. It is the combined work of the beetle boring and the blue stain fungi that kills a tree.
Large outbreaks of this beetle are common, especially in lodgepole pine. Most trees that are attacked are killed, but some may be pitched out or strip attacked. An unsuccessful attack is usually characterized by few pitch tubes (<5) and little to no boring dust. A strip attack is a vertical portion of the bark that is infested by MPB, but the entire trunk is not involved. Trees less than 5 inches in diameter are seldom attacked.
Indicators of Mountain Pine Beetle Attack
It is important to recognize trees infested by MPB before the beetles have left.
The first evidence of MPB attack is the presence of pitch tubes on the trunk. These range in color from white to pink to brown and are about the size of a nickel to a quarter. They can be found at any height on the trunk. Pitch tubes produced in the current year are very sticky. Single, white pitch tubes usually indicate an unsuccessful attack.
|Fresh Pitch Tubes on Lodgepole Pine||Successful attack by MPB
Note reddish-brown resin
|Unsuccessful attack by MPB. Note white resin and beetle trapped in resin|
S. Kegley, USDA Forest Service
Other evidence of MPB presence is boring dust in bark crevices and on the ground immediately around the tree base. This dust is reddish-brown and granular.
|Bark Flaking from Woodpecker||
Pitch Tubes on Mugho Pine - Helena
Ian Foley USDA
Evidence of woodpecker feeding on the trunk may also be present. Patches of bark are removed and bark flakes lie on the ground or snow below the tree.
Foliage turns yellowish to reddish throughout the entire tree crown. This usually occurs eight to ten months (May to July) after a successful MPB attack.
Live MPB (eggs, larvae, pupae and/or adults) as well as galleries can be found under the bark. This is the most certain indicator of infestation. A hatchet for removal of bark is needed to check trees correctly. Under the bark, look for straight, vertical galleries with a hook or "J" at the lower end. They are packed tightly with boring dust.
|Colleen Keyes USDA|
A Field Guide to Diseases and Insect Pests of Northern & Central Rocky Mountain Conifers - USDA Forest Service
Mountain Pine Beetle Identification and Management - Montana State University Extension
Mountain Pine Beetle Infesting Urban and Shelterbelt Trees - Montana State University Extension