The mountain pine bark beetle (Dendroctonus ponderosae) is endemic (usually present at low levels) to Western North America. The adult bark beetle bores into the bark of pines (particularly lodgepole pines) and lay its eggs in the inner bark. The larvae eat the inner bark, sucking nutrients from the phloem, the portion of the tree that transports food. In addition, the beetle carries a fungus that moves into the xylem and affects the water carrying capacity of the tree. The “tracks” of the larvae in the inner bark and the dark fungus “fingers” are readily visible once infected trees are cut down. Severely infected trees with compromised nutrient and water transport weaken and die.
The infected tree produces and extrudes resin at the points where the beetles attack the bark in an attempt to push the infesting beetles back out of the bark. This defense mechanism, along with the health and vigor of the trees themselves, insects that prey on the beetles and beetle nematode diseases normally keep the mountain pine bark beetle population under control. Occasionally environmental conditions (warming and drought) weaken the trees and favor bark beetle epidemics resulting in large die-offs of pines.
Currently the drought in California combined with recent warmer weather has weakened many trees resulting in an explosion of mountain pine bark beetle populations. These bark beetle “symptoms” were photographed on lodgepole pine (Pinus contorta) trees at Medicine Lake (Siskiyou County CA). The US Forest Service is marking and removing infested trees, applying bark beetle pheromones to infested trees and spraying trees with insecticide in an effort to prevent healthy trees from being attacked.
We are all hoping for an end to the drought soon.