Dendroctonus ponderosae, although considered a parasite and nuisance now, has actually co-evolved with Pinus trees for many years. The forest ecosystem actually began to rely on periodic infestations of mountain pine beetles, and the subsequent clearing due to forest fires. Now, however, predators of mountain pine beetles cannot control their populations. The destruction of Pinus trees affects more than just the trees themselves; animals in the forest ecosystem that rely on the trees for protection and shelter, such as deer and elk, are also affected by tree destruction.
Infestations of Dendroctonus ponderosae occur wherever Pinus trees are, whether they are in forests, on mountains, or isolated in yards. Due to the transportation of firewood, isolated stands of Pinus can become infected. After one tree is infected with mountain pine beetles, nearby trees usually succumb to the infestation as well. Trees inevitably die after infestation with the beetles, due to the catastrophic damage caused by the tissue ingestion of mountain pine beetle larvae as well as the introduction of the blue stain fungus by adult beetles. Blue stain fungus invades the phloem and eventually cuts off nutrient supply in the tree, resulting in tree death. Blue stain fungus, which is distributed by mountain pine beetles alone, stops the tree from using resin to remove the beetles from its phloem. In this way, blue stain fungus and mountain pine beetles are mutualists.
Signs of mountain pine beetle infestation include the presence of pitch tubes on the surface of trees in which the beetles have begun to dig egg galleries. Color of pitch tubes ranges from brown to pink or white. An increased presence of woodpeckers could also indicate Dendroctonus ponderosae infestation, as beetles serve as a valuable food source to the woodpeckers. Sapwood - the younger, outer portion of the tree - could also have blue stains, indicating that beetle infestation has occurred, and that the adult beetles introduced the blue stain fungus. The final sign of mountain pine beetle infestation is the transformation in color of the crown of infested Pinus trees to red or yellow. This is the last visible sign of infestation, as it does not occur until eight to ten months after the initial infestation.
There are some ecologists and landowners that do not necessarily see the damage by mountain pine beetles as totally negative. Ecologists point out that as a result of destruction of portions of Pinus forests, canopy overgrowth decreases, allowing understory vegetation to blossom and expand. This increases the plant diversity, which is beneficial for the forest ecosystem. Some landowners, meanwhile, prefer the growth of plants that normally cannot thrive under a dense canopy of Pinus trees.
Ecosystem Impact: parasite
Species Used as Host:
- Ponderosa and lodgepole pines, Pinus trees
- Blue stain fungus, Grosmannia clavigera
- Some nematodes, Nematoda
- U.S. Department of Agriculture: Forest Service. Mountain pine beetle dynamics in lodgepole pine forests - Part I: Course of an infestation. INT-89. Ogden, Utah: US Government Printing Office. 1980. Accessed July 03, 2011 at http://www.usu.edu/beetle/documents/48-Cole-Amman1980.pdf.
- Stone, W., M. Wolfe. 1996. Response of understory vegetation to variable tree mortality following a mountain pine beetle epidemic in lodgepole pine stands in northern Utah. Vegetatio, 122: 1-12.
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