Mountain Pine Beetles Affect Lodgepole and Whitebark Pines

Red Trees Conspicuous in Grand Teton

August 21, 2008
A number of red-colored trees interspersed throughout the conifer forests of Grand Teton National Park have captured the attention of visitors and local residents and prompted many questions about what is happening. These conspicuous red trees stem from a cyclic, natural phenomenon caused by mountain pine beetles, a native insect whose activities kill individual lodgepole and whitebark pine trees by damaging the phloem layer and cutting off the flow of water and nutrients between the roots and needles. Periodic outbreaks of mountain pine beetles play an important ecological role in the Greater Yellowstone Ecosystem and are central to the life cycle of western forests.

The mountain pine beetle—the most common bark beetle and a natural part of forest ecosystems in western North America ranging from Mexico to British Columbia—evolved with lodgepole pines over the millennia. Climate plays a role in the proliferation of beetles, and warmer air temperatures of recent years have allowed bark beetles numbers to expand into higher elevation ecosystems, as well as to flourish where they have historically occurred. Vast numbers of beetles are needed to kill a single tree, and beetle numbers are sustained by the availability of suitable hosts, such as drought-weakened trees, as well as by mild winters that ensure successful beetle reproduction. While hearty pines typically use their resin as a defense against the boring activities of beetles, trees can become overwhelmed by prolific beetle outbreaks. Periodic increases of this insect, and the subsequent tree mortality, are part of a naturally occurring cycle. In fact, similar beetle infestations occurred throughout the Rocky Mountain West and in Grand Teton National Park during the 1930s and 1960s. Biologists and ecologists acknowledge that mountain pine beetle outbreaks help to create a mosaic of forest types and ages, and to maintain nutrient and energy cycling in a natural ecosystem, much like other natural events: fires, avalanches and microburst winds that topple large tracts of trees.

Currently, beetle-affected trees are most prevalent in mid-elevation lodgepole pine forests. This year, Grand Teton National Park is attempting to reduce beetle infestations in some key visitor use and developed areas by hanging white pouches containing a pheromone called verbenone on selected trees. Two pouches per tree (or 40 per acre) send a chemical message that a specific tree, or a general area, is already saturated with beetles; the pheromone application may reduce the likelihood that additional trees will be attacked. Lodgepole pines chosen for this treatment include: trees that are located in high visitor use or administrative areas where lodgepole pine is the predominant tree species; trees that were preserved during construction projects; and trees that contribute to the character of a specific area, such as picnic sites. The success and value of this strategy will be tracked and further assessed in the fall of 2008.

In the Jackson area, mountain pine beetles began attacking high-elevation whitebark pine forests about four years ago. Impacts to this species are extensive and ongoing. Park biologists are also using verbenone pouches to try to protect small numbers of individual whitebark pines that appear to be genetically more resistant to white pine blister rust, thereby preserving the seed source for the future. In the past, the high-elevation environment of the whitebark pine prevented mountain pine beetle from becoming abundant enough to kill more than a few trees at a time. However, warmer winters and longer growing seasons related to climate change have allowed the beetles to produce many more offspring in a faster period of time. Park biologists will also monitor the effectiveness of pheromone use among whitebark pine stands.

While the mountain pine beetle activity in whitebark pine ecosystems is atypical, the effects of beetles on local lodgepole pine forests are more common. There is some concern that an abundance of beetle-killed trees may increase the risk of wildland fires. Although extensive stands of dead trees can be flammable, current research suggests that once the red needles have fallen from the dead trees, the fire risk may be reduced in some situations.