Western Forest Threats

What is killing this pine and how?

If you see red or gray forests in the high elevations of the western mountain landscape, you may be witnessing the effects of white pine blister rust infestation or mountain pine beetle infestation.

Blister Rust

Grav Skeldon

Blister rust cankers prevent water and nutrients from reaching the branch of a whitebark pine. © Grav Skeldon

A pine tree suffering from white pine blister rust can be difficult to identify from a distance. There are three main questions to ask to determine if a tree is infected with this disease:

  1. Is the tree a five-needle pine? White pine blister rust only infects five-needle pines — pines with needles mostly in bundles of five.
  2. Is only a portion or the top of the tree red? The disease attacks one branch at a time, so the tree often dies in stages.
  3. Are there are orange-yellow cankers and flaking or missing bark located on the branch with red needles? The disease produces cankers and increases the bark’s sugar content, making it attractive food for insects and rodents.


How does blister rust kill a five-needle pine?

Blister rust is a non-native fungus introduced into our forests in the early 1900s. Blister rust has a complex life cycle that requires two hosts: a five-needle pine and an alternate host — most commonly a currant or gooseberry plant. First, the fungus enters a pine through its needles, ultimately growing in size and spreading to a twig, then a branch and finally to the trunk of the tree. It forms ugly cankers, or blisters, which girdle the stem and prevent water and nutrients from passing through the diseased area. These blisters rupture and release bright orange-colored aeciospores which infect the alternate host. While on the alternate host plant, the rust produces a type of spore, which is released and infects additional tree when the plant naturally loses its leaves in the autumn.

Branches that cannot receive water or nutrients slowly die, first turning red and then gray. If a canker forms on the trunk, it will kill all of the branches above it, causing “topkill.” Blister rust infections can persist within a tree for years. Though the disease may take nearly a decade to kill a large tree, it can kill small or young pines within just a few years. During the progression of the disease, branch death and topkill can significantly reduce cone and seed production, harming five-needle pine populations by removing their reproductive capability.

Mountain Pine Beetle

Mountain pine beetle is the most destructive of the three types of bark beetles in the Greater Yellowstone Area. Hence, it is American Forests’ focus in restoration and protection.

How do I identify mountain pine beetle infestation?

When trying to identify whether or not a five-needle pine has been infested by mountain pine beetles, one should start with three questions:

1. How big is the tree? Though these beetles are interested in all western pines, they can only attack trees of a certain size. If you are looking at red or gray adult trees 20 cm in diameter or larger, you may be looking at a mountain pine beetle infestation.

2. Is there are series of red or graying trees? Beetles often kill in large swaths.

3. Are there small holes in the bark? This is where the beetles entered.


How do mountain pine beetles kill pine forests?

Millions of acres of forest have suffered from mountain pine beetle attack.

Mountain pine beetles disturb pine trees by laying eggs that hatch into larvae under the bark and by introducing a blue stain fungus into the tree, effectively girdling it and blocking water and nutrient transport. The combination of the fungus and larvae feeding on the tree can successfully kill it. When a tree is first attacked, it remains green. Within a year of attack, the tree will be dying or dead and the needles will turn red. In three to four years after the initial attack, the trees will appear gray.

Mountain pine beetles are actually native to our western forests. In the past, they have played an important role in the Mountain West ecosystem, attacking old or weakened trees, which helps the development of a younger forest. While young healthy trees can sustain an attack from beetles, current beetle levels are unprecedented, causing trees that would normally resist an attack to be at risk. Infestation is estimated to be 10 times greater than previous rates. In addition, many trees are already weakened by white pine blister rust or drought, making them more susceptible to beetle attacks.

These unprecedented numbers of beetles are largely due to rising temperatures. Mountain pine beetles’ eggs hatch into larvae, transform into pupa and grow into adulthood all beneath the tree bark, only emerging as adults to attack a new host tree. For the beetle to thrive, it must complete development in one year. A one-year life cycle places the emergence of new adults and their subsequent attacks on trees in mid-to-late summer. This allows sufficient time for the eggs to hatch and develop to the larval stages before winter, as only the larvae are capable of surviving winter temperatures. Recently, warming mountain-top temperatures have allowed beetles to complete a full life cycle and more than just larvae to survive winter. So we are now seeing two periods of attacks on trees: an early flight of surviving adult beetles in spring, followed by a later flight of the new adult beetles in summer.

Some five-needle pines try to drown or flush out attacking beetles by pitching out their resin. However, certain species like whitebark pine produce little to no resin and can be overwhelmed. In addition, as the beetles bore into trees, pheromones are released attracting additional beetles to the same tree. This continuous cycle can result in a mass attack that overcomes the tree’s defenses and results in attacks on adjacent trees.