Mountain Pine Beetle

Forests across Colorado's Rocky Mountains look very different today than they did twenty years ago. Millions of trees have been killed by mountain pine beetles (Dendroctonus ponderosae). The mountain pine beetle is a native insect that is roughly the size of a grain of rice. Large populations of this tiny bug have had a huge impact on forests in Colorado.

Life Cycle

The beetles live most of their lives in the bark of pine trees, including lodgepole pine (Pinus contorta), ponderosa pine (Pinus ponderosa), limber pine (Pinus flexilis), whitebark pine (Pinus albicaulis), and bristlecone pine (Pinus aristata). Larvae hatch from eggs laid under the bark in early autumn. They spend the winter and spring as larvae feeding on the phloem (the tissue that carries the tree’s nutrients). By midsummer, they become adult beetles and emerge from the bark to find a new home in another tree. There, they mate and reproduce. Beetles generally infest large, fully grown pine trees that are eighty years or older.

In order to reproduce successfully, the beetles must overwhelm pine trees’ natural defenses. Healthy trees can push out and kill beetles with their sap and chemicals called terpenoids. To overwhelm tree defenses, hundreds of beetles attack a single tree. They use compounds derived from the tree’s defense chemicals to create a chemical signal that tells other beetles to join them. Once there is no room for any more beetles in the tree, its occupants send a chemical signal that the tree is full. Beetles then attack a neighboring tree en masse. This pattern leads beetle-caused tree mortality to occur in patches. Trees are killed primarily by a fungus that the beetles carry. The fungus infests the tree’s wood and stops water transport from the roots. Needles of killed trees turn red within one to two years after attack, and fall off within four to seven years.

Most of the time, beetle populations exist at low levels. Under these natural conditions, beetles typically kill small patches of mature older trees. Many of these trees are already stressed, diseased, or damaged. Cold winter temperatures can kill the beetles. It is difficult to pinpoint a temperature at which beetles are most likely to die, but late fall or spring temperatures below -20 degrees Fahrenheit or colder typically kill most larvae. But several years without cold snaps allow beetle populations to grow. Prolonged drought taxes the defense systems of many older pine trees, creating favorable conditions for beetles and resulting in outbreaks that cover large areas. Such outbreaks occur periodically in Rocky Mountain pine forests. They often reoccur every thirty or forty years.

Recent Outbreak

The recent mountain pine beetle epidemic has killed millions of acres of pine forest from British Columbia to Colorado. It is generally thought to be larger and more severe than previous outbreaks observed throughout the twentieth century. The recent outbreak is the result of a perfect storm of conditions for beetle population eruption: several years of warmer-than-normal winter temperatures allowed beetle populations to increase, and drought left trees with reduced defenses. At the same time, large expanses of lodgepole and other pine trees across North America were reaching an age ripe for beetle infestation. This is largely because many forests were cleared or burned during the early days of European American settlement in the late 1800s, and these forests are all roughly 100–150 years old. There is general consensus that both forest structure and climate played a role in the recent outbreak. However, there is disagreement about the relative importance of each.

Effects on Forests

Although mountain pine beetles killed many trees, these forests are recovering and full of life. Trees of all species are regenerating. The trees that survived the outbreak are growing faster than ever, thanks to all of the additional light, water, and nutrients that were previously being taken up by the now-dead canopy pines. Further, the forest conditions created by beetle-caused mortality provide excellent habitat for many wildlife species, such as woodpeckers, pine martens, and snowshoe hare.

Effects on Fire

Mountainsides covered with recently killed red-needled pine raised concern about increased wildfire risk in Colorado. Once the needles fall (three to seven years after outbreak), beetle-killed trees do not change or may actually lower the risk of fire ignition because fire is less able to spread from crown to crown. Fire is limited primarily by climate in lodgepole forests. There is always enough fuel, but weather conditions that allow large fires to burn are relatively rare.

Though beetles do not radically change the likelihood of fire, beetle-caused mortality does affect the way fire burns. Firefighters have seen faster crown fire speeds and consumption of standing woody trees, usually where trees were killed in the previous ten years. The forest structure created after dead trees fall is likely to burn with high intensity. Huge amounts of dead wood lay on the ground surrounded by foliage, twigs, and branches that easily catch fire and act as kindling to ignite the large tree trunks. The wood can smolder for days or even months. Dead trees that remain standing are extremely dangerous to firefighters because those trees are likely to weaken and fall in a fire.

The first priority of forest management following the pine beetle outbreak is to cut dead trees so they cannot fall on people, property, or roads. Reduction in fuels is another major management objective. Reducing fire fuel through tree harvesting has been controversial. On the one hand, people are concerned about fire and want to reduce the chances of forest fires burning valuable assets such as houses, buildings, recreation sites, and infrastructure; on the other, fire is a natural process and when and where it will occur are hard to predict. Removing trees may lessen fire risk to highly valued assets, but it can have negative effects on soils, plants, animals, and a variety of human activities. Balancing these concerns has been and will continue to be a source of conflict.

What's Next for these Forests?

What will the future bring to these forests? As the climate changes, winter cold snaps will be less frequent, and the reduced vigor of stressed host trees could help beetle populations grow. However, forest composition will likely hinder the development of a continent-scale outbreak for many decades. The tree age and species diversity that has been created by the recent outbreak will make forests in the next century much less susceptible to large-scale beetle outbreaks. Though there are many uncertainties about how ecosystems will react to climate change, mountain pine beetles and lodgepole pines will be an integral part of Colorado forests in the century to come.

Forests across Colorado's Rocky Mountains look very different today than they did twenty years ago. Millions of trees have been killed by mountain pine beetles. The mountain pine beetle is a native insect that is about the size of a grain of rice. Large populations of this tiny bug have had a huge impact on forests in Colorado.

Life Cycle

The beetles live most of their lives in the bark of pine trees, including lodgepole pine, ponderosa pine, limber pine, whitebark pine, and bristlecone pine. Larvae hatch from eggs laid under the bark in early autumn. They spend the winter and spring as larvae feeding on the phloem (the tissue that carries the tree’s nutrients). By midsummer, they become adult beetles and emerge from the bark to find a new home in another tree. There, they mate and reproduce. Beetles generally infest large, fully grown pine trees that are eighty years or older.

In order to reproduce, the beetles must overwhelm pine trees’ natural defenses. Healthy trees can push out and kill beetles with their sap and chemicals called terpenoids. To overwhelm tree defenses, hundreds of beetles attack a single tree. They use compounds from the tree’s defense chemicals to create a chemical signal that tells other beetles to join them. Once there is no room for any more beetles in the tree, the beetles send a chemical signal that the tree is full. Beetles then attack a neighboring tree en masse. This pattern leads beetle-caused tree mortality to occur in patches. Trees are killed primarily by a fungus that the beetles carry. The fungus infests the tree’s wood and stops water transport from the roots. Needles of killed trees turn red within one to two years after attack, and fall off within four to seven years.

Most of the time, beetle populations exist at low levels. Under these natural conditions, beetles typically kill small patches of mature older trees. Many of these trees are already stressed, diseased, or damaged. Cold winter temperatures can kill the beetles. It is difficult to pinpoint a temperature at which beetles are most likely to die. However, late fall or spring temperatures below -20 degrees Fahrenheit or colder can kill larvae. Several years without cold snaps can allow beetle populations to grow. Prolonged drought taxes the defense systems of many older pine trees. This creates better conditions for beetles and results in outbreaks that cover large areas. Such outbreaks occur periodically in Rocky Mountain pine forests. They often reoccur every thirty or forty years.

Recent Outbreak

The recent mountain pine beetle epidemic has killed millions of acres of pine forest from British Columbia to Colorado. It is generally thought to be larger and more severe than previous outbreaks in the twentieth century. The recent outbreak is the result of favorable conditions for beetle population eruption. Several years of warmer-than-normal winter temperatures allowed beetle populations to increase. Drought left trees with reduced defenses. Large expanses of pine trees across North America were reaching an age ripe for beetle infestation. This is largely because many forests were cleared or burned during the early days of European American settlement in the late 1800s.  These forests are all roughly 100–150 years old. There is general consensus that both forest structure and climate played a role in the recent outbreak. However, there is disagreement about the relative importance of each.

Effects on Forests

Although mountain pine beetles killed many trees, these forests are recovering. The trees that survived the outbreak are growing faster than ever. Additional light, water, and nutrients that were being used by the now-dead canopy pines is helping the new trees grow. The forest conditions created by beetle-caused mortality provide habitat for many wildlife species.

Effects on Fire

Mountainsides covered with recently killed red-needled pine raised concern about increased wildfire risk. Once the needles fall, beetle-killed trees may actually lower the risk of fire ignition. The fire is less able to spread from crown to crown. Fire is limited primarily by climate in lodgepole forests. There is always enough fuel. However, weather conditions that allow large fires to burn are rare.

Beetle-caused mortality does affect the way fire burns. The forest structure created after dead trees fall is likely to burn with high intensity. Huge amounts of dead wood lay on the ground can easily catch fire and act as kindling to ignite the large tree trunks. The wood can smolder for days or even months. Dead trees that remain standing are extremely dangerous to firefighters. Those trees are likely to weaken and fall in a fire.

The first priority of forest management following the pine beetle outbreak is to cut dead trees. That way, they cannot fall on people, property, or roads. Reduction in fuels is another major management objective. Reducing fire fuel through tree harvesting has been controversial. People are concerned about fire and want to reduce the chances of forest fires burning things like houses, buildings, recreation sites. However, fire is a natural process and when and where it will occur is hard to predict. Removing trees may lessen fire risk, but it can have negative effects on soils, plants, animals, and a variety of human activities. Balancing these concerns has been and will continue to be a source of conflict.

What's Next for these Forests?

What will the future bring to these forests? As the climate changes, winter cold snaps will be less frequent. The reduced vigor of stressed host trees could help beetle populations grow. However, forest composition will likely hinder the development of a continent-scale outbreak for many decades. The tree age and species diversity that has been created by the recent outbreak will make forests in the next century much less susceptible to large-scale beetle outbreaks. Mountain pine beetles and lodgepole pines will be an integral part of Colorado forests in the century to come.

Forests across Colorado's Rocky Mountains look different today than they did twenty years ago. Millions of trees have been killed by mountain pine beetles. The mountain pine beetle is a native insect that is about the size of a grain of rice. Large populations of this tiny bug have had a huge impact on forests in Colorado.

Life Cycle

The beetles live most of their lives in the bark of pine trees. Larvae hatch from eggs laid under the bark in early fall. They spend the winter and spring as larvae. They feed on the tissue that carries the tree’s nutrients. By midsummer, they become adult beetles and leave to find a home in another tree. There, they mate and reproduce. Beetles infest large, fully grown pine trees that are eighty years or older.

To reproduce, the beetles must overwhelm pine trees’ natural defenses. Healthy trees can push out and kill beetles with their sap. Hundreds of beetles attack a single tree. They use compounds from the tree to create a chemical signal. The signal tells other beetles to join them. Once the tree is full, the beetles send a different chemical signal. The other beetles then attack a neighboring tree in large numbers.

Trees are mostly killed by a fungus that the beetles carry. The fungus stops water from getting to the tree from the roots. Needles of dead trees turn red within one to two years after attack. The needles fall off within four to seven years.

Beetle populations usually exist at low levels. Beetles typically kill small patches of older trees. Many of these trees are already diseased or damaged.

Cold temperatures can kill the beetles. Temperatures below -20 degrees Fahrenheit can kill larvae.

Several years without cold snaps can allow beetle populations to grow. Prolonged drought stresses older pine trees. That can result in beetle outbreaks that cover large areas. Such outbreaks can occur in Rocky Mountain pine forests. They often reoccur every thirty or forty years.

Recent Outbreak

The recent mountain pine beetle epidemic has killed millions of acres of forest. It is thought to be more severe than previous outbreaks. Several years of warmer-than-normal winter temperatures allowed beetle populations to grow. Drought left trees with reduced defenses. Large numbers of pine trees across reached an age ideal for beetle infestation.

Effects on Forests

Although mountain pine beetles killed many trees, the forests are recovering. The trees that survived the outbreak are growing faster than ever. Light, water, and nutrients that were being used by the now-dead trees are helping new trees grow.

Effects on Fire

There is concern about increased wildfire risk. Once the needles fall, beetle-killed trees may lower the risk of fire. The fire is less able to spread from crown to crown.

Fire is limited primarily by climate in pine forests. There is always enough fuel. However, weather conditions that allow large fires to burn are rare.

Beetle-kill does affect the way fire burns. Dead wood on the ground can catch fire and act as kindling. That ignites the large tree trunks. The wood can smolder for days or even months. Dead trees that remain standing are dangerous to firefighters. Those trees are likely to fall in a fire.

After a pine beetle outbreak, forest managers cut down dead trees. That way, the trees cannot fall on people, property, or roads. Reducing fuel is another goal. People want to reduce the chances of forest fires. However, fire is a natural process. When and where it will occur is hard to predict. Removing trees may lessen fire risk. Cutting dead trees can have negative effects on soils, plants, animals, and human activities. Balancing these concerns has been and will continue to be a source of conflict.

What's Next for these Forests?

What will the future bring to these forests? As the climate changes, winter cold snaps will happen less. However, forest composition will likely stop a large pine beetle outbreak for many years. The species diversity caused by the recent outbreak will make forests less susceptible to large-scale beetle outbreaks.