Mountain Pine Beetle, Photo by S. Clarkson. Wikimedia Commons
Audio of Mountain Pine Beetles
The mountain pine beetle (MPB) (Dendroctonus ponderosae) is a native insect of western North America. Its two other common names (“Black Hills beetle” and “Rocky Mountain pine beetle”) give a good idea of its range and distribution. The adult form of this beetle is about the size of a grain of rice and is an important component of the natural life (and death) cycles of our western pine forests. The MPB has also become, in the disrupted conditions of our western, mountain ecosystems, an out of control threat to vast stretches of pine forests.
Under normal conditions (and, admittedly, “normal” is such a hard thing to define!) the MPB’s in their brief, late summer adult dispersion phase, seek out large (and, therefore, old … at least 80 years old) pine trees of almost any species (including the very abundant Ponderosa pines (Pinus ponderosa), lodgepole pines (P. contortus) and limber pines (P. flexilis)). Female MPB’s burrow under the bark of their selected tree and begin to release attraction pheromones which draw more and more female MPB’s to the tree. A tree, then, very quickly gets inundated by large numbers of invading beetles.
Pitch tubes. Photo by USDA FS, Public Domain
Male MPB’s are also attracted to the female pheromones and burrow into the tree in order to find and mate with the females who have been busy carving out egg galleries in the wood just beneath the bark. These egg galleries are in the phloem of the pine tree, the transport tissue that moves sugars throughout the tree. Sap from the phloem often oozes out these beetle entry holes making structures called “pitch tubes.” The presence of pitch tubes is an external sign that a pine is being infested with bark beetles.
The arriving males also produce a set of pheromones, but these chemical cues act to repel further invasion of the tree by beetles. They signal to the foraging clouds of bark beetles that this particular tree is “full.” This then leads to the invasion of trees that are adjacent to the initially infested tree generating the typical patchwork clustering of bark beetle damaged trees.
The fertilized beetle eggs hatch in the early autumn and the larvae begin to eat the woody tissue of the phloem. Their consumption of wood continues all through the coming winter and spring. This new generation of bark beetles will pupate and then emerge in the late summer as adults flying off to infest new pine trees. The founding generation of adult bark beetles may also overwinter in their tree and, the next year, produce another batch of eggs and larvae. These adults may also emerge with their mature progeny from this first tree in the summer and go out to find new trees.
Photo by USDA FS, Public Domain
The adult beetles carry with them a substantial load of fungi. One of these fungi, the blue-stain fungus, spreads through the wood of the pine and make the wood more palatable to the larvae. This fungus may also act to dampen down the defenses of the infested pine. The blue-stain fungus also specifically attacks the other type of transport tissue in the pine tree, the xylem. Xylem is involved in transporting water throughout the tree. This damage to the xylem can be so severe that it actually is the cause of death of the bark beetle-infested pine tree. The observed cycle of tree damage in an affected pine (needles turning red after a year or two, loss of needles after four to seven years) are actually symptoms of fungal damage to the tree’s xylem.
Other fungi transported by the bark beetles include wood decaying fungal species. These fungi facilitate the breakdown and recycling of the pine trees’ tissues after its death.
Old growth Ponderosa pine forest. Photo by USDA FS, Public Domain
A healthy, unstressed pine tree is able to defend itself against invasion by bark beetles. It can engulf the beetles in the sap of its phloem, it can produce natural pesticides (including “terpenoids”) that kill the adult and larval beetles or, at least, greatly limit their growth rates and numbers. Also, the signs of beetle invasion and damage to a pine tree (including pitch tubes and boring dust in the fissures of the bark) attracts a variety of avian predators of the bark beetles including woodpeckers. Further, both adult and larval bark beetles are quite sensitive to cold temperatures. The deep, prolonged cold of winter can kill or, at least, substantially reduce bark beetle numbers in a tree, and late summer cold snaps corresponding to the adult bark beetle dispersion flights can significantly decrease the numbers of adults who survive to infest a tree.
The natural cycle of these bark beetles and their host pine species is in a sustainable equilibrium. Old pines are preferentially killed by the beetles. Trees are typically killed in clusters which then opens up broad light gaps in the forest canopy. These gaps allow the growth of sun-loving trees and other plants which greatly increases the vegetative diversity of these forests. These gaps also provide the suitable space for the growth of the early succession stages of the pines themselves. And, the beetle facilitated delivery of wood decaying fungi to the bark beetle killed trees accelerates decay and speeds up the recycling of nutrients that are bound up in the tissues of the pines. The bark beetle-maintained pine forest then is patchy in its age distribution of trees and dynamically renewing itself via succession.
Bark beetles will kill more trees if these trees are stressed by drought or if there are unusually warm temperatures. These conditions have historically cycled in 30 or 40 year intervals and have led to pulses of broad, pine forest die-offs over these time intervals.
Mountain Pine Beetle damage. Photo by USDA FS, Public Domain
So, why have western pine forests from New Mexico to British Columbia experienced over the past 20 years or so such a prolonged and extensive period of tree damage and death from MPB’s? In Colorado alone 70% of the state’s 1.7 million acres of lodgepole pines have been damaged or destroyed by MPB’s. MPB’s in Colorado are estimated to have killed a total of 3.4 million acres of pines! One in fourteen of all of the trees in the state are dead primarily from the activity of MPB’s! Standing, dead trees in Colorado forests have increased 30% since 2010 to a total of 814 million trees. The most obvious causes of this catastrophic decimation of pine trees are the unbalanced age structures of the forests, the past management of fire in these forests, and the ongoing changes that are occurring because of climate change.
Extensive areas of western forests have been logged since the onset of European settlement of the American West, one hundred to one hundred and fifty years ago. The regrowth of these forests has generated broad areas of very evenly aged trees. Further, long standing state and federal policies that mandated that all forest fires be suppressed has allowed these forests to build up large numbers of trees that are of optimal ages for bark beetle infestation. This suppression of fires has also caused large amounts highly flammable, woody debris to build up on the forest floors.
Over the past few decades average yearly temperatures all around the world have steadily risen. These rising temperatures and altering distribution of seasons has been very apparent in western North America. Winters are shorter and less cold which allows more and more bark beetles to survive inside infested pine trees. Also, the chances for dispersing-beetle-killing, late-summer cold snaps have declined. More beetles are surviving the winter in trees, and trees at higher and higher altitudes are being successfully invaded and killed by beetles. Further, the warmer temperatures have generated drier conditions throughout the west. This ongoing drought has not only stressed trees making them more vulnerable to the bark beetles, but it has also dried out the accumulated forest floor debris and all of the beetle-killed standing tree trunks and fallen limbs making the system increasingly vulnerable to severe wildfires.
So, the ongoing bark beetle problem is due to both local and global factors and both historical and current events. The beetle damaged pine forests, though, are astonishingly vulnerable to catastrophic wildfires. These incredibly intense fires fueled by a century of forest floor material accumulation and piles of freshly killed trees, could quite literally erase these forests from our mountains! Optimistically, it might take hundreds of years for the process of succession to replace these forests, or, pessimistically, they might not come back at all!