Dendroctonus ponderosae

Geographic Range

Dentroctonus ponderosae (the mountain pine beetle) inhabits a large portion of western North America. This species ranges from British Columbia in the north to northern Mexico in the south, as well as from North Dakota west to the Pacific coast. Since D. ponderosae infests Pinus ponderosa , as well as other trees in the genus Pinus , the range of the mountain pine beetle is mostly coincident with that of forests containing these trees.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Mountain pine beetles infest Pinus trees in western North American forests. While they can reach relatively high altitudes, their preference is for lower altitudes with suitable temperatures. Because instars (stages of larvae) of Dendroctonus ponderosae are susceptible to the cold, the beetles prefer to reside in areas with moderate temperatures. This limits the range of D. ponderosae with respect to both altitude and northern expansion.

With the increase of temperatures due to global warming, mountain pine beetle larvae are now capable of living through winter in areas farther north that were formerly too cold for their survival. In addition, the beetles have moved to higher elevations. The expansion of the mountain pine beetle range due to global warming has resulted in damage to Pinus forests in previously unaffected locations.

Mountain pine beetles preferentially infest trees that are under stresses such as injury or disease, fire damage, old age, and overcrowding. These trees are also the first to die. If the beetle population gets large enough, D. ponderosae will infest healthier Pinus trees in the area. As these trees die as well, entire populations of Pinus trees become kindling for fires that can have drastic effects on the forest ecosystem.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• forest forest

Physical Description

Dendroctonus ponderosae is black and cylindrical and, on average, 5 mm long. The gradual curve of the hind wing of the adult D. ponderosae distinguishes it from other bark beetles that normally have sharp spines along the hind wing.

Mountain pine beetle eggs are normally white, while the larvae typically have white bodies and brown heads. The larvae are also legless, as they remain under the bark of the Pinus trees for the duration of their development and have no need for legs.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Development

An adult female mountain pine beetle deposits her eggs in egg galleries within the phloem of a Pinus tree, and the eggs lay dormant for 10-14 days before hatching. Larvae then hatch from the eggs, appearing white with brown heads and no legs. These larvae develop through their instars for approximately ten months. Development time varies, depending on the temperature of the phloem in which the larvae are found; colder phloem results in prolonged development, while warmer phloem can shorten the duration of development. By the time winter arrives, the larvae have reached their third or fourth instar, which are much more durable in cold weather. The instars metabolize glycerol during this time, which prevents them from freezing. At the end of their development, around June of the following year, Dendroctonus ponderosae larvae build oval cells stemming out from their egg gallery. Within the oval cell each individual larva develops into a pupa. This stage is normally complete by the end of June or July. After a month or so this pupa develops into an adult. The adults can feed on bark within reach of their oval cells until they break into other oval cells or penetrate the bark to emerge from the tree. For the beetles infesting Pinus trees, this normally takes place in mid-August. At that point the adult females fly to new Pinus trees, secrete pheromones to attract males, and begin penetrating the tree bark to form new egg galleries. When males arrive, they also secrete pheromones to attract more beetles to the location, then fertilize the females beneath the bark of the Pinus tree. From here, the cycle starts again.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

Upon emerging from the oval cell in which they completed development, adult female Dendroctonus ponderosae fly to a Pinus tree that is suitable for breeding and maintaining progeny. Here they build an egg gallery by burrowing straight through the bark of the tree and into its phloem. These females then secrete pheromones in order to attract male D. ponderosae to the site. The males, upon arrival to the site, secrete their own pheromones to attract both male and female beetles to the location, initiating a local infestation.

At this point, the males proceed to exhibit mate selection by preferentially choosing females whose egg galleries are of larger size but within smaller trees. Smaller males are more likely to enter galleries than larger males, most likely because of some kind of size-dependent sexual selection dependent on the females. Large females in turn show sexual selection by allowing larger males to enter their galleries much more quickly than smaller males. To enter the galleries, males must first stridulate (rub their legs together in order to produce a certain sound). Once a female grants access to the male, the male enters the gallery and fertilization occurs. The female will then lay approximately 75 eggs. Males will remain with the females anywhere from a few days to three weeks after fertilization occurs.

• Mating System
• monogamous monogamous

Adult mountain pine beetles leave the trees in which they developed during summertime; this is usually sometime during July or August, but could potentially be anytime from mid-June until the beginning of September. At this time, they seek out new trees in which to reproduce. Males fertilize females within these new trees, then females lay about 75 eggs. The eggs hatch into larvae within about 10 to 14 days. Although the parents might remain within the tree for a few days afterward, there has been no parental involvement observed in Dendroctonus ponderosae . Around June or July of the following year, after having developed within the egg gallery throughout winter, the eggs develop into pupae. The pupae then become sexually mature adults sometime around mid-August, and can then leave the Pinus tree to find a mate.

• Key Reproductive Features
• semelparous semelparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

No parental involvement has been observed in Dendroctonus ponderosae .

• Parental Investment
• pre-fertilization
  • protecting
    • female

Lifespan/Longevity

The life cycle of Dendroctonus ponderosae lasts approximately one year, but it is highly climate dependent. At higher latitudes or elevations, the colder temperatures lengthen the larval development stages to about two years. Normally the adult stage last only a few days, and consists of leaving the tree in which they developed, flying to a new tree, and reproducing under the bark there.

Behavior

Mountain pine beetles are solitary throughout their developmental stages, but interact more with other beetles as they reach sexual maturity in the adult stage. Although most of their lives are spent beneath the bark of the Pinus trees, mountain pine beetles do develop the ability to fly and spend a few days of their life flying from their tree of origin to a new host tree. The arrival of a female at a new host tree and signaling others to join it through pheromones is characteristic of this species.

• Key Behaviors
• arboreal arboreal
• flies
• parasite parasite
• motile motile
• nomadic nomadic
• hibernation hibernation
• social social

Home Range

The home range of Dendroctonus ponderosae centers at its tree of origin and expands only as far as the host tree that it comes to upon emerging as an adult. The new host trees are limited in distance to a few days flying from the tree in which the beetles hatched.

Communication and Perception

Once a female adult beetle exits the tree from in which it hatched, it will attack a new tree, meanwhile releasing a pheromone that attracts other beetles to the site. These other beetles are then able to attack the target tree, as well as adjacent trees. Males that have been attracted to these trees then attack and release their own pheromones, thereby attracting more females to the site. Once a critical density of beetles per tree has been reached, males and females then release more pheromones, signaling to others to attack adjacent trees instead.

One of the pheromones produced by female D. ponderosae at critical density is called verbenone. Because verbenone at one site signals to D. ponderosae to avoid that site and seek out another, verbenone has been considered as a repellent the mountain pine beetle.

• Communication Channels
• acoustic acoustic
• chemical chemical

• Other Communication Modes
• pheromones pheromones

• Perception Channels
• acoustic acoustic
• chemical chemical

Food Habits

Dendroctonus ponderosae larvae survive mainly on the phloem of the tree which they inhabit. They feed in lines perpendicular to the egg galleries in which they were hatched. Directly after pupation within the oval cells, adults feed on fungal spores that other beetles have introduced into the tree, as well as additional tree tissues. Adults later feed on the bark of the tree as they make their way from the oval cells out into the open.

• Primary Diet
• herbivore herbivore
  • lignivore
  • eats sap or other plant foods
• mycophage mycophage

• Plant Foods
• wood, bark, or stems
• sap or other plant fluids

• Other Foods
• fungus

Predation

Woodpeckers ( Picidae ) are a natural predators of mountain pine beetles, as they penetrate the bark of Pinus trees to feed on adults and larvae. Several other beetles, including two species of checkered beetles ( Cleridae ) also feed on the adults and larvae beneath the bark. Dolichopodid flies also feed on Dendroctonus ponderosae .

Other predators, however, target Dendroctonus ponderosae as the adults fly from their tree of origin to a new host. These predators include nuthatches ( Sitta ) and other birds.

Ecosystem Roles

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 parasite

Economic Importance for Humans: Positive

Mountain pine beetles are not known to provide any benefits for humans.

Economic Importance for Humans: Negative

Mountain pine beetle infestations cause the destruction of huge numbers of trees. To curb the massive losses due to these infestations, governments in the United States and Canada have supplied large amounts of money toward eradication efforts. Without these efforts or any future change, the death of Pinus trees leads to increased carbon emissions. In addition, the destruction of Pinus forests leads to an increase in wildfire fuel, which could be potentially harmful to humans. Especially significant are the forest fires which occurred in Yellowstone National Park in 1988 and were the result of mountain pine beetle damage to Pinus trees. Deforestation due to mountain pine beetles can also negatively affect local timber industry.

Conservation Status

Dendroctonus ponderosae is not an endangered species, nor does there seem to be any concern about conserving the species as a whole. In fact, due to the nature of D. ponderosae , the focus is shifted more to eradicating the pest and restoring forest ecosystems rather than maintaining the species.

Other Comments

With the increase of temperatures due to global warming, mountain pine beetle larvae are now capable of living through winter in areas farther north that were formerly too cold for their survival. In addition, the beetles have moved to higher elevations. The expansion of the mountain pine beetle range due to global warming has resulted in damage to Pinus forests in previously unaffected locations.

Eradication efforts currently in place focus on short-term treatments, such as insecticides, long-term prevention, and landscape restoration. Specifically, larvae hidden beneath the bark are targeted with treatments such as stripping away the bark in order to expose the larvae to harsher environmental conditions. In addition, solar techniques can be used to heat up the phloem of the trees until they reach temperatures unbearable to the beetles. A great deal is being spent on these techniques in order to preserve the forest ecosystems that have been present for so long and that now run the risk of complete destruction.

Additional Links

Encyclopedia of Life

Contributors

Monica Muzzin (author), University of Michigan-Ann Arbor, Phil Myers (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.

References

Alfaro, R., R. Campbell, P. Vera, B. Hawkes, T. Shore. 2004. Dendroecological reconstruction of mountain pine beetle outbreaks in the Chilcotin Plateau of British Columbia. Mountain Pine Beetle Symposium: Challenges and Solutions: 245-256. Accessed July 03, 2011 at http://www.for.gov.bc.ca/hfd/library/MPB/alfaro_2004_dendro.pdf .

Eaton, C. 1941. Influence of the mountain pine beetle on the composition of mixed pole stands of ponderosa pine and white fir. Journal of Forestry , 39/8: 710-714.

Kurz, W., C. Dymond, G. Stinson, G. Rampley, E. Neilson, A. Carroll, T. Ebata, L. Safranyik. 2008. Mountain pine beetle and forest carbon feedback to climate change. Nature , 452: 987-990.

Leatherman, D., T. Mehall, I. Aguayo. 2007. "Mountain Pine Beetle" (On-line). Colorado State University - Extension. Accessed July 03, 2011 at http://www.ext.colostate.edu/pubs/insect/05528.html .

Logan, J., J. Powell. 2001. Ghost forests, global warming, and the mountain pine beetle (Coleoptera: Scolytidae). American Entomologist , 47:3: 160-172.

Lynch, H., R. Renkin, R. Crabtree, P. Moorcroft. 2006. The influence of previous mountain pine beetle ( Dendroctonus ponderosae ) activity on the 1988 Yellowstone fires. Ecosystems , 9/8: 1318-1327.

Mock, K., E. O'Neill, J. Chong, J. Orwin, M. Pfrender. 2007. Landscape-scale genetic variation in a forest outbreak species, the mountain pine beetle ( Dendroctonus ponderosae ). Molecular Ecology , 16/3: 553-568.

Pureswaran, D., R. Gries, J. Borden, H. Pierce, Jr.. 2000. Dynamics of pheromone production and communication in the mountain pine beetle, Dendroctonus ponderosae Hopkins, and the pine engraver, Ips pini (Say) (Coleoptera: Scolytidae). Chemoecology , 10/4: 153-168.

Reid, M., O. Baruch. 2010. Mutual mate choice by mountain pine beetles: size-dependence but not size-assortative mating. Ecological Entomology , 35:1: 69-76.

Safranyik, L. 2001. Seasonality in the mountain pine beetle: causes and effects on abundance. Boreal Odyssey, NOR-X-381: 150-151.

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.

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 .

U.S. Department of Agriculture: Forest Service. Mountain pine beetle. Forest Insect & Disease Leaflet 2. Portland, Oregon: USDA Forest Service. 2009. Accessed July 03, 2011 at http://www.fs.fed.us/r6/nr/fid/fidls/fidl-2.pdf .