Global Warming Science -


Mountain Pine Bark Beetle Devastating Western Pine Forests


[last update: 2011/03/26]




The Alarm


Across western North America, from Mexico to Alaska, forest die-off is occurring on an extraordinary scale, unprecedented in at least the last century-and-a-half — and perhaps much longer. All told, the Rocky Mountains in Canada and the United States have seen nearly 70,000 square miles of forest — an area the size of Washington state — die since 2000. … These large-scale forest deaths from beetle infestations are likely a symptom of a bigger problem, according to scientists: warming temperatures and increased stress, due to a changing climate.





Mountain Pine Beetle


The Mountain Pine Beetle (MPB) is the most important pine killing insect in western North America.


Mountain Pine Beetle – USDA Forest Service []

The following points are from the above referenced “Forest Insect and Disease Leaflet 2 - 1989” (written prior to the 2000s epidemic).


  • Outbreaks frequently develop in lodgepole pine stands that contain well-distributed, large- diameter trees or in dense stands of pole-sized ponderosa pine. When outbreaks are extensive, millions of trees may be killed each year.


  • The beetle is native to North America. It is found in an area from the Pacific Coast east to the Black Hills of South Dakota and from northern British Columbia and western Alberta south into northwestern Mexico (fig. 1). Its habitat ranges from near sea level in British Columbia to 11,000 feet (3,353 m) in southern California.



  • The beetle usually takes 1 year to complete its life cycle. However, at high elevations where summer temperatures are cool, 2 years may be required to complete the life cycle. And in California, two generations may be produced in 1 year in low-elevation sugar pines.


  • Unseasonably low temperatures may retard outbreaks. Early autumn or mid-spring temperatures of about 0 oF (-18 oC) and winter temperatures below -34 oF (-37 oC) may affect outbreaks



Mountain Pine Beetle – Colorado State University []

The following points are from the above referenced “Fact Sheet”.


  • Mountain pine beetles (MPB) are the most important insect pest of Colorado's pine forests. MPB often kill large numbers of trees annually during outbreaks. Periodic outbreaks of the insect, previously called the Black Hills beetle or Rocky Mountain pine beetle, can result in losses of millions of trees.


  • MPB prefers forests that are old and dense. Most mature Colorado forests have about twice as many trees per acre as those forests which are more resistent to MPB.


  • For winter mortality to be a significant factor, a severe freeze is necessary while the insect is in its most vulnerable stage; i.e., in the fall before the larvae have metabolized glycerols, or in late spring when the insect is molting into the pupal stage. For freezing temperatures to affect a large number of larvae during the middle of winter, temperatures of at least 30 degrees below zero (Fahrenheit) must be sustained for at least five days.


  • Chemical control options for MPB larvae have been greatly limited in recent years. At present, there are no labeled pesticides for use on MPB.



USDA Research Paper RM-235 “Ponderosa Pine Mortality Resulting from a Mountain Pine Beetle Outbreak” 1982 []:


  • From 1965 to 1978, mountain pine beetles killed 25% of the pines taller than 4.5 feet in a study area in north-central Colorado. Average basal area was reduced from 92 to 58 square feet per acre. Mortality increased with tree diameter up to about 9 inches d.b.h. Larger trees appeared to be killed at random. Mortality was directly related to number of trees per acre and presence of dwarf mistletoe, but not to site index, elevation, or percent Douglas-fir in the stand


Mountain Pine Beetle Epidemics – Congressional Research Service Report for Congress 2009  []


  • Mountain pine beetle epidemics have occurred in lodgepole pine forests for thousands of years. Epidemics lasting 5 to 20 years occur at irregular intervals, affecting large areas and often killing more than 80% of the trees of more than 10 centimeters (about 4 inches) in diameter.


  • The current mountain pine beetle epidemic in Colorado and Wyoming is extensive, but it is unclear whether the current level is unprecedented. Most researchers note that mountain pine beetle epidemics are known to have occurred in lodgepole pine forests, but that the current epidemic is more extensive than has been seen in the past century. However, one source noted the loss of 15 billion board feet of lodgepole pine timber from mountain pine beetles in Idaho and Montana from 1911 to 1935. Although the current epidemic is extensive, it may be normal and natural. Some researchers have stated: Even though insect outbreaks greatly affect forest ecosystems, they may not be detrimental from a long-term ecological perspective. Such disturbances may in fact be crucial to maintaining ecosystem integrity, a situation ... described as “normative outbreaks.”


  • The interior forests of British Columbia (BC … the mountain pine beetle is an endemic (native) species, with periodic epidemics. However, the current epidemic is the most severe ever recorded, and is expected to kill up to 80% of the mature lodgepole pine in BC within the next decade.




British Columbia


September 2009: “After more than a decade of devastation, B.C.'s Forests Minister says the plague of the Pine Beetle may finally be over. … the pine beetle is really on the decline. Unfortunately, that's as a result of it running out of food at this point … The pine beetle infestation has ravaged nearly 25 per cent of B.C.'s pine trees []


That’s the same percentage killed in Northern Colorado between 1965 – 1978 (when global cooling was the scare).


Council of Forest Industries


  • Successful fire suppression over the last century has resulted in an abundance of mature pine susceptible to attack. Combined with recent favourable climate conditions (warm winters and hot dry summers) ideal conditions exist for the Mountain Pine Beetle to extend its natural range and for the beetle population to explode.  The only known way the spread of the beetle will be stopped is through prolonged periods of very cold weather, a sustained period of -25 degrees Celsius in the early fall or late spring and – 40 degrees Celsius in the winter.


Canadian Forestry Service – Mountain Pine Beetle Synthesis Report, 2006



  • mountain pine beetle outbreaks generally occur in stands that are more than 80 years old, containing many trees of large diameter (Safranyik et al. 1974; Shore and Safranyik 1992). Thus, forest composition and age-class structure are the primary factors influencing host susceptibility and outbreak severity. The second condition comprises a sustained period of favourable weather over several years (Safranyik 1978). Insect development and activity are dependent upon temperature and seasonal weather conditions. Specifically, summer heat accumulation must be sufficient to allow development and reproduction followed by winter minimum temperatures that do not fall below thresholds that cause significant mortality.


  • The beetle’s preference for breeding in larger diameter trees results in proportionately more small-diameter trees surviving each year and following the collapse of the infestation. In general, during epidemics the percentage of trees killed is proportional to tree diameter above a minimum diameter of ca 10 cm.


  • Where extensive tracts of susceptible lodgepole pine dominate, outbreaks may last 10 or more years and kill most large-diameter pine trees on hundreds of square kilometres.


  • Forest fire suppression began in western Canada approximately 100 years ago. The effectiveness of fire suppression has steadily increased, especially with greater availability of aircraft since the 1950s. This effectiveness is evident in the decreasing trend of area burned in pinedominated forests in British Columbia between 1920 and 2002 (Taylor and Carroll 2004) (Fig. 10)



  • With the introduction of fire management, a large proportion of the stands which regenerated after these fires have matured and entered the susceptible age-class for mountain pine beetle. The result is a threefold increase in the area of pine susceptible to mountain pine beetle during the 20th century. … the area increase of mountain pine beetle outbreaks since the 1970s are related to an increasing amount of susceptible pine



The various reports refer to “warming trends” making the pines more susceptible, but they do not provide local region data – they simply refer to global trends.


The following figures shown monthly average temperature anomalies for the two 5x5 degree grids covering south and central BC for Oct through March – the critical months for pine beetle mortality. (Plots of HadCRU CRUTEM3 data plotted at Plots for 1910 through 2009.









The above temperature anomaly plots correspond to the 5x5 degree grids shown on the map below right.

The figure below right shows the summer precipitation at Kamloops – no drying trend.







The following figure shows the MPB tree kill extent by 2006.




The following figure shows Colorado winter temperatures (left) and summer temperatures (right) from the NOAA NCDC data []





A Colorado State University Study found that the amount of uniform mature stands of pine has increased over the last century []


  • The climate of the Colorado Front Range appears to be influenced in part by very broadscale atmospheric processes, such that local climate is statistically correlated with indices of atmospheric and oceanic conditions in the tropical Pacific (El Nino - Southern Oscillation), the northern Pacific (Pacific Decadal Oscillation), and the northern Atlantic (Atlantic Multidecadal Oscillation). Climatologists are just beginning to fully understand the ways in which variability in these broad-scale atmospheric processes affect the climate of Colorado (and consequently fire occurrence) at time scales of a few years to centuries.


  • evidence exists that tree planting was emphasized in some areas during the 20th century when assuring sustainable timber production was a national goal. Some transplanting occurred even where historical forest densities may have been low or zero.


The following figure shows the Sugarloaf area near Boulder



Another 2006 study states: “There is no evidence to support the idea that current levels of bark beetle or defoliator activity are unnaturally high. Similar outbreaks have occurred in the past



A historical perspective of drought in Colorado:



There is a lack of long-term rural stations in the area of Colorado affected by MPB (area shown on map below) in the NOAA US climate network. (Map from


The following figures show average January and minimum January temperatures for Steamboat Springs (top - location indicated by the red oval) and Cheesman (bottom - location indicated by blue oval). The temperatures are within natural variance.








The following figure shows the area of Montana affected by the MPB (area shaded in red – forested area shaded in green).




The following figure shows the acreage by age class of lodgepole pine (from the same source as above). Most of the trees are in the most susceptible age class of 80 years or older. The above source states: “In some areas of Montana, wildfire suppression and other factors have led to overstocked forests, in which many trees are growing in close proximity, competing for water, sunlight and nutrients.




It also states: “in Montana, an estimated 506,000 acres of ponderosa pine are in the “fully stocked” and Overstocked” categories and thus at high risk. … Given the factors of age and stocking conditions for lodgepole pine and ponderosa pine, more than 2.8 million acres of Montana’s forested lands are at high risk for mountain pine beetle infestation. The last major outbreak in Montana occurred in the 1970s and early 1980s, when some four million acres were affected. … Montana’s current outbreak has not yet reached the levels seen in the early 1980s.




The graph below shows acreage affected by mountain pine beetle over the past 33 years (through 2008) in the USDA Forest Service’s Northern Region, which includes the Idaho Panhandle, all of Montana, and North Dakota (from the same source as above).




January 21, 2010: the Billings Gazette reports: “The acreage of pine forest in Montana infested with the mountain pine beetle more than doubled in 2009, but a forester says the epidemic is losing steam in some areas. Pine beetles infested 1.2 million acres of forest in 2008 and 2.7 million acres in 2009, based on aerial surveys. Most of the outbreak has occurred in forests near Butte, Anaconda and Helena.



The following figure shows the average minimum January temperature for Anaconda and Butte from the NOAA GHCN database (plotted at: No long-term warming trend is evident in the January minimum temperatures.





See also Western Montana regional study:




Historical Newspaper Articles


Spokane Daily Chronicle – June 4, 1948





Berkeley Daily Gazette – June 6, 1930