Pine beetles and conifer forests have long coexisted in British Columbia. Around the turn of the twenty-first century, a combination of factors, including fire suppression and mild winters, allowed beetles to destroy unusually large tracts of forest. So many trees have sickened and died that the damage is visible in satellite imagery.
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite observed part of British Columbia on October 8, 2006. This image is made from a combination of light visible to human eyes and light our eyes cannot see (near- and shortwave infrared). Similar to a true-color image, deep blue indicates water and green indicates vegetation. While bright green indicates healthy vegetation, damaged forest appears in pink-tinged hues. Cleared land (logged or naturally bare) appears pinkish brown.
The lifespan of a mountain pine beetle, Dendroctonus ponderosae, is generally one year. Pine beetle females typically seek large-diameter, mature trees in which to lay eggs, although during a serious outbreak, beetles may settle for younger, thinner trees. (A lodgepole pine tree is considered mature after about 80 years.) After hatching, beetle larvae spend the winter under the bark, tunneling as they eat, and cutting off the tree’s nutrient supply. The larvae usually transform into pupae in June or July, but adults can emerge as early as June or as late as September. Larvae leave tell-tale tunnels in the bark, and several months after the onset of an attack, a pine tree’s foliage turns orange or brown.
Numerous outbreaks of pine beetle infestations occurred throughout the twentieth century, but toward the end of the century, conditions were just right for an epidemic. One contributing factor was a history of fire suppression, which enabled large numbers of trees to reach maturity. Another factor was warming temperatures. Several straight days of temperatures below -35 to -40 degrees Celsius (-31 to -40 degrees Fahrenheit) can kill the larvae, and cold snaps are especially effective in the autumn. If, however, temperatures fail to reach those lows often enough, the larvae survive until the following spring, perpetuating the beetle infestation. Furthermore, hot, dry summer conditions can weaken trees’ defenses.
NASA image created by Robert Simmon, using data from NASA/GSFC/METI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team. Caption by Michon Scott.
