The western pine beetle,
Dendroctonus brevicomis LeConte, can aggressively attack and kill ponderosa and Coulter pine trees of all ages and vigor classes that are 6 inches (15 cm) or larger in diameter, including apparently healthy trees. Group killing of trees is common in dense, overstocked stands of pure, even-aged, young sawtimber, but also occurs among dense clumps of pine in stagnating mixed-conifer stands. One million or more trees containing more than 1 billion board feet of timber may be killed each year during an outbreak. Such extensive tree killing may deplete timber supplies, adversely affect levels and distributions of stocking, disrupt management planning and operations, and increase forest fire danger by adding to available fuels.
http://www.na.fs.fed.us/spfo/pubs/fidls/we_pine_beetle/wpb.htm