They aren't what kills the tree.  The Blue Stain Fungus introduced by them infects the tree and blocks the water and nutrient conducting vessels through the tree, much like our circulatory system, starving the tree of nutrients and resulting in its death. Affects mature Ponderosa, Lodgepole, Bristlecone and Limber Pine Trees.  

These creatures eat the needles of your Blue Spruce and Douglas-fir trees causing defoliation.  If it's a healthy tree it can fend off the defoliation for a few years in a row but eventually, it will become weakened and become highly susceptible to beetle attack. The best remedy is a pesticide applied to the worm during its larval stage. 

A close relative to the Mountain Pine Beetle, the Spruce beetle only affects Blue Spruce or Engelman Spruce trees.  The adult bores into the tree and eats away at the cambium layer creating galleries of destruction and infecting the tree with fungus.  It lays its eggs there, which will hatch, fly away and perpetuate this cycle to neighboring Spruce Trees. 

Much like the other two beetles shown, the Douglas-fir Beetle keeps it in the family too.  This relative's food source of choice is the Douglas-fir.  It is a copycat to it's not so distant relatives attacking these trees in the same fashion with the same result.  

Dwarf Mistletoe is a parasitic plant living off the host Pine tree slowly weakening it and killing it.   It hits the tree with a 1-2 punch by making it susceptible to attack from beetles resulting in a more rapid death than the Dwarf Mistletoe alone.

This is a Bristlecone Pine that is trying to defend itself from an infestation of Mountain Pine Beetles.  We removed this tree before the eggs were able to hatch, mature and spread to neighboring trees.