Lodgepole pine killed by mountain pine beetle.

Once attacked there are usually no dramatic changes to a pine for some time.  There might be visible signs of "Pitch Tudes" on the trunk but the tree will initially remain green.  Over the course of the winter and spring browning will occur as the tree dies until a year later it will appear red as seen here.  It is important to remove infected trees before they reach this stage as they contain insects that were hatched out from eggs laid the previous year.

Limber pine killed by mountain pine beetle and white pine blister rust

Lodgepole pine killed by mountain pine beetle with subsequent woodpecker predation.

White spruce defoliation by spruce budworm.

Warren root collar weevil damage (and larvae) on lodgepole pine.

Poplar borer damage on trembling aspen.

Spruce cone maggot damage internally on white spruce cone.

Forest tent caterpillar defoliation damage.

This is probably the most overt sign of immediate Mountain Pine Beetle attack.  As the beetle eats through the bark the tree naturally exudes sap to try and push out the invader and seal the wound.  As the beetle keeps eating it's way in and the sap keeps getting pushed out a "Pitch Tube" of dried sap is formed.  Each tube represents at least one beetle invasion and individual trees can have from just a few to several thousand. Trees can survive small numbers but this tree will undoubtably die.