Spruce Aphid are preferential to Norway, Sitka, and Blue spruce.  Most insects are very particular about their target species.  I don't think the leaf miners have much to worry about when it comes to wasp predation, since they are encased within the leaf itself.  they might be attacked by parasitoid wasps (the life cycle of Sigourney Weaver's Alien is based off of those guys), but as far as I am aware, the primary biological control for leaf miners are soil nematodes and fungi.  The leaf miners overwinter in the duff layer, and are vulnerable to attack until they emerge as adults.  

As far as the general system goes, Beetles and Fire are the natural cleansing agents of the forest.  think of the forest as a herd, and those two agents as wolves, picking off the weak and sick trees.  The system functions in that manner rather well under normal conditions, but when the climate began a major regime shift here in the 80's, it triggered the largest bark beetle outbreak ever recorded (cont'd)