I know bag worms attack many evergreens like Spruce. Actually the caterpillar of a moth:
http://en.wikipedia.org/wiki/Bagworm_mothThere are also other moth caterpillars that attack evergreens, but the bag worms seem to be the most common. They can wreak havoc on unhealthy trees.
Being of the order Lepidoptera, BT would probably kill them in their early instars, but I would guess the people who grow trees for commercial purposes use something a little more durable than BT. Just a guess, I don’t grow Christmas trees, I just like a few in the yard. I just pick the bagworms off the trees when I see them.