Could use some tannerite too!
Can't speak for your fir trees but ants seem to be the #1 killer of trees around here.I'm not sure what kind but red ants could be the name of them . they have trails going to my large fir trees . some of the ant hills are right at the base of my old 5+ft trees.
Poisons and chemicals have their own secondary effects that I might not like.
Natural methods are for me but .........
Chickens would be my first choice to transform ants into tasty and beautiful eggs.