Spruce bark beetles
I lived on the hillside above Anchorage during the 80's - 90's. We had several mature trees attacked by the beetles. We sprayed the lower 40 feet of each tree, but the tops died, At the time, it was thought the the beetles wouldn't attack above 40'. I kept them well watered, so that helped. I did notice that one mature tree which had a 2'x2' plywood sign on it with 2 each 3" galvanized nails into the tree, was never attacked. I'm wondering if the nails had anything to do with fending off the beetles?
Anchorage Alaska spruce bark beetles