I have actually killed one tree at my house by screwing in tree steps that I would use for testing out tree stands for deer hunting. I thin the issue I had was that I would remove the screw-in steps occasionally. I believe the holes were a conduit for bugs and disease to get in to the tree. I have other trees that I have left in trees for years. Those trees are still healthy, but I don't trust the integrity of the steel inside the tree that I can't see. I won't climb these trees any more and have gone away from screwing anything in to a tree that is meant to support my weight. The tree will expand and contract with temperature changes allowing water to get in to the holes and as it freezes and thaws the hole can expand. This could lead to the step (or eye bolt) slipping out of the hole.