What kind of critter does this?
I have a dying/dead tree in my back yard that I think was a silver birch. I noticed a considerable amount of sawdust collecting at the bottom of the tree and inspected the tree more closely. The trunk was bristling with many little extrusions of some form of pest waste about 1/32" in diameter and various lengths of up to nearly an inch. The extrusions are basically a "stool" of sawdust that rinses off to the bottom of the tree when it rains. I chiseled a small window into the trunk around one of these extrusions and found a small hole going radially into the tree wood. I stopped after excavating about a 1/2" deep window without finding the source of the hole. Do you have any idea what pest is causing this condition? Is it a threat to other live trees? See attached pictures.
The pictures show "frass toothpicks" that are evidence of successful attack by bark beetles referred to as ambrosia bark beetles. Most Ambrosia beetles attack stressed or dying trees but there is evidence that some may species may attack healthy trees. I would suggest removal of infested trees and destroy them by chipping or burning. If you can find a sample of the insect, either larva or adult, you can submit it to the entomology lab for proper identification through your local extension office.