At my parents home I noticed a ring of sawdust around the base of one of thier plum trees, also 2 of the larger branches appear to have died off. The dead branches are "sapping" and look like they are cracked also they have small holes maybe 1/16 of an inch bored into them. The question is anyone know/want to guess what is doing this? I have inspected the tree at different times of day and have not seen any activity(insect). Also I have removed the dead/ infested parts of the tree but I am concerned that whatever is doing this may have already spread further. Any help would be greatly appreiciated.