I recently looked at some oak in a timber frame construction that has been without a roof and exposed to the elements for about one year. The oak timbers were still in a green state, and several of them had large grub cavities, which usually measured 1/2" to 5/8" wide and 1 - 4 inches long.
1. What type of insect made those holes?
2. When did the infestation occur?
Get a copy of the book that NC State Extension published--Lumber Defects Caused By Insects, Fungi, and Stains by Moore. Pub No. AG-425.
The borers in oak are present in the log and tree. They are no longer active when the wood dries down. An on-site inspection would be best, but my answers based on what I typically see are:
1. White oak borer.
2. In the living tree.
Gene Wengert, forum technical advisor