Q: I have never seen this type of frass from a borer’s entrance hole. They have hit every beech on my property that is over 5″ caliper. They make a vertical slit in the trunk, close to an inch in length, and then pack it with frass. It looks like a zipper when their hole is closed up.
A: Astute entomologist Jule-Lynne Macie says it’s very probably a beech borer, Goes pulverulentus.
Notes at www.bugwood.org say the borer is a roundheaded larva, white, legless, cylindrical, about 1.5 inches long. The adult is a brownish-gray longhorned beetle with a life cycle of 3 to 5 years.
It could also be an oak branch borer, Goes debilis, because one characteristic of this borer is the yellowish frass they eject.
In either case, I don’t think you’ll have much damage to your trees.