What makes holes in walnut trees?

This morning I notied a couple of sap weeps on the trunk of
a seedling walnot tree planted about fifteen years ago. It
looks like something chewed holes in the trunk about two feet
above the ground, leaving a residue of cuttings in the entrance
hole and a bit of sap around it, with the excess dripping down.

Photos are in
http://www.zefox.net/~bp/walnut/

If anybody recognizes the pattern I'd be curious to know
what it is. The tree seems healthy, at least for now.

If anybody has a clue what it might be please let me know.

Thanks for reading,

bob prohaska