Occasionally I have had problems with borers. Probably the way to avoid them is by taking off the bark and sapwood soon after the wood is cut and then sealing the back and ends of the staves with sealer, polyurethane, or some similiar sealer. If you aren't wanting to remove the bark immediately there are poisons that can be purchased (I haven't used them so someone else will need to chime in here). Another option would be to leave the staves submerged in moving water fot several months--evidently this is a method the English used when curing yew staves, this would probably eliminate the pests as well.