Search borer bees or carpenter bees.
The chemical treatment is not real effective.
Nasty buggers. Their Holes in wood are not pretty.

A good option is to keep the wood painted well. If it's natural color you want, use linseed oil or thompson's water seal and several coats of either. The holes are very close to 3/8ths inch usually so if they have already drilled, insert a short piece of a 3/8 dowel rod dipped in glue, cut off flush and paint over.