Bora-Care is the preferred choice to treat wood for any insect issues if most of the wood is exposed and it is raw wood you are treating, meaning it is not painted, stained, or sealed, it is just normal wood. If your wood is stained, then you would either have to sand off the finish to use Bora-Care, and then you can re-stain it after 24 hours of applying Bora-Care, or your other option is to drill holes every 6 inches apart into the center of the wood and inject Bora-Care to get it past the outside stain. Bora-Care is made only for wood, and you do not have to know exactly where the termites are for it to work which is a plus. Bora-Care is sprayed evenly over all exposed surfaces of the wood and actually penetrates through the entire piece of wood. When the termites in the wood try to consume the wood after it has been treated, they ingest the Bora-Care with the wood and die. No matter where they are in the wood the Bora-Care will find them as long as you treated the exposed wood that you can see. The other main benefit is that Bora-Care stays in the wood forever, so you will not have to worry about termites or beetles infesting the wood that you treated ever again. Also, Bora-Care dries clear so you can paint the wood, stain it, or leave it looking natural.
Bora-Care: http://www.domyownpestcontrol.com/boracare-p-100.html
Bora-Care would be a great choice for treatment of powderpost beetles on raw natural wood. I have included the product label link below for additional information.