I'm building a live edge cherry table & under the bark are what looks like PPB holes. There are just a few of them. but wonder if I need to do something about them. The wood is kiln dried but I don't know the details of the process. If I need to kill whatever is in the holes, is there some chemical that will kill them? I can turn the planks on edge & let gravity help me. Is this a worry or can I just varnish over them & forget about it.
You might want to check out a product called Timbor /Tim-bor.
Actually should have said Bora-care. They are both made by the same company.
You need to find out what the final temperature of the kiln was and if an actual heat treatment procedure was done. Generally PPB's are considered safely killed if the HT process was 160F for 24 hours. That is exceeding what the USDA requires, but you don't want to take any chances and introduce PPB's to your home as a result.
The holes are where the bugs have exited the wood. The concern is if their kin are still in there chomping away and waiting to exit. If the wood has been properly kilned, any grubs left in there are dead, and the holes are historic, and you don't need to worry.
Otherwise, yeah, it's cause for concern. There are products like bora-care, but they usually work to coat the outside of the wood with Borax. That cramps the style of any new bug that wants to move in, but might not kill any that are already deep in the wood. Should take care of their offspring though.
My concern with a liquid treatment is the fact the lumber is already dry. I don't see much if any of the boron getting into the lumber at all in those conditions.
It might not be powderpost beetles. There are a number of wood boring beetles, most of which cannot live in dry wood. Powderpost beetles are one of the few that can. First, I would identify which type beetle you are dealing with before spending a lot of time and money if is one of the non-important ones that cannot live in dry wood.
Post a pic or describe the holes in the wood.
I think PPB often get the blame for any frass or holes when most of the time the damage occurs by other insects attacking green wood before it is sawn.
It could well be ambrosia beetles that did it while the wood was still green. Ambrosia will not go in dry wood, only wet green wood.
Shell guard from Permanchink is alcohol based and will penetrate a little better than other products, but after it dries, the film on the surface needs to be removed in order for a finish to be applied. Washing it as noted by Permachink or sanding.
Like has been said, you might not have an issue at all If it was kiln dried with a sterilization cycle. A sterilization cycle will get the internal temperature of the wood up to a temp that will kill any living beetle larvae. I think that is 133F or more, for at least 2 hours. That's why it was mentioned a kiln temp of 160 for 24 hours. It takes time and a higher temp than 133 to get the internal temp of the wood up to 133+.