Bugger, could we have a better picture? If it is so bad the timber needs replacing a new picture would be nice before that happens.

I know if I leave boards, flitch, with live edges in piles there is a worm which gets in under the bark and will subside for a number of years. I believe in the cambium layer just under the bark. They will move around some. They are a different bug than the pine borer which will work its way through the log. They are smaller in size and leave a very fine power. Or they could be a stunted borer. How big are the bug channels?

Was there bark left on any of the timber? They could have migrated and came out there by the brace?

Keep an eye out for more frass.

If you look at old timber, usually in the cellar, you will find bug tracks on the wane, well some of them.