Quoting from Janet's reference, "The white substance you see on the leaves beneath the hole is frass, or insect excrement". My understanding was that the poop often reflected the diet, so if it is munching its way in through the pith then the "end result" will potentially be white. Either way it is the sign of a borer of some kind.
So you're thinking it may be an exit hole, Janet? I thought that usually there was only frass/sawdust/whatever below the entry holes because with the exit holes the debris would be left inside the stem. Need to read up on that again too.
