David,
There are quite a few stem-boring larvae; could be lepidoptera or beetle.
Systemic insecticide seems worth a go.
But of course, part of the issue will be the local population, waiting to in-infest.
If the particular culprit could be indentified, maybe we could see if there is a known pheromone trap for it.
An aside:
There's a stem borer that goes for my Cupressus sempervirens, working along the twig towards the tip until its tunnel is the same size as the twig, whence the trig drops off. I expect other folks have noticed such twig-tip-drop. Next time, pick up the twig and look at the end. I bet you'll see a hemispherical hollow - the end of the tunnel.
Mike