If you carefully, with a sharp knife, split the stem open on one side lengthwise and find the boring caterpiller, pull it out, and spray BT on the area. It may recover. If the stem is mush all the way through it won't survive.
I tried using a large syringe and injecting water with BT into the plant stem a few years ago, with mixed results. Instead, now if I find a split or weakened stem I use a needle nose pliers and pry the split open a bit and pour water with Garrett Juice, BT, etc over it and let it dribble into the stem. That seems to have worked as well or better on those borers. And I do go back regularly and water with some BT in the water. Be careful not to use BT on the flowers if you can avoid it, it kills butterflies. (You want the butterflies to pollinate, you don't want their larva because they kill your plants. Ironic!)
_________________ Northwesterner
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