To eliminate these pests, the gardener needs to remove all the old foliage in the fall where the little buggers overwinter, which I somehow failed to do. So now, I have to go on seek-and-destroy missions before the borers mature and make their way into the rhizomes. The tiny larva start out up on the foliage and chew their way inside.
Yes, it's tedious to search every leaf, but it must be done and is much less unpleasant than digging full-fledged borers out of stinking, rotting rhizomes. Is it worth it? Here's my answer: