Answer: What you describe sounds more like crown rot than insect feeding, but there's always the possibility that slugs or snails might be munching on the leaves. Crown rot causes the leaves to wither and die, then the stems, and finally the whole plant. You may be able to slow the process by cutting off the obviously infected stems. However, it's late in the season and frost will kill the plants outright so you may simply want to enjoy your impatiens while you can and replace them next year. Wish I had something more positive to share with you.
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