Oh yeah, those stem borers are here, too. There are two major groups: one are beetle grubs (often weevils/Curculio sp.) and the others are caterpillars. Once inside the stem, it’s hard to control them. I cut the stem off, split it and find, kill the offending larvae. There are also full-grown beetles with this nasty habit.
The key to control, unfortunately, is to keep the plants covered with an insecticide, especially coating the stems. Once inside the stem, the insects are mostly not in contact with the insecticide. If there are very few, you may want to use the prune and squish method. Often, these pests are not very specific, boring into sunflowers, peppers, Echinacea, Lilies, etc.
If you identify the pest as a lepidopterous caterpillar, it may be possible to keep the plants coated with Dipel, or other organic Bacillus thuringiensis var. Berliner (Bt). This bacteria only kills caterpillars. If you find a larvae, examine it to determine if it is a caterpillar (six front legs and rear, fleshy legs) or a beetle (three front legs only).
