butterfly bush wilt
I planted a butterfly bush (one of the medium-size types) in a bed last year and it thrived; this year about a dozen stalks have wilted and died off (by gently pulling, they came out at the root). Most of the plant looks healthy and is blooming. We have rabbits in the area, but it doesn't show signs of being eaten. I have used an organic natural anti-rabbit spray in that bed, although not on or around that bush. We've had a lot of rain over the past month or so. Also, the yard abuts on a farm (where the fence is in the picture), and the farmer does spray. Thank you very much for your help!
Butterfly bush borer can infest individual shoots of butterfly bush. The treatment is simply to remove the infested shoot, which you have already done. Put them in the trash, not the compost, as you want to get the borer off the property.
These are plants that do not like to be sitting in soggy soil. Be sure not to mulch up to the base of the plant. Mulch should never be piled on, or touching, any plant. Stressed plants tend to attract insect pests.