Susan:
Sorry to hear of your problem. You can use a product
which is organic (not a chemical pesticide). It is
sold under several names locally. See if you can find
this Thuricide or BT (Bacillus Thuringiensis var kurstaki)
it only targets what it is sprayed on will not hurt good
insects like lady bugs, praying mantis, etc.. washes off
and has no long term activity.So as not to harm your butterfly larva. As to were your to put your B&B plant is leave it there or move it but the moth larva will still find it.Eliminate the moth larva and they should not come back. I have not had this problem with my B&B but onther problems.
I almost forgot it does not act like a poision (because it is not one) the larva will still be there for a short time but they will stop eating and then die.

Thank you so much for the suggestion! i will just have to be cautious of drift and not do on a windy day. My plant is still green and this is the latest it has stayed looking so nice (foliage). The worms are not on it now due to the cooler temps and angle of sun in fall, and, no blooms for some time now either, so won't worry til spring and will give it another chance. You may have contributed to the salvation of my poor B&B! Thanks so much!

Susan