Tough, since whatever will kill them will prolly also kill the butterfly's. Maybe manual removal of the "bad" insects? Or you could simply remove the infected leaves and dispose of them so the infection doesn't spread.. (wrapped in several plastic bags, toss then wash your hands twice b4 touching the rest of the plant so you don't spread the whatever it is..)

Or... Not having a picture of the infection it's hard to make suggestions.. Perhaps take & post one?