Senetti attacked by leaf miner fly

I have two Senetti plants whose leaves are covered in ugly, brown, wriggly lines. A look on the net tells me that this is leafminer fly, and most sites say that it is rarely that damaging to plants. However both senetti are now dying, and it looks like so many of the leaves are affected that the plant might not be able to photosynthesise properly.Is there any way of treating the plants at this stage or should I just chuck them? Picking off the infected leaves as suggested by some sites would mean almost all the leaves at this stage. I am also worried about the rest of my garden becoming infected.