Keith is right, you have tachinids, and yes, they are always lethal. Sometimes the maggot parasites emerge from the pupa, and other times they kill the larva before it can even get half-grown. They fluctuate wildly: i.e. in a strong local Monarch colony (near Los Angeles) I frequently monitor, in early September 4/5 larvae were parasitized. Now, in early October, all 25 larvae I found were parasite-free.
You... all... stare... but you'll never see, there's something inside me