Does this look similar to the "white, stringy discharge" you observed:
If so, then your caterpillars were parasitised by a Tachinid fly. The female flies seek out and oviposit on the larvae. The tiny brown casings that you may also observe are fly puparia. The flies are smaller than house flies and are gray in color. The female flies are quite persistent and are difficult to keep out of cages made from netting. They may also have the ability to lay eggs on larvae through the netting e.g., when the larvae are crawling on the netting rather than the plants. Fortunately, unlike most Lepidoptera, the monarch has relatively few fly and wasp parasites to contend with. One reference indicates that 5 tachinid species have been reared from monarch larvae and pupae. In some locations the flies kill more than 90% of the larvae not found and harvested by spiders, wasps, earwigs, lady bugs (beetles), lacewings, ants, stink bugs and other predators.
You can read more about this parasitoid (and view more photos) in the July-November 2003 Updates at
The best way to protect developing larvae from predators and parasites outdoors is to find them early in development and enclose the plant they are feeding on with a tightly sealed mesh bag - we offer Insect Rearing Sleeves for just this purpose (or you can make your own):