Hey there Floridians!
The best way to protect developing larvae from predators and parasites 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 via the Monarch Watch Shop for just this purpose (or you can try to make your own):
Perfect for raising monarchs and other critters on plants indoors or out! These white mesh sleeves have a clear panel for observation, one full-length zipper that allows the sleeve to open flat, and long strings for sealing the ends around branches or tree-trunks. Sleeves may be zipped together to increase the diameter so as to enclose sections of tree trunks or other large objects. Sleeves are approximately 24"' x 27" (unzipped).
As harpo mentioned, your screened porch could be a good spot to raise your monarchs. Tropical milkweed (Asclepias curassavica) does very well as a container plant - you could keep some of the containers outside and then cycle the plants after they have been chewed down by the caterpillars. Harpo's tip of clipping the leaf is a good one - keeping handling of the larvae to a minimum will decrease the chance of inadvertantly damaging them.
Harpo, you're full of all sorts of good info...yes the larvae tend to wander a bit before pupation. If you don't mind finding pupae in all sorts of weird places on your porch you can let them roam, but if you want to keep them together you'll need to either keep a close eye on your 5th instar caterpillars and move them to an enclosed space prior to pupation or use rearing sleeves or something similar to contain them.
Here's a fun photo of a monarch that apparently had free reign inside a classroom:
BTW - "pupae" is the plural form of "pupa"
Hope this helps!