I assume because the level of toxins in the leaves is less on new growth (like seedlings), but I'm only guessing, I'm not a scientist, nor a mama monarch, nor did I stay in a Holiday Inn Express last night.

I'm thrilled you have monarchs, I haven't seen ONE this year.

It's odd how that works. Last year I was over run with Red Spotted Purples. Later in the year I had Giant Swallowtails. I did not see a single Monarch until September, which I'm assuming were migrators just stopping by for a drink.
This year I have Monarchs stopping by daily. I saw ONE Tiger Swallowtail fly through but it never stopped. I wish I could share some of the cats, as I have so many!
I also planted flowers that are supposed to be good for the Monarchs (Liatris). I have yet to see them nectar on any flowers other than the Milkweed and Butterfly Bushes.

Yeah, oddly people say that what was their #1 nectar go to plant last year, is ignored this year in favor of something else.

Perhaps we should just assume they have individual tastes and desires ?

I told my husband I am loosing sleep over this! I worry about those caterpillars all day and night. ;)
The first thing I do in the morning is go outside and check on them. I would be more than willing to share eggs and caterpillars. I had eggs on almost every leaf of a milkweed plant. And I find that the female Monarchs lay eggs on the flowers all the time.
I would love to share the seeds this fall if anyone would like some. I have the white, and the pink Swamp Milkweed. Although I am finding that every seedling that comes up is pink.
I told my hubby that I need to get a cage for our screened in back porch. I tried looking through the files for other peoples set ups but didn't find many pictures.

Lol, relax....sit down...get a good night's sleep.

The caterpillars will be fine (or mostly fine)..Nature's been doing this a long time. You're doing admirably to provide host plants for them, to counter balance all the habitat destruction we're doing.

Here is a photo I took of one of my Monarchs.

Beautiful! Mitten, the easiest and quickest thing to do is buy yourself a couple pop-up enclosures. They are cheap and last for at least a few seasons. They are so lightweight and easy to move around, clean, fold up and store, and would be perfect on a screened porch (outside a squirrel or raccoon could probably rip them apart).

I bought one at Educational science about 5 years ago, that has lt. blue trim, approx. 14 x 24 inch, which is still usable but has torn a little in the corners. 2 years ago I bought a couple at the store in Livemonarch.com that I like very much - good quality. Got both the small (12 x 12) and large (14 x 24) castle.

I usually put the cats in the popups when they are about 3rd instar or larger. I've fed and raised up to 50 at one time in the size of the large castle. Right now I've got 5 chrysalises in the small castle.