Funny you should ask this!
I was just reading this article-

http://www.ecosystemgardening.com/what-did-black-swallowtails-eat-before-we-brought-in-parsley-dill-and-queen-annes-lace.html

First Q: Depending on region, BSTs will use native members of the Carrot Family including Golden Alexander, Queen Anne's Lace and Mock Bishop's Weed
Q#2: Swamp Milkweed, without a doubt. I also get quite a few hits on my white Butterfly Bush and Phlox.
Hopefully, your "tree-top hoppers" are laying eggs:)

Leafhead,
As mentioned in the article Scorpgirl posted, Queen Anne's Lace is not native to the US.

Umm...My bad. Delete that.

Hi
I have the hole life cycle of the black swallowtail eggs caterpillars chrysalis and a female that is laying eggs on rue inside a cage. I'm so happy :)

Berger, the Eastern Tiger swallowtails is probably the most abundant species I see in my yard in the summer. This is probably because I've got lots of Prunus serotina, and many nectar plants. I've observed a female ovipositing on the black cherry trees a couple times, however they were doing so about 20-30 feet up in the canopy!

In my garden they particularly love Cup plant (Silphium perfoliatum) as a nectar plant. Also Echinacea, Buddleia, and the tall zinnias.