They DO use other prunus species, according to what I've read, and I don't doubt it, because, I know from my personal experience that they use a variety of host plant trees. Although they prefer p. serotina around here, they've also used my pteleas, and I've seen one lay an egg on a tulip tree in Alabama.
What about common chokecherry/p. virginiana or any plum/prunus? Or what about any type of malus/apple or crabapple? Red-spotted purples prefer prunus serotina, also, but they use my crabapples from time to time, and I've read that tigers use malus pumila. Tigers also reportedly use willows and cottonwoods/salicaceae, camphor tree/cinammomum, catalpa, American hornbeam/carpinus caroliniana, cottonwood/populus, mock orange/styrax americana, sassafras, sweet bay/magnolia virginiana, spicebush/lindera, common lilac/syringa vulgaris and ashes/fraxinus.
The plants in our FAQ host plant list are just plants that somebody on this forum has had actual personal experience with - we didn't see any reason to just copy lists from books. Our FAQ doesn't get updated very often, which reminds me, if you're reading this LarryGene, ptelea trifoliata should be added as a host plant for tiger swallowtails - they've laid eggs on mine, and I've raised some on it.
MissSherry

If you want something smaller, try the spicebushs, but they need a lot of water. The camphor and cinnamon is not hardy in New Hampshire. Mock Orange would stay fairly small as well as the lilacs. I think you can even find dwarf types. However, I would try to stick as close to native sources as possible. Many of the newer cultivars aren't as attractive as host plants.

Crabapples are not large trees, either. But, don't plant them close to a cedar tree, or you can get really bad cedar rust on them. I don't know that it deters egg-laying, but I wouldn't want to take a chance. I have a crabapple that someone planted long before I moved into my house - and they planted it about 15' from a cedar, so naturally, it gets really bad rust. I've never found caterpillars on it.

I think the chokecherries get fairly large, but check them out, too, as MissSherry suggests. I'm growing WBC in pots so it doesn't get too out of control. When it gets too big, I'll start some other seedlings. The pot is one of those gigantic plastic pots.

Susan

I have the best luck attracting Tigers to lay eggs on my Sweet Bay Magnolia. The blooms smell heavenly and my 7 year old plant is a very reasonable 6 feet tall with a sort of lanky, upright shrubby growth to it... slow to grow even in full sun and very easy to control the size.

Judy (www.ButterflyNature.com)

Judy, I just planted a Sweet Bay, but I'm in zone 7 and wasn't sure if they were hardy to zone 5 in NH. Are they? If so, that's great, and means they have a very broad range.

Susan

According to what I've read about it, the Sweet Bay is hardy in zones 5 - 9. We sometimes get temps below zero here in Kentucky during the winter, and mine does great even without being mulched.

~ Judy

Thanks for the responses, Sorry it took so long for me to reply, I forgot about this post(due to all my seed starting efforts) I know the forum facts is for just hosts that members have had success with, that's why I wanted something listed there. I was considering P.virginiana but I think I went with the american plum (P. americana) because it's smaller and I haven't heard of it having spines which the chokecherry does? I am thinking I won't be able to check it for babies if it's spiny. I have a dozen spicebush but I've only had Spicebush swallowtails on those, so I don't think the tigers around here like those. That's interesting that ptelea has worked for you Penny, I wanted to plant some of those in hopes that a rare Giant in my area might find it, now I have a little more incentive to try them.

Just wanted to add that I have read all the different types of hosts they'll "supposedly" use in books and online but it just seems their favorites were P. serotina, and as Penny mentioned RSP's seem to favor them as well. So I was hoping maybe others could share if they've had good experiences with other Prunus species as well (since they are probably the most like them). I like the blooms on the prunus too and I've heard the butterflies love them too, so I'm thinking that may be the lure for them laying eggs on them. I am always trying to kill two birds with one stone so I figure what's better than a host plant that has good nectar source as well. Plus I like that they'll have berries for the birds, maybe that'll keep them full enough to stay away from the cats, Lol! Oh yeah I forgot before to mention that I think that sweet bay magnolia is marginally hardy here, but it might be taking a chance for me. I think there are others that unfortunately aren't as showy that would be a safer choice for me.