Here's a few small trees to consider:

Prickly-ash, Zanthoxylum americanum, is a shrub or small tree. It is the host plant for Giant Swallowtail, P. cresphontes, and Spicebush Swallowtail, P. troilus. It is very spiny and sprouts alot from the roots so not everyone likes it. However, the only places I've seen giant swallowtails are areas with prickly-ash nearby. And they are AWESOME butterflies!

Wafer-ash, also known as hoptree, Ptelea trifoliata, is a tall shrub or small tree. It is another host plant for the giant swallowtail butterfly. I think this is a great small tree. However, some people like the flowers' scent and others don't. For giant swallowtails, I'd take my chances.

Wild Black Cherry, Prunus serotina, might be good, also. It is the host plant for larvae of the
Red-Spotted Purple, Limenitis arthemis astyanax, Coral Hairstreak, Strymon titus, and Tiger Swallowtail, Papilio glaucus. The caterpillars of many moth species feed on Wild Black Cherry including the Promethea Moth, Callosamia promethea.

This grows into a large tree. You can severely prune it back every few years and keep it small. The caterpillars like it just fine even when it's small.

Never seen a butterfly on Seven Son flower (Heptacodium miconioides). Lots of other pollinating insects though.

I have prickly ash. Saved it from the bulldozers when they developed the 20 acres behind me for a subdivision back in the early 1990's. I harvest the little fruits every summer for use as Szechuan peppercorns. True S.P. comes from a closely related Asian species of Zanthoxylum that isn't hardy in my zone. The spice tastes the same -- lemon pepper with a neat cooling, numbing effect in the mouth. I can't say as I've ever noticed butterfly larvae on mine, but that doesn't mean they aren't there. I usually see a few giant swallowtails every summer. I have paw paws, but have never seen a Zebra swallowtail in Michigan. Where my dad grew up in S. Illinois they are common, extremely pretty.

You may already know this, but there's a lot of info about this on the Butterfly Forum and on their FAQ page.

Here is a link that might be useful: Butterfly Forum FAQ Page

ok.. i give up ... what does LF mean in the title???

wait.. Looking For???

anyway ... i never understood this butterfly thingee .. ANY flower is an attractant for such ... to suggest that because you plant one given plant.. a butterfly on its migration from mexico will find your plant seems a bit far fetched to me ....

all you need is a vast swath of diverse flowers.. that bloom in sequence.. so that they are attracted to your garden all season long ....

it seems to me, IN MY ZONE.. that most of my flowering trees.... bloom in early spring.. long before butterflies are in the area .... so you might want to make sure that the trees flower IN YOUR ZONE at a time when they are here ...

just my 2 cents.. whatever makes you happy is good enough for me ....

good luck ...

ken

Ken...just a bit of info from a butterfly fan. As you say, most flowers may be visited by butterflies, but some contain much more nectar and are like loaded buffet tables to them. Some hybrid flowers have very little nectar and are of little use to butterflies and are seldom visited. I see lots of butterflies visiting my zinnias, butterfly bush, and various species of milkweed. Many other flowers are in the area and few butterflies stop by them. When I survey for butterflies in our county parks I repeatedly find butterflies concentrated in areas with high quality nectar plants. You're right, planting certain plants doesn't guarantee butterflies will come, but it ups your chances tremendously.

Other plants/trees are planted to provide food (leaves) for baby butterflies (caterpillars). These may or may not be plants that the adults use for nectar sources. If you plant these hoping to attract butterflies, you should be glad to see chewed up leaves and not use pesticides or you will kill the baby butterflies (caterpillars). Most species of caterpillars can only feed on one or a few closely related species of plants. Plant these "host plants" and there is a good chance the adults will find them and lay eggs there, and you may see the young and adults later in the season....or see birds, spiders, etc. snacking on the critters you tried to lure into the garden :) If you don't have the exact food the young need, you will only see adults that happen to make their way into the area for nectar. More adults show up if you have host plants sending out their scents to the prospective parents looking for egg laying spots.

Denninmi, I haven't seen a zebra swallowtail in Michigan either...or anywhere else. Friends of mine have seen a few in Michigan....usually in low areas near rivers where paw paws are growing. Someday, I'm hoping one will fly by as I kayak through their river bottom.

Sweetgum is a great choice for lunas; in this area they will also feed on hickory and birch.

I have occasionally seen butterflies (monarch, giant swallowtail, and a few others) visiting my Heptacodium, but it is especially attractive to bees. Perennialfan - I could probably send you some seed - contact me through gardenweb email.

Sassafras and spicebush are very attractive as foodplants for spicebush swallowtail (as well as promethea moth). You may have trouble finding sassafras at nurseries, but the larger nurseries sometimes carry spicebush.

You'd probably also have trouble finding prickly ash or hoptree. I used to have a Euodia (=Evodia) that often attracted giant swallowtails. Like prickly ash and hoptree, it's in the Rutaceae family.

A discussion of trees & shrubs as larval hosts is really appreciated. This highlights another dimention of woody plants that isn't talked about enough. W/O the plant species mentioned above, butterflies & moths would quickly vanish.
Lunas are one of several kinds of silk moths found in my area. All of them have underdeveloped proboscis and as such the adults do not feed. The caterpillars require specific host plants. Lunas seem to favor hickory and oaks here. A friend always has Lunas on his shagbark hickory in the Summer. I have them on my pecan. You can find them flat against the bark in the daytime. Quite often we see the wings under the tree, a screech owl gets them during the night.
As noted above, you can't have Zebra Swallowtails W/O Pawpaw. Sassafras/Spicebush/Red Bay should be a fixture in every so-called butterfly garden.
On the DelMarVa the beautiful Great Purple Hairstreak larva feeds exclusively on Mistletoe.