The monarch needs the swam milkweed to host on...no milkweed: no monarch. Tell your friends to grow milkweed, The swamp milkweed and Butterfly milkweed are beautiful. The common milkweed is a bit invasive in your backyard. Above is the black swallowtail caterpillar.
Tulip tree is a great host for the swallowtail.....I picked up 2 host plants on the way home from her house.