The Pale Swallowtail looks much like a Tiger Swallowtail. Except is is pale yellow, whereas a Tiger is bright yellow. They can sometimes be hard to tell apart. But they do lay their eggs on different plants. A Western Tiger Swallowtail will lay eggs on willows and plums, among others. Host plants for Pales include ceanothus and hollyleaf cherry.
Hollyleaf cherries bloom in spring. The bees love the flowers! If I stand under mine when it is in bloom, the buzz is quite loud. Could be scary, but the bees have no interest in me.
I don't see Pale Swallowtails very much. But they do visit my backyard occasionally. I was even able to rear one from an egg collected from the hollyleaf cherry a few years ago. Very exciting!