The principal hostplant for Papilio bairdi (western black swallowtail) is Artemisia dracunculus (tarragon sage.)  It generally is montane and grows near rivers and along hillsides.  It's most distinctive, consistent character in identification is the sweet smell of the leaves. 

Tarragon sage is an aster; not an umbel.  As such, there are other asters that superficially look like A. dracunculus; but are not.  Again, you can end all confusion as to whether a certain aster is A. dracunculus through smell. 

Papilio bairdi have also been known to feed on Lomatium junceum and Lomatium grayi grayi.  There is little doubt that they will be found on other umbels.