Answers from Helpful People
SelecTree from CalPoly 
SelecTree is an incredible searchable database for trees grown in California naturally, or under cultivation. A trusted resource for cities, universities, students, utility companies and non-profit organizations, we can be thankful for California ingenuity and the dedicated professors and graduate students who continue to build this useful database. Arboriculture and horticulture departments from universities throughout California, Florida, and others, contribute and teach from this website, and it is simple enough to understand, that the amateur and master gardeners alike can find it useful too.
Plant Answers 
Found on the way to photos of a plant known in California as "Sky Flower", this resourceful site is hosted by Dr. Jerry Parsons -- "professor and extension horticulturalist - retired" -- and Milberger's Landscaping & Nursery. The website provides a phenomenal online library of plant information and gorgeous photos showing plant interaction with birds and butterflies, deer, squirrels and the like. Although not from California, but Texas, this abundant information comes from a state with a university system that has made substantial contributions to both agriculture and horticulture. Perhaps start here: PlantAnswers.com. And yes, there are Sulfur Butterflies in California.
SDNHM Butterflies and Plants of San Diego 
A quick list of butterflies making their home in San Diego, rare or otherwise. This list's author is the San Diego Natural History Museum. Founded in 1874 by amateur naturalists during the John Muir era and coincident with the creation of California's state parks, and partnering with several other cultural institutions, SDNHM's preservation tradition continues. Copy and paste their butterfly names into your favorite search engine, or Google, to learn more about, and how to attract, California butterflies. The list is here, a subpage of SDNHM's Entomology page. Check out their list of plants that are either native or naturalized in San Diego County here, SDNHM's Botany page. Or, learn where to find San Diego plants where they occur naturally in SDNHM's Plant Atlas.
