The monarch (Danaus plexippus) is native to North America and is well-known for their mass migrations to warmer climates, especially coastal California.
The best way to attract monarch butterflies (and help the species) is to to plant native milkweed, plants in the Asclepius genus. Besides being the larval food source for monarchs, milkweeds are loved by bees and other beneficial insects. The milky sap of the milkweed is poisonous–not to monarchs, only their predators. Their milkweed diet makes both the larvae and adult monarch a poisonous meal to many predators.
Monarch butterflies aren’t in danger of extinction, but their North American populations have declined. This is in part because of the decline in popularity of native milkweeds in the garden (in favor of non-native butterfly plants such as butterfly bush, Buddleia davidii, which don’t have poisonous sap) as well as the elimination of milkweed habitat (through development).
Native milkweeds are a must for your garden if you want to attract monarch butterflies and help them out by giving them the best diet. Shown above is a popular member of the milkweed family called butterfly weed, Asclepias tuberosa. Swamp milkweed, A. incarnata, is shown below.
Photo credits: A. tuberosa courtesy of Wikipedia. A. incarnata courtesy of AB Native Plants. The monarch photos were taken by me at Tower Hill Botanical Garden.