Larval Host Choice of the Monarch Butterfly (Danaus plexippus L.) on Four Native California Desert Milkweed Species

The monarch butterfly, Danaus plexippus L. (Lepidoptera: Nymphalidae), is found across North America during the summer months. Adult populations from western North America migrate to roost along the central coast of California. Monarchs that disperse in the spring to the arid regions of southern California encounter widely dispersed individuals of desert milkweed species, including Asclepias fascicularis Decne. (Narrow-Leaf Milkweed), A. californica E. Greene (California Milkweed), A. eriocarpa Benth. (Indian Milkweed), and A. erosa Torrey (Desert Milkweed). The four species represent a range in suitability as hosts for monarch larvae. In laboratory assays of plants grown from wild-collected seeds, neonate larvae showed the least preference for A. fascicularis in host-selection assays and there were no differences in preference among the other three species. Previous research has shown that A. fascicularis has the lowest level of cardiac glycosides and the lowest latex flow of the four species examined. The leaf hairs of A. californica and A. eriocarpa provide a significant barrier to neonate larval success; larvae feeding on shaved leaves were significantly larger than larvae feeding on leaves with intact hairs. First instar larval behavior of cutting hairs before beginning to feed may improve their success by modifying plant quality and defenses.