Of all groups of insects, butterflies, due to their popularity among amateurs and professionals alike, have received the most attention in ecological studies. Research on this group has influenced the formation and development of ideas in such areas as coevolution, nutritional ecology, population structure, and mimicry. Despite these influential studies, there is not much published information on butterfly ecology, which could be important in developing models and theories of plant/insect interactions and their evolution. One area with a paucity of available data concerns host plant location by adult females. The behaviors associated with this process are integral in determining the host plant range of a species, yet in only a handful of cases have any details of this process been uncovered. In this dissertation, I examine host plant location in the Baltimore checkerspot butterfly, Euphydryas phaeton, and study its consequences, in terms of host plant choice and larval ecology. These results are used, along with previously published information on other species, to formulate models of pre-alighting host plant location and host range expansion in butterflies.