The influence of host plant phenology on the univoltine specialist lepidopteran herbivore Mitoura nelsoni Boisduval was investigated using incense cedar, Calocedrus decurrens Torrey. The hypothesis that new spring growth is an optimal resource for M. nelsoni was tested by rearing larvae on plants collected along an elevational gradient at two times in the spring (both before and during the typical flight period of M. nelsoni). The oviposition preferences of females were assayed with the same plants. M. nelsoni pupae grew to consistently greater pupal weights when reared on incense cedar branches in the earliest phenological stages (largely consisting of branches taken from trees that had not initiated new growth), although females avoided ovipositing on prenew growth branches. Trees in the earliest phenological stages, which resulted in the highest larval performance, were collected before the typical flight season of M. nelsoni. The phenology of M. nelsoni does not seem to be synchronized to host conditions that are optimal for larval development. These results are discussed within the context of host-associated speciation in the genus Mitoura and temporal isolation that may be an important component of reproductive isolation between M. nelsoni and a closely related species in northern California.