Mating behavior, host preference, developmental heat requirements, induction, termination of diapause, and conditions influencing spring emergence were studied in apple, walnut, and plum populations of the codling moth, Laspeyresia pomonella (L.), from Tulare County, Calif., in an attempt to determine whether these different host populations were distinct host-determined races. Mating activity is apparently restricted to the orchard environment; males were not attracted beyond the orchard periphery by a synthetic pheromone source. Moths of the apple population demonstrated a strong preference to oviposit on apple trees provided with fruit rather than walnut or plum trees similarly provided with their respective fruit. Moths of both the walnut and plum populations distinctly preferred to oviposit on walnut trees, with their 2nd preference being apple, the plum population's preference for walnut being stronger than that of the walnut population. Apple, walnut, and plum fruits alone were not suitable to accurately determine oviposition preferences. The oviposition responses of walnut and plum populations conditioned on apple fruits and of hybrid crosses made among all 3 host populations indicated that the differential oviposition preferences exhibited by these populations were based on a combination of preimaginal conditioning and genetically inherent behavior. Developmental heat requirements were found to be the greatest for the apple and walnut populations, and the least for the plum population, depicting adaptive synchrony with host phenology. Initiation of diapause under field conditions occurs in the order of plum, walnut, and then the apple population. Responses to different critical photophases and the effect of fruit maturity were responsible for this diapause initiation sequence. In the laboratory, colder temperatures minimized differences in photoperiodic response while warmer temperatures maximized them. Spring emergence was found to occur about 1½ weeks earlier in the plum population than in the walnut or apple populations, which were both quite similar in their emergence patterns. This differential spring emergence was the result of the plum population's earlier diapause initiation and its reduced developmental heat requirement. Earlier diapause induction and longer diapauseterminating photophases were found to be capable of hastening diapause termination in all 3 host populations. The differences among the apple, walnut, and plum populations and their statistical relativity indicate 2 major distinguishing features. First, these Tulare County populations constitute well defined host-determined races rather than simple normal variation within the species. Second, the phylogeny of the races is such that the apple race is the progenitor of the walnut race and the walnut race the progenitor of the plum race, thus giving a hostdetermined succession, with the plum race being the furthest evolutionarily removed from the original apple the race.