The pest status of Liriomyza species in California has changed over time and in different geographic regions, with at least two examples of displacement of one species by another occurring. In the most recent example, beginning in the 1990's, L. huidobrensis (Blanchard) has become the predominant Liriomyza species in valleys along the central coast, while L. trifolii (Burgess) remains the predominant species in southern California. To investigate possible reasons for this change in status, differences in host plant use and reproductive success of intraspecific populations were examined for L. trifolii (Burgess) and L. huidobrensis. Results of these studies indicate that central and southern California populations of each species are distinct biotypes. Analyses of genetic differences corroborate the behavioral data. Furthermore, the results suggest the possible existence of cryptic species within L. trifolii. Equally important, when coupling these results with data from S. Scheffer et al., the L. huidobrensis species from California is now considered to be a separate species, L. langei Frick.