Four species of Diorhabda (Coleoptera: Chrysomelidae) have been approved for release as biological control agents of invasive saltcedars (Tamarix spp.) in the U.S. Two species, D. elongata (Brullé) and D. carinata (Faldermann), were evaluated for their ability to establish populations in northwest Texas (from 32.5^°N to 34.0^°N) and increase in numbers sufficient to defoliate saltcedar trees. Following releases of adults during 2 years, D. elongata overwintered, established, and increased sufficiently to defoliate saltcedar trees at all three release sites. In contrast, D. carinata overwintered in the field at only one of three sites and then only in very low numbers. D. carinata did not defoliate saltcedar trees outside of cages at any release site. After 2 years of release efforts, D. carinata could not be detected at any of the three release sites during the third year and was not considered to have established. Survival of overwintering adult D. carinata was significantly greater than survival of overwintering D. elongata adults. However, D. carinata adults terminated overwintering quiescence earlier and were observed on saltcedar trees earlier in February and March than were D. elongata adults. These results suggested that emergence of adults from overwintering quiescence in late February and early March exposed D. carinata to freezing weather and contributed to the failure of the species to establish in northwest Texas. In contrast, D. elongata terminated overwintering quiescence latter and therefore experiences less risk of exposure to spring frost. These results suggested that D. elongata will be more effective than D. carinata for biological control of saltcedar in northwest Texas (32.5 to 34.0°N).