Technical Abstract: The tamarisk leaf beetle Diorhabda elongata Brulle deserticola Chen was collected in Northwestern China and has been released in the Western U.S. to control tamarisk (Tamarix spp.). Characteristics of diapause and reproductive development in D. elongata were examined to improve management as a biocontrol agent. Under long days, 16:8 (L:D) h, males began to emit aggregation pheromone within 2-3 days of adult emergence, mating occurred and females oviposited within 7 days of adult emergence. Under short days, 12:12 (L:D) h, males did not emit pheromone, mating did not occur, and both males and females entered reproductive diapause marked by inconspicuous gonads and hypertrophied fat body. Ovaries of diapausing females lacked vitellogenic oocytes and the ovarioles were clear and narrow, while reproductive females had enlarged ovaries with 2-3 yellow oocytes per ovariole. Diapausing males had thin, transparent accessory glands and ejaculatory ducts, while reproductive males had thick white accessory glands and white opaque ejaculatory ducts. Sensitivity to diapause-inducing photoperiods extended into the adult stage. Reproductive females ceased oviposition, resorbed oocytes, and entered diapause when switched from long to short days. Diapause-destined insects ceased feeding and entered the leaf litter 10 to 20 days after adult emergence, while reproductive insects remained on the plants and fed for at least 30 days. Reproductive insects exhibited dispersal behaviors, such as attempted flights, while diapause-destined insects did not show dispersal behaviors. Information gained from these investigations will be used to better manage populations in the field and to improve rearing and storage in the lab.