Drought stress on two Tamarisk populations (WY and MT) in containment: effects on Diorhabda carinulata survival and adult size

Several Diorhabda spp. beetles (Chrysomelidae) have been released and established as biological control agents of salt-cedar plants (Tamarix spp.) in the western USA, whose defoliation over several years begins to kill Tamarix plants. Although Diorhabda carinulata has established in northern WY (Lovell), limited or no established has resulted at multiple locations in Montana (including a 250K beetle release near Ft. Peck MT Reservoir). Cage studies were conducted in 2007-2008 to examine how drought stress to Tamarix plants influenced beetle size and survival. Limited survival of D. carinulata in field cages made it difficult to examine the influence of Tamarix drought stress treatments on beetles. In 2010 the Tamarix-Diorhabda related lawsuit forced the drought experiment to be moved into containment since CO Diorhabda (interstate transport) was provided; plant population (Lovell WY vs. Ft. Peck MT) was added as a factor. The survivorship of CO Diorhabda (20 1st instar larvae placed onto each plant with 16 replicates of 4 water treatments with 2 plant populations) were lower in well watered plants. Although the study in containment makes it more difficult to compare to field conditions and lacked predation pressure, it was a useful environment to isolate water and plant population treatment effects in this experiment, since upwards of 100% adult beetle survival occurred on some treatment plants. Our results will be combined with predation surveys and experiments to try to answer a simple question: why has D. carinulata failed to establish strongly in MT, given successful establishment on Tamarix in Northern WY?