The Northern tamarisk beetle (Diorhabda carinulata) was introduced to the Colorado River Basin in 2004 as a biological control agent for the invasive shrub: tamarisk (Tamarix spp.). Since 2004, D. carinulata has colonized much of the Colorado River Basin, defoliating tamarisk and adapting to local abiotic cues as it has spread across the landscape. I studied the interplay of abiotic cues, tamarisk defoliation, and the population dynamics of D. carinulata along portions of the Colorado, Dolores, and San Juan rivers from 2007-2012. My results suggest that the timing and location of tamarisk defoliation can be predicted based on the abiotic cues of a location (specifically temperature and day length) and the spatial distribution of D. carinulata across the landscape. In contrast, I also found that the spatial distribution of D. carinulata was often a result of D. carinulata abandoning areas where it had defoliated tamarisk at high intensities. I found that larval abundances from the first new generation of D. carinulata produced in a year were positively linearly correlated with defoliation intensities one month later. Comparatively, generations of larvae produced later in the season were correlated with defoliation intensity along a bell curve, in which the number of D. carinulata declined in areas>50% defoliated. The timing of defoliation during the active season was correlated with the arrival of each new generation of larvae. I found the number of generations of D. carinulata produced in a year varied based on spring temperatures and fall day lengths. The timing of when spring temperatures rose above 15°C dictated when D. carinulata could begin reproductive activity, and this in turn resulted in how early in the year tamarisk could become defoliated. Day length cues governing overwintering in D. carinulata appear to have shortened by>30 min. compared to populations of D. carinulata first released in North America in 2001, resulting in longer duration of activity into the fall. We found the range of D. carinulata could grow as much as 62.8±5.6 km in a year along a linear riparian system, and populations of D. carinulata could defoliate between 24±11.2 and 116±11.2 km of river corridor tamarisk in a year.

Patterns of woody-plant mortality have been linked to global-scale environmental changes, such as extreme drought, heat stress, more frequent and intense fires, and episodic outbreaks of insects and pathogens. Although many studies have focussed on survival and mortality in response to specific physiological stresses, little attention has been paid to the role of genetic heritability of traits and local adaptation in influencing patterns of plant mortality, especially in non-native species. Tamarix spp. is a dominant, non-native riparian tree in western North America that is experiencing dieback in some areas of its range due to episodic herbivory by the recently introduced northern tamarisk leaf beetle (Diorhabda carinulata). We propose that genotype x environment interactions largely underpin current and future patterns of Tamarix mortality. We anticipate that (i) despite its recent introduction, and the potential for significant gene flow, Tamarix in western North America is generally adapted to local environmental conditions across its current range in part due to hybridization of two species; (ii) local adaptation to specific climate, soil and resource availability will yield predictable responses to episodic herbivory; and (iii) the ability to cope with a combination of episodic herbivory and increased aridity associated with climate change will be largely based on functional tradeoffs in resource allocation. This review focusses on the potential heritability of plant carbon allocation patterns in Tamarix, focussing on the relative contribution of acquired carbon to non-structural carbohydrate (NSC) pools versus other sinks as the basis for surviving episodic disturbance. Where high aridity and/or poor edaphic position lead to chronic stress, NSC pools may fall below a minimum threshold because of an imbalance between the supply of carbon and its demand by various sinks. Identifying patterns of local adaptation of traits related to resource allocation will improve forecasting of Tamarix population susceptibility to episodic herbivory.