Overview

Tamarisks are small trees or shrubs native to Eurasia that were introduced into western North America in the mid 1800’s, mainly to provide windbreaks and to prevent soil erosion in semi-arid and arid areas. The plants became naturalized in late 1800’s (Shafroth et al. 2005), and some species are still planted as ornamentals and shade trees. Approximately 1-1.6 million hectares of land in North America is infested today (Shafroth et al. 2005).

The plants are considered to be among the world’s 100 top invaders and one of the most damaging invasive weeds in the western US. The species that are present in the U.S. include Tamarix ramosissima, T. parviflora, T. aphylla, T. chinensis, and T. canariensis, and several hybrid forms between the different species (Gaskin & Schaal 2002). Similar to Arundo donax (giant reed), tamarisk invades riparian systems and responds positively to flooding which seems to facilitate establishment. The plants form dense stands, and invaded areas often become completely dominated by tamarisk with few other plant species present.

Tamarisk Biological Control

Tamarisk has proven extremely difficult and labor intensive to control with mechanical and chemical methods. Consequently, a tamarisk biocontrol program was initiated in the 1960’s. In North America, the plants are subject to very little herbivory. As with many introduced plant species, the full suit of plant feeding insects and pathogens that may feed on the plants in the native ranges was not imported at the time of introduction.

One leaf hopper species, Opsius stactogalus (Homoptera: Ciccadelidae), and two scale insects, Chionaspis spp. (Homoptera:Diaspididae), do occur on tamarisk in North America, but these species do not exert significant pressure on the plant (Weisenborn 2001, Lewis et al. 2003). After extensive exploration of potential biocontrol agents in the plants native ranges (Europe, Asia and North Africa), and subsequent testing to ensure safety and efficacy (e.g. DeLoach et al. 2000,
DeLoach et al. 2003, Lewis et al. 2003, Dudley & Kazmer 2005), the leaf beetle Diorhabda elongata (Coleoptera: Chrysomelidae) was released into the open in 2001 (DeLoach et al. 2003). The results of these releases were
mixed (DeLoach et al. 2004), and the most successful establishment took place at five sites in Colorado, Nevada, Utah and Wyoming.

At sites in northern Nevada, the beetles have defoliated vast stands with over 10 000 ha damaged at one site in 2004 (Geraci et al. 2006). By the 4th year of defoliation (2005), plant mortality reached 40%. Even without plant mortality, biocontrol may provide substantial benefits. Sap-flow measurements and evapotranspiration indicated that groundwater losses were reduced by over 70% during the first year of defoliation in Nevada, with greater savings in subsequent years (Pattisson et al.; unpubl. data). In addition, recent studies suggest that both avian (Hitchcock et al.; in review) and spider (Dalin & Dudley; unpubl. data) diversity and abundance have increased in stands where D. elongata is present.