Salt Cedar Problem

Salt cedar is a shrub or small tree that is native to Asia, the southern Mediterranean, and northern Africa. It comprises several species within the genus Tamarix, commonly called tamarisks, which were introduced in the United States beginning in 1823 (DeLoach, 2004). During the Dust Bowl of the 1930s, salt cedar was planted widely for windbreaks and to control stream erosion in the Great Plains and western United States. Since then, however, salt cedar has spread rapidly becoming an invasive plant throughout arid and semi-arid portions of the western United States (Fig. 15-1)

FIGURE 15-1 Salt cedar (Tamarix) growing beside an irrigation canal near Fallon, Nevada. Salt cedar is a large bush or small tree, up to 4 m tall, with attractive pink flowers, often used as an ornamental garden tree. Salt cedar has become an invasive plant that grows in dense thickets along streams, rivers, and wetlands in the western United States. Taken from Aber et al. (2005, fig. 1).

FIGURE 15-1 Salt cedar (Tamarix) growing beside an irrigation canal near Fallon, Nevada. Salt cedar is a large bush or small tree, up to 4 m tall, with attractive pink flowers, often used as an ornamental garden tree. Salt cedar has become an invasive plant that grows in dense thickets along streams, rivers, and wetlands in the western United States. Taken from Aber et al. (2005, fig. 1).

and northern Mexico, where it is responsible for a major ecological disaster (Deloach et al., 2007).

Salt cedars form dense thickets along waterways; these thickets crowd out native vegetation, degrade wildlife habitat, harm some 50 endangered or threatened native species, cause increases in wildfires and soil salinity, and impede recreational use of parks and natural areas. In addition to its disruption of native ecosystems, salt cedar is also a phreatophyte that consumes large amounts of ground water—a valuable resource in the western United States (Zavalata, 2000).

Most conventional methods for controlling salt cedar infestations are generally ineffective. Once established, Tamarix is extremely tenacious and difficult to eradicate through mechanical (cutting) or chemical (herbicide) means. Because of this growing problem, the U.S. Bureau of Reclamation (USBR) is participating with several other governmental agencies in research on the biocontrol of salt cedar at one of several approved field research sites in the Saltcedar Biological Control Consortium (DeLoach and Gould, 1998).

A leaf-eating beetle, Diorhabda elongata, imported from Eurasia, is a natural enemy of salt cedar (Fig. 15-2). In their larval and adult stages, D. elongata eat the leaves and outer layers of stem tissue of salt cedar and may defoliate the plant to such an extent that it eventually dies (Fig. 15-3). The carefully managed international biocontrol program has proceeded in several steps (DeLoach et al., 2007).

• Ten years of testing D. elongata under quarantine conditions in Texas and California, starting in 1986.

• Release of D. elongata imported from China into secure field cages at authorized research sites in 1997.

• Field release into the open within circumscribed areas at 10 approved sites in six western states in 2001.

• Additional field releases of D. elongata imported from Crete, Greece in two southwestern states in 2003.

• Release of Greek D. elongata at study sites in northeastern Mexico along the Rio Grande in 2007.

The intent eventually is to distribute D. elongata widely to the public for biocontrol of salt cedar, once thorough research and review of its effectiveness and safety are demonstrated. Remote sensing has been employed as part of

FIGURE 15-2 Diorhabda elongata deserticola, imported from northern China, on salt cedar. Third instar larva (~8 mm long). Taken from Aber et al. (2005, fig. 2).
FIGURE 15-3 Salt cedar largely defoliated by beetles at Pueblo, Colorado study site. This ground picture was taken near the end of the growing season in October 2003. Salt cedar bush stands about 4 m high. Taken from Aber et al. (2005, fig. 3).

this interdisciplinary research, primarily for identifying stands of salt cedar and evaluating the effectiveness of the biocontrol efforts (e.g., Everitt et al., 2006; Pu et al., 2008). A variety of methods has been applied including standard color photography, videography, hyperspectral imaging, and vegetation indices—data and imagery acquired from conventional manned aircraft at medium altitudes.

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