A River System to Watch: Documenting the Effects of Saltcedar (Tamarix spp.) Biocontrol in the Virgin River Valley

Throughout riparian areas of the southwestern United States, non-native saltcedar (also known as tamarisk; Tamarix spp.) can form dense, monotypic stands and is often reported to have detrimental effects on native plants and habitat quality (Everitt 1980; Shafroth et al. 2005). Natural resource managers of these riparian areas spend considerable time and resources controlling saltcedar using a variety of techniques, including chemical (Duncan and McDaniel 1998), mechanical, and burning methods (Shafroth et al. 2005). Approximately one billion dollars are spent each year on river restoration projects nationally (Bernhardt et al. 2005), and a majority of these projects focus on invasive species control in the Southwest (Follstad Shah et al. 2007).

A technique that has drawn much attention is the use of the saltcedar leaf beetle (Diorhabda spp.), a specialist herbivore, as biological control of saltcedar (Lewis et al. 2003). Research testing was conducted with beetles housed in secure enclosures in six states in 1998 and 1999 (Dudley et al. 2001), followed by open release at some of those sites starting in 2001 (DeLoach et al. 2004). By 2005, full-scale saltcedar biocontrol was implemented in 13 states, led by the USDA Animal and Plant Health Inspection Service (APHIS), the agency that oversees biological control programs, and with the participation and support of the U.S. Fish and Wildlife Service (USFWS). Despite the widespread application of Diorhabda, however, only limited research has quantified the consequences (benefits and costs) on biotic communities and ecosystem services. Alterations to riparian areas caused by various non-native species control activities have the potential to affect a variety of habitat types used by wildlife (Bateman et al. 2008a); processes like water availability, fluvial deposition, and erosion; and the establishment of other non-native species (Carruthers and D'Antonio 2005, Shafroth et al. 2005, DeLoach et al. 2006). Similarly, biocontrol is expected to modify riparian ecosystems, and it is imperative to document and evaluate both the environmental benefits and the potential costs of this tamarisk management method.

Policy History of Saltcedar Biocontrol and Southwestern Willow Flycatcher

The Virgin River, with both extensive saltcedar stands and native vegetation, flows from Zion National Park in southwestern Utah through the northwest corner of Arizona and into Nevada, where it flows into Lake Mead (Figure 1). Downstream from Lake Mead is the lower Colorado River, another system heavily affected by saltcedar and where managers and biologists are interested in the impacts of biocontrol. In 2006, weed managers moved a biological control agent (Diorhabda carinulata) from central Utah south to the town of St. George (intrastate transport is not under the jurisdiction of APHIS) in an effort to control saltcedar infestations along the Virgin River.

Along this portion of the river and downstream from St. George, can be found the southwestern willow flycatcher (Empidonax traillii extimus), a passerine bird and federally endangered subspecies managed by the USFWS. The southwestern willow flycatcher builds its nest in saltcedar across a much of its range, but it also utilizes willow (Salix spp.), its historic breeding habitat. Flycatchers likely select nesting habitat based on the structure of the vegetation and not necessarily plant species composition (USFWS 2002). In a summary of what is known about willow flycatcher breeding and territorial behavior, Durst and others (2007) found nearly half (43%) of flycatcher nests were in native-dominated riparian vegetation such as willows; in other locations, the bird nested in a range of habitats from monotypic saltcedar (6%), saltcedar-dominated (22%), to mixed native-saltcedar (28%) habitats. Thus a potential conflict exists when an invasive species (saltcedar) provides habitat for an endangered subspecies (southwestern willow flycatcher). Beetle releases prior to the 2006 introduction to the Virgin River basin had all occurred outside of flycatcher territory, so this was a new situation.

The early observation of southwestern willow flycatcher nesting in saltcedar led to consultation between the USFWS and APHIS (and its research collaborators) in 1999, with the outcome that Diorhabda research...