- Animals: bovine
- Animal Production: range improvement
- Pest Management: biological control, weed ecology
Saltcedars are exotic, invasive small trees which form dense stands along rivers and streams in west Texas and degrade water and land resources. Leaf beetles, introduced into the US for the biological control of saltcedar, consume saltcedar leaves. Defoliated trees transpire less water and allow sunlight to reach the soil, favoring growth of forage grasses. In cooperation with ranchers and NRCS, beetles were collected and released in five counties in 2010-12. In 2012, large populations of beetles defoliated extensive areas of saltcedars at three locations. Beetles are now well established and should continue to suppress saltcedar growth and naturally disperse.
Saltcedars (Tamarix spp.) are exotic and invasive small trees or shrubs which commonly form dense stands along rivers, streams and riparian areas of west Texas. Saltcedar stands of 700-1000 plants per hectare have been documented and an estimated 500,000 acres of saltcedar occur in Texas (Hart et al., 2003). The deep root system of saltcedar allows it to tap ground water resources beyond the reach of many native plants. Dense stands along river banks, floodplains and reservoirs can result in a lowering of water tables due to saltcedars’ high evapotranspiration rate (Smith et al., 1998, Hart 2004). The high water use rate and extensive stands of saltcedar are of critical concern to ranchers in west Texas, a semi-arid region where water resources are especially limited.
In addition to riparian areas, saltcedar readily invades adjacent rangeland and crop land. Monotypic stands of saltcedar degrade rangeland quality by shading out desirable grasses and forbs, reducing surface and ground water resources and increasing soil and water salinity due to salt released from leaf glands and in fallen leaves (Hart et al., 2004). The accumulation of salts beneath trees further inhibits vegetation.
The Texas Farm Bureau has adopted as its state policy recommendations for controlling saltcedar and other brush species to improve grazing conditions for cattle and improve wildlife habitat and the American Farm Bureau policy urges the USDA to promote eradication of saltcedar (Nicolette 2007). Also, the Natural Resource Conservation Service NRCS in Texas ranks saltcedar a “high priority” for cost-sharing of herbicidal and chemical control methods for rangeland and agricultural land through the NRCS Environmental Quality Incentive Program EQIP (NRCS 2008).
Saltcedar stands can be rapidly removed by mechanical means and herbicides, but these methods are very expensive, can damage non-target plants, and treated areas are subject to re-infestation by wind and water-born seeds from nearby, untreated areas. Since 1999, $5.7 million of public funds has been invested in applying herbicide for saltcedar control along the Pecos and Colorado Rivers in Texas to improve water quality and quantity. Of the estimated 500,000 acres of saltcedar in Texas, these programs treated only 19,000 acres and no new large-scale herbicide programs are planned.
During 2004-2008, ranchers and farmers in Texas expended $2,250,000 in cost-share funds from the NRCS Environmental Quality Incentive Program (EQIP) to control saltcedar using mechanical or herbicidal methods (Bade 2008). Total cost of this control effort was $3 million as the EQIP cost-share is 75% of the actual cost and the landowners paying the balance. During this time, saltcedar was removed from 14,600 acres of rangeland and cropland enrolled in the Conservation Reserve Program at a cost of about $200/acre. This five year, $3 million effort treated only 3% of the estimated half-million acres of saltcedar in Texas. Clearly, the cost and challenge of managing saltcedar in Texas with mechanical or chemical methods is overwhelming and additional management tactics such as biological control are needed.
The USDA-Agricultural Research introduced the leaf beetle, Diorhabda elongata, into the US (Lewis et al. 2003). This species, collected from Crete, was released in 2004 near Big Spring, TX. In 2009, this population defoliated nearly 500 acres of dense saltcedar and dispersed across a fifty mile area in two west Texas counties. The Texas AgriLife Saltcedar Biological Control Implementation Program collected beetles from this site and re-located them to new sites on the Colorado River in 2007-2009 (Knutson et al. 2009).
Beetle larvae eat saltcedar leaves and populations are often so great that all of the leaves are consumed, resulting in complete tree defoliation. Without leaves, the trees slowly starve to death. Research by Texas A&M found that saltcedar trees begin to die after 4-5 years of repeated defoliation (Hudgeons et al. 2006). At Big Spring, TX, about 75% of the trees at the original release site are now dead, the remaining trees are much smaller and native grass cover has greatly expanded (DeLoach et al. 2007). Following the first year of beetle defoliation, trees begin to dieback, the canopy is reduced, and more sunlight reaches the ground, allowing grasses and forbs to grow. Also, without leaves, trees translocate much less water. Thus, the benefits of biological control begin to accrue within the first year beetles defoliate trees. Once established, beetle populations increase without further assistance or cost and naturally disperse throughout the watersheds, benefitting ranchers throughout the region.
Objective 1. Establish Self-Sustaining Populations of Saltcedar Leaf Beetles.
Release sites are located on properties with large saltcedar infestation and on major river systems in north central Texas. Once beetle populations are established, releases are discontinued and beetles naturally reproduce and disperse throughout the county.
Objective 2. Document Recovery of Grasses and Forbes Following Saltcedar Defoliation.
Once beetles begin to defoliate trees, competition for sunlight and water should be reduced and grasses and forbs should increase beneath the canopy of defoliated saltcedar trees. Vegetation will be sampled along permanent transects to measure this recovery.