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.
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, in Howard County TX. In 2009, this population defoliated nearly 500 acres of dense saltcedar and dispersed across a fifty mile area in two west Texas counties. Beetles were collected from this population and released at the five counties for this project. Beetles were collected by shaking saltcedar branches into a white, five gallon bucket equipped with a funnel at the bottom. Beetles shaken from the branches were funneled into a one liter plastic jar at the bottom of the funnel. The collection site was periodically monitored to determine when a generation of adults would appear. Once beetles were present, usually in late July and again in late August, large numbers of beetles could be collected. Beetles were placed in paper bags containing a small saltcedar branch, held in coolers and released at the new site within 24 hrs. In some cases, coolers where shipped overnight to NRCS cooperators who then released the beetles.
Beetle establishment was determined by surveying trees for adults and larvae. The survey was conducted by marking ten trees spaced 30 meters apart along a transect extending from the release point. Two people visually searched each marked tree for 2 minutes and recorded all adults and larvae observed. The percent defoliation of each tree was also recorded. Once beetle populations increase sufficiently to defoliate trees, the beetle count survey was discontinued and the area occupied by defoliated trees was estimated. Each site was surveyrd the following spring to determine overwintering success of beetles. If beetles did not overwinter, additional beetles were then released. A beetle population was considered established at a site if the beetles overwintered (were present the spring following a release), and increased to numbers sufficient to defoliate trees without additional releases. One a population was established, no further releases were made as the population was expected to increase and disperse naturally.
At the Mitchell County site, changes in vegetation following saltcedar defoliation were monitored using four permanently marked transects. In 2012, grasses and forbs in each transect plot were identified and the percent ground cover occupied by each plant species recorded. Also, the percent canopy cover of saltcedar and other trees and shrubs was recorded.
During the four year study, a total of 105,000 beetles were released at the five sites (Table). Following release efforts in 2009, leaf beetles populations were established at King County and Mitchell County sites in 2010. In 2010, 46,000 beetles were collected and released and as a result addition populations were expected to be established in 2012. However, a late spring freeze in early February, 2011, apparently killed most of the leaf beetles at these and other sites throughout west Texas. Record-breaking cold swept through west Texas and high temperatures were below freezing for 2-4 consecutive days as far south at the Pecos River. Surveys of these five sites in the summer of 2011 found no beetles at any site. Late in the summer, a remnant population of beetles increased and defoliated about 5 acres of saltcedar at the Mitchell County site. The large leaf beetle population at Howard County was also lost so there was no source of beetles from which to collect and release beetles again at the project sites during 2011.
In 2012, beetle populations again increased and defoliated saltcedar trees at four of the five sites. The most successful sites was in King County were the beetle population dramatically increased in July and by early September had defoliated dense stands of saltcedar along eight miles of the Wichita River. Beetles also recovered at the Mitchell County site following the 2011 freeze and defoliated about 5 acres of saltcedar trees. Many trees at this site have very little foliage due to repeated defoliation. Populations did not establish at the sites in Knox and Garza County. Additional beetles, 17,000, were released in 2012 at the Garza County site in hopes of establishing a population. In 2012, leaf beetles rapidly increased, dispersed throughout the county and defoliated all of the saltcedar, an estimated 125 acres, along tributaries of the Pease River.
Educational & Outreach Activities
A publication titled “Biological Control of Saltcedar” was written and published by Texas AgriLife Extension. This publication, L-5444 describes the three species of saltcedar leaf beetles, their biology, life cycle and impact as biological control agents for saltcedar. This bulletin was distributed at Extension educational programs dealing with brush control and invasive species and is available online at:
A newsletter titled “Beetle-Mania” was written and distributed to about 150 readers representing landowners, water and land managers, and agencies (NRCS, etc.). The newsletter described current progress on releasing and establishing leaf beetles in Texas and current issues relevant to biological control of saltcedar. These newsletters are posted online at: http://bc4weeds.tamu.edu/
Presentations were made at the statewide Texas Brush Control Symposium attended by about 275 people in Abeline, TX, in 2012 and at the annual meetings of the Saltcedar Biological Control Consortium held in El Paso, TX in 2010, 2011 and 2012. About 45 people attend this annual meeting. Those attending the 2012 meeting represented two international agencies, five federal agencies ARS, NRCS, U.S. Fish and Wildlife Service, National Park Service and Bureau of Reclamation, four Texas state agencies two state universities, and two nonprofit organization The program included presentations by 18 speakers, breakout sessions and reports by four subcommittees (Science, Wildlife/Environment, Federal, State and Private Liaison, and Mexican Cooperative Relations).
In 2012, all of the saltcedar along about ten river miles was entirely defoliated by leaf beetles by late July at the King County site. The beetle population at the Mitchell County site also recovered and again defoliated about 45 acres of large saltcedar trees. Many of these trees now have very little green canopy as a result of repeated years of defoliation and grasses and other plants are increasing beneath the saltcedar canopy. Beetles at the Motely County site also increased dramatically and defoliated saltcedar trees along more than 50 miles of the Pease River. The NRCS cooperator reported that ranchers had decided to delay and possibly forgo herbicide spraying for saltcedar due to the impact of the beetles. The beetles at the Motely County site were identified as D. carinata, another species of leave beetles released several years earlier but that had not been later recovered. No beetles were present at the two release sites in Kent and Garza Counties. An additional 17,500 leaf beetles were released at the Garza County site in 2012.
A series of plots were established at the Mitchell County site to document recovery of grasses and other vegetation as the saltcedar is defoliated. Due to the record-breaking drought in 2011, very little plant growth has been recorded in the study plots. Vegetation cover and plant species data were collected at this site in 2012 to continue to monitor vegetation changes.
Saltcedar leaf beetle populations are now well established at three of the five ranches and are widely and naturally dispersing to new saltcedar infestations. As beetles repeatedly defoliate saltcedar, the trees dieback, cease flower and seed production, and becomes less competitive, allowing grasses and other vegetation to return. The process requires several years, and beetle populations can be temporarily reduced due to cold winters. Results show that biological control has the potential to provide a very inexpensive, target-specific and sustainable method of suppressing saltcedar infestations and is especially suitable for use in the rangelands and natural areas of West Texas.
As a result of this program, ranchers and land managers have seen how saltcedar leaf beetles defoliate and suppress saltcedar growth in their counties and have a better understanding of the benefits and limits of these biocontrol agents. Beetle populations are difficult to establish due to the need to collect and release a very large number of beetles. However, once established in a county, beetles disperse naturally and ranchers and landowners need to take no action as beetles will soon attack saltcedar throughout the region. Texas AgriLife Extension with the cooperation of NRCS has provided the expertise and assistance necessary to establish local beetle populations which will then disperse naturally throughout the area.
Areas needing additional study
Several years will be necessary to fully document the recovery of grasses and forbs following defoliation of saltcedar. The extreme drought of 2011 slowed all vegetation growth at the Mitchell county study site. Thus, documenting the response of vegetation as saltcedar growth is suppressed will require a long term study and is an area needing additional study.