Biological Control Successful in Managing Invasive Shrub in Texas
DALLAS, Texas – In Texas, where cattle ranchers from the Rio Grande to the panhandle battle a persistent, invasive shrub that chokes out forages, robs sunlight and moisture, and degrades the land, researchers have found a sustainable method of suppression and environmental restoration.
Texas A&M AgriLife Extension entomologist Allen Knutson, along with the Natural Resources Conservation Service, received a Southern Sustainable Agriculture Research & Education (SSARE) On-Farm Research Grant to study leaf beetles as a biological method to control saltcedar. Over the four-year study (2009-2012), Knutson found that beetles released in five counties in west Texas were successful in establishing populations and defoliating saltcedar trees – an estimated 230 acres.
“Biological control is the most practical and economical control of saltcedar available,” said Knutson, who works at the Texas A&M AgriLife Research & Extension Center at Dallas. “Saltcedar can be controlled using herbicides, but it’s costly – about $135 or more an acre – because the chemicals must be applied by air due to the impenetrable nature of the saltcedar thickets. Saltcedar can also be controlled using mechanical means, but sites are subject to reinvasion because the roots can re-sprout and the tiny seedlings can be easily carried by wind and water.“
Between 2004 and 2008, roughly $3 million was spent through the NRCS Environmental Quality Incentive Program (EQIP) to control saltcedar using mechanical or herbicidal methods. Saltcedar was removed from 14,600 acres of rangeland and cropland enrolled in the Conservation Reserve Program at a cost of about $200/acre, but the effort treated only three percent of the estimated half-million acres of saltcedar in Texas.
Knutson said that saltcedar was introduced to the United States from Eurasia as an ornamental plant, but quickly spread in the absence of natural enemies. The shrub grows in dense thickets along streams, rivers and riparian areas, its deep roots stealing water from native plants, grasses, and forages.
“The high water use rate and extensive stands of saltcedar are of critical concern to ranchers in west Texas, a semi-arid region were water resources are especially limited,” said Knutson. “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.”
Leaf beetle species were introduced to the U.S. in early 2000, and since then have proven effective in defoliating saltcedar with no impacts documented on other plant species.
“Saltcedar is a good target for biological control as the plant has no close relatives in the U.S., thus the risk of beetles feeding on another plant is very low,” said Knutson. “Four species of leaf beetles have been imported and released into the U.S. One species is common in Colorado, Nevada, and Utah. Three other species are present in Texas and adjacent Oklahoma and New Mexico. These species are from southern Europe, northern Africa, central Asia and northwestern China.”
The leaf beetle larvae eat saltcedar leaves and tender bark, resulting in tree defoliation. Without leaves, the trees slowly starve to death.
“The biological control of defoliation, branch die back and tree death is a slow process,” said Knutson. “Once a thriving beetle population is present, which may take one to three years, the beetles must feed on trees for another two to three years before defoliation begins to significantly reduce canopy size such that forages can begin to recover.”
However, once established, leaf beetle populations are self-sustaining, naturally disperse throughout the area with no landowner assistance, and can increase in such large numbers that entire saltcedar stands are defoliated along many miles of streams and rivers.
“During this time, trees are using less water because of the lack of leaves. Also, once defoliated, trees have little energy to bloom and produce seeds, greatly reducing the threat of seedling infesting new areas,” said Knudson.
In the Southern SARE study during 2012, leaf beetles at sites in one county defoliated an entire stand of saltcedar, an estimated 125 acres, and in another county, the beetles defoliated all of the saltcedar along 10 river miles. The repeated defoliation by multiple generations of beetles over the four-year process has allowed grasses and other vegetation to return to the areas.
Knutson said that research on the leaf beetle species continues. Current work includes determining why beetles at some sites didn’t establish successful populations.
“In some cases, there is evidence that red imported fire ants and native ants are preying on the beetle in the pupa stage. Where these ants are abundant, their predation may be so great that the leaf beetle cannot survive,” said Knutson.
Researchers are also looking at the recovery process of grasses and forages following the defoliation of saltcedar.
Texas AgriLife Extension has created a publication, “Biological Control of Saltcedar”, and Knutson maintains a newsletter, “Beetle-Mania” that describes current research efforts to educate farmers and ranchers on using the leaf beetle to manage saltcedar.
More information on the SSARE project, “Biological Control of Saltcedar on West Texas Ranches Conserves Forage and Water Resources” (OS10-053), can be found in the SARE projects database.
Published by the Southern Region of the Sustainable Agriculture Research and Education (SARE) program. Funded by the USDA National Institute of Food and Agriculture (NIFA), Southern SARE operates under cooperative agreements with the University of Georgia, Fort Valley State University, and the Kerr Center for Sustainable Agriculture to offer competitive grants to advance sustainable agriculture in America's Southern region.