Saltcedar Biocontrol Team, 2014 Friends of IPM Award Winner
Pulling Together

Saltcedar Biocontrol TeamThe invasive saltcedar now has a formidable foe, thanks to a team of Texas A&M AgriLife Entomlogists and efforts by over a dozen stakeholders and agencies. Once a giant that lapped up riparian water resources and caused flooding by altering stream flow, saltcedars are now losing ground to three species of leaf beetles that are literally eating them to death. Behind the success of this pest management effort is a regional collaboration of federal, state and private agencies led by the Saltcedar Biological Control Team consisting of Dr. Allen Knutson and Dr. Mark Muegge, Texas A&M AgriLife Extension and Dr. Jerry Michels and Erin Jones, Texas A&M AgriLife Research. In February, these four entomologists received the Friends of Southern IPM Pulling Together award at the meeting of the Southwestern Branch of the Entomology Society of America.

Introduced in the 1800s as an ornamental, saltcedar (Tamarix spp.) ranges between 5 and 20 feet in height. It prefers riverbeds and desert riparian areas and grows quickly, often in dense thickets that block out sunlight and compete with native vegetation for water resources. Saltcedar has invaded riparian areas in the southwestern states, including Texas, New Mexico, Nevada, Colorado and Utah. In Texas alone, saltcedar infests an estimated 217,000 acres of riparian habitat.

In Texas, farmers and other stakeholders expended more than $6 million in spraying herbicide to control saltcedar; $2.3 million of that was in federal cost-share funds from 2004 to 2008.

In 2006, the Saltcedar Biological Control Team began working with scientists from USDA Agricultural Research Service to evaluate and release several species of saltcedar leaf beetles for the purpose of controlling saltcedar through biocontrol. Saltcedar leaf beetles feed only on the leaves and tender bark of the saltcedar reducing transpiration and weakening the trees’ ability to thrive and reproduce. Many saltcedar trees die after 3 to 4 years of repeated defoliation.

“The selection of species to release was initially based upon availability of beetle species,” says Allen Knutson, one of the members of the Saltcedar Biocontrol Team. Later, USDA-ARS entomologists developed maps that matched the climate of the beetles’ native range to similar climates in the US. This research helped determine where in Texas the different species were best adapted and helped direct release efforts.

The northern Tamarisk beetle, collected from northern China, was first available and released in Texas and other western states in 2004. Although it is very effective in other states, this species failed to establish in Texas.

The USDA-ARS then introduced three other species into the US, which were released in Texas by ARS and the Saltcedar Biological Control Team. One species, from Crete, initially established in west Texas but populations were greatly reduced by a late spring freeze in 2011. This species is currently limited to a few Texas counties and is not spreading rapidly.

A species from Uzbekistan was released in northwest Texas during 2005-2009 but was not recovered for a number of years. Suddenly in 2012, large populations of the Uzbek species defoliated extensive areas of saltcedar throughout northwest Texas and dispersed into adjacent Oklahoma. This species continues to be very effective, defoliates saltcedar annually and is expanding its range in Texas.

 Another species collected from Tunisia was released on the Rio Grande and Pecos Rivers in far west Texas. This Tunisian beetle quickly established and defoliated dense stands of saltcedar along several hundred river miles from Big Bend National Park to El Paso. This species is also very effective and continues to disperse into New Mexico.

Early in the project, research was conducted to determine how effective the beetles could be as a biocontrol agent for saltcedar. Research by Knutson and his graduate student Jeremey Hudgeons discovered that after four consecutive years of defoliation, carbohydrate levels, which are necessary for plant metabolism, were significantly depleted. Trees use reserved carbohydrates to survive the winter and to regrow new leaves in the spring.

“We wanted to know if beetles could be an effective biological control agent, or if they would only defoliate trees, and the trees would recover and continue to grow without any long term impact,” says Knutson. “Once we documented that indeed the repeated defoliation reduced food reserves necessary for tree growth, I was convinced the leaf beetle could be effective, and all of the efforts to establish beetles could then be initiated with confidence.”

“Early efforts to establish beetle populations involved releasing small numbers of beetles on a saltcedar tree in field cages, about 8X8 feet in size, and recording the numbers of eggs, larvae and adults of the subsequent generation,” Knutson says. “As numbers increased in the cage, the cage was removed and beetles allowed to move to adjacent saltcedar trees.”

Once a beetle species became established at the initial location, the team collected large numbers of them—20,000 or more at one time—to bring to new sites. These beetles were released directly onto trees without cages. From 2006 to 2012, the Saltcedar Biological Control Team collected and released 1.4 million beetles at new sites throughout west Texas. Today, three species of leaf beetles are now established on all of the major river systems in west and north Texas.

Beetle populations are established on an estimated 100,000 acres of saltcedar along riparian areas in Texas, or about 45 percent of the total infested area. As a result of the beetles’ defoliation, saltcedar thickets are brown and often leafless during most of the summer. Continual stress reduces the trees’ ability to bloom, so they have stopped producing seeds. Seeds are viable for less than a year, so the seed bank is quickly depleted, limiting saltcedar invasion of new sites. Trees that have endured defoliation for several years suffer branch dieback and some trees are dying. Large areas of trees are dying along the Rio Grande River where the Tunisian beetle has been defoliating trees for several years. Some of the native understory plants have begun to return.

“Observers unfamiliar with the beetles assume the defoliated saltcedar has been sprayed with an herbicide or are dying of disease,” Knutson says. “After one to two years, trees begin to die back as upper branches die, and epicormic growth is sometimes seen. Later, growth is limited to a few green shoots that emerge from the root crown as trees slowly starve to death. So the expanses of brown, defoliated saltcedar trees, and later dying trees, are changing the appearance of the landscape.”

Caption for photo: L-R: Jesus Esquivel, Allen Knutson, Erin Jones, Mark Muegge, Jerry Michels, and Jim VanKirk