Saltcedar Beetle and the Rio Grande by Kristi Drake

 

Saltcedar (Tamarix spp.), native to Central Asia and the eastern Mediterranean area, was introduced into the United States in the late 1800's as an ornamental and for use in erosion control. In 1920, saltcedars started to spread rapidly and today they infest some 2 million acres of important riparian habitat that is vital to fish and wildlife. In Texas, the largest infestation is along the Rio Grande, also known as the "Forgotten River" that stretches 285 miles from El Paso to Lajitas, TX.

Saltcedars are drying up desert springs and arroyos, displacing native plant communities, driving out native cottonwoods and willows, and degrading fish and wildlife habitat. They increase soil salinity and wildfire frequency, lower water tables and reduce the recreational value of parks and natural areas.

The reproductive strategy of saltcedar makes it extremely difficult to control. Mechanical and chemical controls require repeat applications, due to copious resprouts and reinvasion of seeds, This causes an increase in cost and damage to native plants and aquatic organisms. As a fire-adapted species, saltcedar also limits the use of prescribed fire - a less expensive management tool.

The best hope for long term, cost effective control lies with a tiny olive-green beetle: the saltcedar beetle (Diorhabda elongata). Imported from China, Crete, Tunisia, Karshi, and Uzbekistan, the vivacious beetle looks out over the sea of green saltcedar and thinks "buffet"! No bigger than a ladybug, the saltcedar beetle (both adult and larvae) feeds on the saltcedar's scale-like leaves and bark of small twigs, completely defoliating it. Overwintering under the leaf litter and soil, the beetle returns the next spring and begins to feed again. After 3 or 4 years of defoliation the saltcedar begins to die from starvation. Sound simple? Not entirely.

Research and monitoring potential biological control (bio-control) agents can take years, even decades. For the saltcedar beetle it took decades. Extensive studies of biological control agents for Tamarix started in the 1970s in Italy. Some 300 species were found to be natural enemies of saltcedar but further investigations were needed before permission was granted by USDA-Animal and Plant Health Inspection Service (APHIS) to introduce any of them (DeLoach and Knutson, 2004). The federal agency, along with advisory boards, must be certain that any introduced bio-control agent is host specific and safe for the environment.

In 1986, C. Jack DeLoach of USDA - Agricultural Research Service (ARS) along with collaborating scientist began reviewing literature and testing promising host-specific species for saltcedar overseas. Six years later, ARS was granted permission by APHIS to introduce seven species of insects into quarantine at the ARS Facility in Temple, TX for final host range testing. In 1999, the saltcedar beetle was chosen as the top candidate for the job and APHIS granted permission for the beetles to be released in secured cages. Two years later approximately 60,000 beetles were released into the open field across six western states, 500 of these being released in Texas.

The beetles did not establish in Texas. ARS discovered that in order for the beetles to survive, day length/diapause characteristics needed to match that of their native habitat. The saltcedar beetles that had survived in other states were collected north of the 38th parallel (China/Chilik, Fukang, Kazakhstan) and required a summer day length period of at least 14 hours 45 minutes (DeLoach and Knutson, 2004). Texas needed a beetle that was adapted to a day length period of less than 11 hours to avoid premature diapause (overwintering). The search began and by 2005 beetles from Uzbekistan, Crete, and Tunisia were discovered and approved for release, establishing well in central (mimicking Crete) and northern (mimicking Uzbekistan) areas of Texas.

Beetle release and establishment along the Rio Grande still had hurdles to jump. From 2003-2007, national biological control meetings and an international symposium were organized between Mexico and the U.S. to address the concerns of Mexican scientists about releasing beetles along the international boundary (DeLoach et. al., 2009). Their uncertainty was based on the concern that the beetles would damage the Athel (Tamarix aphylla), a close relative of saltcedar valued for its shade and wind breaking ability in Mexico. Field surveys showed that saltcedar beetles would feed slightly on athel, but would cause no more than minor damage to them.

By June of 2007, the Mexican officials approved the release of saltcedar beetles along the U.S. side of the Rio Grande. In 2008, three cages were erected at Ruidosa, TX, becoming the first site with saltcedar beetles collected from Tunisia. To populate this and other sites, roughly 30 beetles were placed in a mesh cage enclosing saltcedar. Once beetle populations reached 500-1000 they were released into the open, with a few reserved to populate other sites.

The release of beetles in the spring of 2009 became the start of something unpredictable. With the help of Big Bend Ranch State Park, Big Bend National Park and private landowners, several release sites were selected along the Rio Grande River between Candaleria and Big Bend National Park. By the end of October, ARS observations concluded that the beetle populations appeared likely to establish and were predicted to defoliate five miles of river frontage within the next year (DeLoach et. al., 2009). Weather patterns of 2010, along with the all-night buffet of saltcedar, played in favor of the little warriors and their population increased exponentially. By late August, the saltcedar beetles had defoliated 20 river miles of the Rio Grande and 10 river miles of the Rio Conchos, blowing predictions out of the water.

Take a drive down the River Road (FM 170) and you'll see remnants of the saltcedar beetles' work as brown, leafless shrub-like trees that line the river's banks. While only time will tell the final story of the Rio Grande, the saltcedar beetle gives us hope - hope that in time, native cottonwoods and willows will once again flourish along the grassy banks and the river will still be enjoyed by generations to come.

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