Four promising biological controls that could curb the impact of the invasive plant giant reed in the United States have been found in Spain by Agricultural Research Service scientists.

The giant reed, Arundo donax, has been particularly destructive in the Southwest United States, where it's an exotic and invasive weed of riparian habitats and irrigation canals. But in its native Spain, the giant reed is kept under control by a host of insects, and that's where the scientists have made numerous trips to in search of biological controls to bring back to the United States.

At the helm of the U.S. project is John Goolsby, an entomologist at the ARS Beneficial Insects Research Unit (BIRU) in Weslaco, Texas. He was accompanied to Spain by scientists Alan Kirk and Walker Jones from the ARS European Biological Control Laboratory in Montpellier, France. The ARS scientific team tested various insects to see if they attack only giant reed and not other plants. Helping out with Goolsby's efforts at Weslaco is entomologist Patrick Moran, who has documented the traits, or biologies, of the candidate biocontrol agents.

One of the biocontrol candidates, a scale insect called Rhizaspidiotus donacis, attacks the reed's root. This insect's release has been recommended by the Technical Advisory Group (TAG), a North American organization that oversees releases of weed biological control agents.

Another of the biocontrol candidates, the Tetramesa romana wasp, was released in Texas in April 2009. This wasp attacks the weed's main stem, weakening the plant, reducing its overall height, and causing it to form galls and put out side shoots.

The third promising biocontrol agent, the Arundo fly (Cryptonevra spp.), eats the inside of new shoots of the plant, while the leaf sheath miner, Lasioptera donacis, destroys the plant's leaves.

The scale insect -- which has an outstanding reproductive capacity and feeds on the part of the root known as the rhizome, where most of the plant biomass occurs -- shows the most promise of the four biocontrol candidates. Debilitating the rhizome could have a big impact on the plant's growth and spread.

This biological control approach is sustainable over the long term and complements mechanical and chemical control strategies. BIRU research leader John Adamczyk notes that the potential release of the scale is a major accomplishment for the research unit.