Green Invader

BY JOHN HOLDER

It’s a giant reed. It grows 10 meters tall, like a huge grass, and it grows … everywhere.

Well, not quite everywhere. But it’s appearing in more and more of North America, and that’s a problem. It’s an invader—like kudzu, or water hyacinth, or walking catfish … or fire ants.

It’s called Arundo donax, and it’s an invasive plant species that is spreading through Texas, California and other areas in the United States. In fact, Arundo can be found all over the Texas A&M campus, and there is a large infestation in the Navasota River bottomlands around Highway 6, just outside Navasota.

Texas A&M University research teams are seeking more information about Arundo in hopes of finding ways to control this plant, as well as assessing its effect on the Rio Grande Valley and other parts of the state. Associate Professor James Manhart, Associate Professor Alan Pepper, and graduate student Daniel Tarin, in Texas A&M’s department of biology, are especially interested in determining exactly where the Arundo found in the Rio Grande basin originally came from—because that may provide the key to controlling the runaway reed.

Like any invasive species, Arundo is not a problem in the places where it originated. In its original habitat, there are natural checks and balances called biocontrols that keep it from growing out of its natural ecological niche. According to Manhart, samples of Arundo taken from multiple locations in North America show that the plant was introduced from the same geographic location, very possibly southeastern Spain. Arundo is also found elsewhere in the Mediterranean littoral region.

But in New World locales such as the Rio Grande Valley, where sizable invasive stands of Arundo swathe riverbanks, the reed is causing significant problems. In its adopted North American homes, Arundo crowds out or overwhelms native species. The plant’s dense monocultural stands clog waterways and irrigation ditches, use more water than native species, increase the risk of fires, and ultimately lead to the loss of native habitats and biodiversity.

Working with genetic markers called microsatellites, Manhart and his colleagues are trying to narrow down the original habitat of Arundo, and its natural adversaries, as precisely as possible. The team’s task is somewhat easier thanks to an important characteristic of Arundo donax: The plant is sterile, not reproducing sexually. That means that the species is evolving more slowly than organisms that do reproduce sexually usually do, which in turn leaves Arundo more vulnerable to the same biocontrol agents over time.

The biologists are looking closely at several candidate biocontrol organisms. One is a wasp called Tetramesa, which like Arundo is parthenogenetic (reproduces asexually). So far, the scientists have developed four genetic markers tying the wasp to Arundo. Tetramesa is found in many of the same areas in Texas as Arundo, including San Marcos, Austin and Laredo. The wasp controls Arundo by damaging the plant’s leaves and other growing surfaces.

Another likely biocontrol agent candidate is one species of a tiny insect called a scale. Like the wasp, the scale feeds on portions of the Arundo plant, diminishing the plant’s dominance. And like Tetramesa, analysis of the insect’s genetic makeup suggests that the scale originated in the same area as Arundo, making it a promising candidate as a biocontrol for the plant.

Manhart hopes the information that he and his colleagues are gathering with microsatellite markers will lead to collecting just the right biocontrol insects.

“This information has allowed our collaborators to collect insects from this region, and these insects are more likely to be effective in controlling Arundo in Texas,” Manhart says. To determine whether the insects can control Arundo, and to protect other plant species from potential insect attack, caretakers are raising the insects in a controlled environment.

“Insects that pass these rigorous tests will eventually be grown in large quantities, released and monitored for their effectiveness in controlling Arundo,” he continues. “We are also using genetic markers to investigate variability in the insect biocontrol agents, and they will be used to determine which, if any, of the insect genotypes are most effective in controlling Arundo.”

The Manhart–Pepper–Tarin team is not the only Texas A&M research group interested in Arundo donax. Georgianne Moore, assistant professor in the Department of Ecosystem Science and Management, is looking at Arundo’s effect on water relations and management. Edward Rister, professor and associate head of the Department of Agricultural Economics, is focusing on Arundo’s economic influence, with help from graduate student Emily Seawright.

John Goolsby, a research entomologist (Biological Control of Pests and Weeds)  in the U.S. Department of Agriculture’s Agricultural Research Service, serves as “the major driving force,” as Manhart puts it, behind Texas A&M’s collaborative projects involving Arundo. Goolsby obtained funding for the work and has served as a coordinator for various aspects of the project.

Together, these biologists, ecosystem scientists, agricultural economists and entomologists are working to make sure that Texas—and the rest of the nation—gains some effective natural defenses against this “green invader.”