FAFDL Columnist: Patty Johnson | Patty is owner operator of Pannill’s Gate Farm, Orange, VA.
In many parts of the country, mechanical and chemical controls of non-native plants species are a practical response. But controlling invasive plants that threaten waterways and other marginally accessible areas requires a more imaginative approach, especially when the water way is the Rio Grande, on the US/Mexico border. Add to that a wide variety of stakeholders, with potentially competing needs, and the prospect of a solution seems improbable. But in the case of Giant Cane (Arundo donax) dozens of local, state, tribal, national and international agencies collaborated to develop a management technique that minimizes the impact to wildlife and water quality, while ensuring that the physical security of the border can be maintained.
Giant Cane (Arundo donax) is a wild grass native to the Mediterranean that has become invasive pest throughout the Southwestern United States. It was first brought to the New World via California in the 1820s where it was used as a roofing material and for erosion control in drainage canals in the Los Angeles area. What didn’t make the journey were any insects that might keep this grass in check. As a result, Giant Cane has become one of the most aggressive invasive plant species in the South, out-competing native species in riparian areas, where traditional methods of control by tillage or herbicide applications are impractical.
The USDA explains the challenges Giant Cane presents:
Giant reed, also known as “carrizo cane” or “Spanish reed,” forms dense thickets that clog streams and irrigation channels, weaken river banks, stifle native vegetation, affect flood control, and reduce wildlife habitat. This invasive weed can grow 3 to 7 inches a day, reaching heights of up to 30 feet. “The weed also creates an ideal habitat for cattle fever ticks and makes it more difficult for inspectors to detect tick-infested cattle and deer,” says John Goolsby, an entomologist with the ARS Tick and Biting Fly Research Unit in Edinburg, Texas.
Eradication of an unwanted species is often unrealistic, but by inflicting enough stress on an unwanted species, more desirable species can become competitive. IPM – Integrated Pest Management – is the approach of applying several, wide-ranging controls – mechanical, chemical and biological – on a targeted species. The USDA has been developing biological controls as part of an IPM strategy for Giant Cane.
The use of beneficial insects is a logical aid in this process, but it’s anything but simple. Identifying, testing, replicating, releasing and assessing the effectiveness of candidate controls can literally take decades. Two species of insects, in this case a wasp and a scale, are showing promise as part of an effective scheme to control Giant Cane on the Rio Grande. The USDA has been combining the biological control with the mechanical method of “topping”:
Goolsby’s recent research shows that the release of the insects has significantly helped control giant reed over more than 558 river miles between Del Rio and Brownsville, Texas. He and his team measured biomass-the amount of cane above ground-at 10 field sites before the insect release in 2007. At the same time, they measured sites where the insects were not released. Measurements taken in 2014 showed an average 22-percent decrease in plant biomass across the sites where the insects were released.
“We’ve thinned the cane out significantly,” Goolsby says. “The biggest decline in plants correlates with the greatest number of our biocontrol agents—the wasp and scale.”
The reduction of giant reed will save more than $4 million worth of agricultural water per year, he adds. Biocontrol of giant reed can also benefit Cattle Fever Tick Eradication Program efforts.
To accelerate giant reed removal, scientists developed an integrated pest management approach that combines “topping,” or cutting, the cane (mechanical control) with wasp releases (biological control).
“Topping the cane suppresses growth for more than a year,” Goolsby says. “It causes the plant to bush out like a shrub. We believe the insects love the bushed-out growth. Not only do the plants stay small in stature, but they are also more susceptible to attacks by the insects.”
The combination of topping and insect releases gives a high, long-term suppression of cane, Goolsby says. It allows native trees to grow and start shading giant reed.
Often, controlling invasive species in not merely a matter of biology, but of resolve as well. And in the case of Giant Cane, this multi-agency, innovative approach, helps ensure that the long-term prospects of control are favorable.