MERCEDES — A small insect is proving to be the key to combating an issue that has plagued the border for centuries: carrizo cane.
That was the news according to a pair of entomologists who shared the results of more than a decade of work fighting the invasive plant during a meeting of the International Boundary and Water Commission in Mercedes this week.
Research entomologists John Goolsby, with the U.S. Department of Agriculture, Agricultural Research Service, and Maricela Martinez Jimenez, with the Mexican Institute of Water Technology, led a multinational effort by scientists from the United States, Mexico, Spain, France and others to combat the invasive plant, which has become endemic throughout the Americas.
Known by the scientific name Arundo donax, the cane has become the bane of binational water management officials, federal immigration officials, wildlife conservationists and even cattle ranchers.
Traditional eradication and control measures have often resulted in the plant, which grows in dense thickets up to nine feet tall, coming back thicker and stronger than before.
“Environmentally, it’s been a big problem all throughout the Rio Grande Basin,” Goolsby explained during his presentation at the IBWC Field Offices in Mercedes Wednesday.
That hardiness and resilience ultimately led scientists to a novel approach in fighting it by employing the use of several species of insects which serve as its natural predator in its native Europe, including a species of wasp whose larvae feed exclusively on the plant.
“All the infrastructure of the USDA was put to bear on this project,” Goolsby said.
Twelve years after he began his research, the wasp’s impact on the plant has been nothing short of impressive. “It’s less than half of what it was at the start of the program,” Goolsby said of current measurements of carrizo biomass.
CARRIZO AND ITS IMPACTS
Endemic to both the United States and Mexico, Arundo donax’s range spreads as far as Central and South America. Some 46 states list it as a noxious weed.
It’s believed the plant arrived in the Americas in the 16th century, when Spanish and Portuguese colonists brought it with them as a building material.
“The Spanish brought it to Mexico probably about in 1520 because they brought it for use as roof thatching,” Goolsby explained. “Everywhere a colonist went, they took the cane with them. That’s why it’s so widely distributed.”
It thrives in subtropical climates — like that found in the Rio Grande Valley — and produces thick mats of hollow, woody stems just over an inch in diameter with long, sword-like leaves. The bamboo-like canes are topped by hairy tufts where flowers produce infertile seeds. At its base, a tangle of roots known as rhizomes, crowd out other plants.
Those rhizomes enable the plant to reproduce by cloning itself. In fact, all the carrizo cane in the Americas is made up of the same clone.
“It’s one single clone in the New World, even though in Europe you have more than 120 different clonotypes,” Goolsby said. “Even down in South America, it’s the same clone,” he said.
It grows rapidly and soaks up water at a higher rate than the drought-hardy plants native to this arid region, ultimately outcompeting those species and taking over vast swaths of riparian habitat — that lush, thickly vegetated habitat often found near the Rio Grande, its resacas and other waterways.
In 2000, the plant’s impact on water resources became starkly apparent when the mouth of the Rio Grande went dry during a drought that affected the river’s watersheds in the Valley and far west Texas. “The River completely closed off at the mouth at Boca Chica and it set off alarm bells that we needed to figure out ways to conserve water,” Goolsby said.
Drought became a glaring concern again in the spring of 2010, when the state experienced its most severe drought since the federal government began keeping track via the U.S. Drought Monitor in 2000. That drought persisted until the summer of 2015.
Federal scientists began looking around the world for innovative water conservation measures and stumbled upon research occurring in South Africa, which feared that its second largest city, Cape Town, could run out of water completely by 2018 in what became known as the countdown to Day Zero.
There, scientists had begun tackling the issue of water conservation through a battle of an invasive plant species of their own, a type of pine tree native to North America. Those efforts led to as much as a 40% increase in rainwater runoff in South African watersheds as the trees were removed from the landscape over a 10-year period.
“This idea of controlling water using weeds to save water is an idea that’s catching on worldwide that comes from research that was conducted in South Africa,” Goolsby said.
But, it’s not just the strain on limited water resources which concerned officials on both sides of the border.
U.S. federal immigration officials tasked with patrolling the borderlands here find their work hindered by the impenetrable walls of green that obscure views of the narrow band of river separating the U.S. from Mexico and give cover to those engaged in illicit activity along that corridor.
Goolsby called it a “big problem” for Border Patrol. At the beginning of Goolsby’s research, visibility through the cane was as low as 6 feet, he said.
The cane’s dense thatch also serves as protection for another invasive species, the cattle fever tick, by creating a humid environment conducive to the tick’s survival, but which also hinders the prevalence of the tick’s predators, Goolsby said.
A WAR ON CARRIZO
The fight against the noxious plant has gone through several evolutions. Efforts to clearcut it, apply herbicides to winnow it down, and burning it have all failed.
“Border Patrol also tested everything — herbicides, burning, digging out the whole riverbank, sieving all the plant material out, replanting,” Goolsby said.
Eric Bautista, who tested several mechanical methods for controlling the cane, spoke about the misconception that burning it served as an effective treatment.
“When you burn it, it comes back twice as strong, twice as fast,” Bautista said.
“It’s brutal. This cane is brutal,” he said, describing how one border fire chief learned that the hard way after a plot of burned carrizo returned in just three months.
In Mexico, where the cane chokes the southern banks of the Rio Grande, as well as the sides of major highways, and the shores of Mexican lakes, officials are opposed to combating the plant using means that can be just as destructive to the environment as the plant itself.
“Mexico is opposed to using herbicides,” said Sergio Alberto Sanchez Garcia via a translator. Sanchez Garcia serves as an engineer for the IBWC’s Mexican counterparts.
Dr. Maricela Jimenez, who headed up the Mexican research team, echoed those sentiments. “It’s very important to use another kind of control of weeds,” she said.
“If we use this biological control, we reduce the cost to control this plant and at the same time if we use bio control agents, in time we can control the plants without any impact on the environment,” she added a moment later.
Researchers found that biological control in the form of several insects. The most successful thus far has been a tiny, non-stinging, plant- eating wasp native to Spain, Tetramesa romana, or the Arundo wasp.
But, before the wasp could be introduced in North America, it had to be approved by officials in the U.S., Mexico and Canada. Scientists tested it on over 40 different species of plants to ensure the wasp would not impact any other species besides carrizo cane. They found it fed exclusively on the cane.
Researchers also successfully tested two other species of insect — Rhizaspidiotus donacis, known as Arundo scale, and Lasioptera donacis, known as Arundo leafminer. A fourth species is still undergoing testing in Europe.
The Arundo wasp and Arundo scale have been deployed in both the U.S. and Mexico with success. The leafminer has been released on this side of the border, but it’s too early to tell if it’s had an impact.
THE BIG BITE OF A SMALL WASP
At just a few millimeters in size, it’s hard to imagine how the Arundo wasp can impact such a gargantuan plant. It’s the larvae that wreak the most damage.
The wasps, all of which are female, lay eggs inside the plant. The larvae then eat the plant from the inside out, weakening it.
“You see dead stems. You see dead side shoots. The plant it just trying to grow, but the insects just keep attacking it,” Goolsby said.
They live for less than a week, but each wasp can produce as many as 66 offspring.
“This thing is really doing great, it’s reaching a high damaging population here,” Goolsby said.
Researchers introduced the wasp in the U.S. as early as 2009. At one test site in Los Indios, the insects had destroyed one-third of the cane by 2014.
The introduction of Arundo scale in areas with the Arundo wasp has increased the rate of carrizo decline, however, as a nearly stationary insect, that impact is slower than the wasp’s.
The scale — so called for its resemblance to a fish scale on the surface of the plant — has a much shorter lifespan than the Arundo wasp. It lives for approximately two days and produces only two generations per year, Goolsby said.
Researchers released Arundo scale by hand every 25 miles along the river from Del Rio to Boca Chica. For the hardier Arundo wasp, releases occurred by hand, but also via air drop.
The insects were chilled in cardboard boxes and then dropped around Lake Amistad, Eagle Pass, Laredo and other locations.
The insects have removed millions of tons of the plant since testing began. “Just the wasp by itself has really had a really big impact,” Goolsby said.
Their success has been aided by man-made measures, as well.
Though clearcutting and complete removal of the plant has proven beneficial to its regrowth, partial removal of the cane has shown to instead benefit the Arundo wasp.
Bautista explained how topping the plant to within three feet of the ground benefits the wasp.
“We’ve had much higher densities of the insect after topping,” he said.
“The topping technique just speeds up the whole process. That’s the bottom line.”
Since the biocontrol measures began, visibility through the cane has increased from 6 feet to 33 feet, allowing Border Patrol better access to their patrol routes. And now that the combination of insects and mechanical topping has proved successful, Border Patrol has taken over the continuation of topping efforts, Goolsby said.
The smaller biomass of carrizo has also resulted in a savings of 6,000 acre feet of water, or $4.4 million of water — enough to supply the city of McAllen for a year.
The gradual decrease of the plant has also spared the riverbank from the erosive effects of total removal, which has allowed native species time to re-emerge while the riverbank remains stable. “We’ve recorded 54 different native plant species returning in these plots,” Goolsby said.
“The insects … have done a good job of bringing this plant back under control,” Goolsby said.