Someone wandering along the banks of the Rio Grande, on the American side, in the summer of 2009 might have been startled by a small cardboard box plummeting to the ground. Neatly sealed with blue packing tape, its paper wrapping crisp and clean, the box would have felt light, even empty. But then, on further inspection, an observer would have noticed the platoon of tiny black insects exiting a slit on one end. Slightly larger than a gnat, the insects would have buzzed off, one by one, into the South Texas heat.

The insects were wasps, which hours earlier had been taken from a refrigerator at the Moore Air Base, in Edinburg, Texas, where they’d been maintained in a soporific state, and placed, with utmost care, in the belly of a Cessna 206 by John Goolsby, an entomologist for the U.S.D.A. Agricultural Research Service. Between 2009 and 2012, 1.2 million wasps—at least a hundred per box—rained down along a five-hundred-and-eighty-eight-river-mile stretch of the border from Brownsville to Del Rio, as part of a joint program between the U.S.D.A. and Customs and Border Protection.

The wasps—which are not the stinging kind—are harmless to humans, but they do inflict damage on a very particular target: Arundo donax, an invasive grass that lines the banks of the Rio Grande in thickets, looking like a hybrid between a corn field and a bamboo forest. Brought to the New World as a building material, the grass has, over the last four hundred years, ruthlessly colonized the waterways of the southern U.S., driving out the native plants in its march to botanical dominance. It flourishes along the rivers of Texas and the Southwest like nowhere else; as much as a hundred thousand acres of the Rio Grande Basin are infested with it. Arundo wasps, which the U.S.D.A. imported from France and Spain, insert their ovipositors into the growing cane shoots to lay eggs, which soon hatch into hungry larvae that burrow through the vigorous plant, rendering it somewhat less savagely aggressive.

Arundo donax, also known as giant reed, carrizo cane, and border bamboo, can grow as tall as a two-story house. Along the border, it forms a grassy forest, within which visibility is roughly twelve to twenty-four inches, and makes an excellent hiding place. This accounts for why Customs and Border Protection put up $9.7 million to send a bunch of boxes of wasps plummeting to earth. People who are trying to enter the U.S. illegally across the Mexican border often hide in the cane. It’s not a hospitable environment—the cane scratches and is difficult to walk through—but it is effective cover. Matthew Hudak, the acting chief patrol agent for the C.B.P.’s Del Rio sector, told me in a phone conversation that his team is sometimes ambushed as they enter the cane in pursuit of a suspect. “The danger in a physical altercation escalates when you’re entangled in an environment like that,” he said. The border-patrol agents would like nothing more than to remove that particularly porous wall of vegetation.

On a recent canoe trip down the Rio Grande in Big Bend National Park, I paddled past green curtains of the cane. Here and there were tattered bits of white plastic tied to cane stalks, marking, I presumed, the spots where immigrants’ trails exited the water and disappeared into the riparian thicket.

In Big Bend I visited a restoration site, where thousands of native coyote willow had been planted. Fred Phillips, the landscape architect who designed the project, told me the local C.B.P. agents love it. The native riparian forest is more open and park-like; agents can see the people crossing through. In southwest Arizona, where the Colorado River forms a portion of the U.S.-Mexico border, Phillips has worked with the C.B.P. since the late nineties to return swaths of cane-infested land to native habitat. In various stretches of floodplain in the Yuma area, long used as a vegetal funnel to smuggle undocumented immigrants from the border to downtown, Phillips and his colleagues would venture in for soil samples, and the like. They would don goggles and dust masks, duct-tape cardboard to their forearms and shins for protection, and “crawl” over the nearly solid mat of vegetation with soil augers in hand. There they discovered a motley community hiding out in the cane: destitute families fresh from the border, smugglers’ camps, meth labs housed in plywood shacks, sandbag bridges that drug runners used to drive across the swampy earth.

Now a four-hundred-and-seventy-acre tract is a nature reserve, a popular spot for wedding parties and birdwatchers. The laws of gentrification, presumably, apply both to unwanted people and plants: kicked out of one neighborhood, they inevitably crop up somewhere else.

The cane is the bane not only of border agents and ecologists but also of farmers and ranchers. It sucks up three times the moisture of native vegetation and harbors a tick that transmits a deadly fever to cattle. I spoke with several farmers and ranchers with property on or near the river, and, though none were eager to see President Trump’s brick-and-mortar wall (despite having voted for him), they would love federal funds to help get rid of the cane. Representative Henry Cuellar, a Laredo native, has been pestering his fellow-legislators on the matter for the past decade, with little progress. His current ask is for fifty million dollars. But by all accounts it’s going take far more money, if not a miracle, to win the battle.

Bulldozing the cane out of existence works reasonably well, but inevitably, a hunk of rhizome floats down the river and reëstablishes a new patch. Cutting it repeatedly with a giant mower is another option, but it would cost huge sums to do so consistently on the full hundred thousand acres, much of which is inaccessible by roads. In 2008, just as Goolsby was preparing to release his first batch of wasps, the C.B.P. hatched a plan to begin aerial spraying of herbicides to control the cane, but residents of Laredo sued, successfully, to stop the agency. The herbicides, they pointed out, might have killed the grasses, but would have also poisoned their drinking water.

Memories of that sordid affair haven’t stopped Texas from continuing to pursue aerial spraying on behalf of its own border police. In 2015, the Texas Legislature passed a bill authorizing an eradication program along the Rio Grande, though only a hundred and eighty thousand dollars has been allocated to the cause thus far. Those funds were exhausted in the summer of 2016 by aerial spraying along nineteen miles of the river—1.5 per cent of the twelve hundred and fifty-five miles that comprise the U.S-Mexico border in Texas.

A spokesperson for the Texas State Soil and Water Conservation Board, the agency charged with carrying out this mission, wrote to me in an e-mail that the sprayed cane is expected to regrow this year and then “die completely” by next year. That doesn’t remotely comport with what several other experts in cane eradication told me, however. Thomas Vaughan, a professor emeritus at Texas A. & M. International University, in Laredo, who specializes in Arundo donax biology, says the plant has immense energy reserves in its root system, which allow it to sprout again and again, whether cut, sprayed, or burned to the ground. “Two weeks later, it’s three or four feet tall,” he says. “Many, many sprayings are required to deplete the energy stored in the rhizomes.”

The C.B.P.’s estimate, in 2008, that eradicating the cane with aerial spraying would likely cost $2.3 million per river mile hints at the absurdity of this approach, especially considering the added environmental costs. In Vaughan’s opinion, the notion of a cane-free country is half-baked at best: “The words ‘Arundo donax’ and ‘eradication’ do not belong in the same sentence. I always emphasize that to people, especially people from Homeland Security.”