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Combating an Invasion

In South Texas, scientists breed super wasps to control a water thief.
by Published on
photo by John M. Randall, The Nature Conservancy, Bugwood.org

Arundo donax, the giant cane that is taking over the Rio Grande Valley, is virtually indestructible. Burn it out, and it sprouts from the ashes. Spray it with pesticide, and within weeks it springs back. The cane grows everywhere and is absurdly thirsty, siphoning off, by some estimates, between 30 and 40 percent of the water in the Rio Grande Valley. That, John Adamczyk explained to me as we rode through the Valley in his four-wheel-drive SUV, is why the federal Department of Agriculture is resorting to biological warfare.

Adamczyk is a head entomologist at the USDA’s Kika de la Garza research station in McAllen. An affable Midwesterner, Adamczyk oversees an audacious and controversial plan to breed, test and release swarms of Arundo wasps, tiny Mediterranean insects that make their home in the cane’s leafy stalks and help slow its growth.

I wanted to see the cane, so we’d driven down U.S. 281 looking for some. It wasn’t hard to find. The cane was everywhere, thick, green stands 15 to 20 feet high, anywhere its roots could pull up water. It was along the highway. It was beside irrigation ditches. It flourishes on the banks of the Rio Grande.

“Man, the stuff’s all over,” Adamczyk said, steering toward a gap in the border fence next to a Border Patrol station. “The Rio Grande Valley is already arid, and that’s not going to get any better. The world is getting warmer, and agriculture on the river is expanding. We really can’t afford to be losing that much water to a weed.”

Mexicans call the cane el ladron de agua, the water thief. It grows so dense and pulls so much water out of the river and irrigation ditches that parts of the Valley are going dry. It’s drained habitats and helped make certain species of fish extinct.

While Adamczyk kept an eye out for Border Patrol agents, I got out of the SUV and walked across a farmer’s field toward the Rio Grande. I could hear the river, but I couldn’t see it. The Arundo cane was too dense. When I stepped into it, it was so thick I could barely move. It took 10 minutes to wrestle my way 30 feet to the water. The cane stretched up and down the river on both sides as far as I could see.

This unnatural density is one hallmark of an invasive species. The Arundo cane has spread so fast in Texas because nothing is feeding on it. In Europe, where it originates, specialized insects keep it under control. When the Spanish brought the cane to colonies in South Texas in the 1500s for basket-weaving, they didn’t bring anything that would eat it. Now, 500 years later, we are paying the price.

“Every native plant species in the Rio Grande Valley has spent millions of years in constant competition with every other plant and animal species in the region,” Adamczyk said. “They have bred to keep each other in check. But there’s nothing doing that for the Arundo. Nothing is eating it or slowing its spread, and it’s able to out-compete everything.”

There are three primary ways to control invasive plant species: Kill them with herbicides, clear them with bulldozers and machetes, or attempt to introduce a new predator. The least controversial approach, clearing the cane, is not going to work. There are thousands of square miles of the stuff, and Arundo cane is nearly impossible to cut out. Each stalk has a thick taproot that sends shoots in every direction. You can bulldoze or chop the cane down, and it will grow right back. Worse, any stress on the plant—say a machete blow—causes it to send out more root stalks. Every chopped-up joint of cane that floats downstream can sprout another stand.

Killing the cane is not going to be easy, and until recently, the USDA was considering spraying the Rio Grande and its tributaries—an area known for its intensive agriculture, which drains into the Gulf—with Monsanto’s Roundup herbicide. The drawbacks are obvious. In 2009, scientist John Goolsby of the USDA’s Beneficial Insects Research Unit proposed using “biocontrol” on the cane. The approach involves fighting one invasive species with another invasive species. In this case, wasps. The Arundo wasps are what entomologists classify as “primitive wasps”—they don’t live in colonies, they don’t build nests, and they can’t sting. They can, USDA scientists hope, help control the cane.

The wasps are from the same part of the Mediterranean as the cane, and the female wasps have sharp, tapered abdomens that they use to inject their eggs deep into the green stalks. The eggs cause the cane to form galls and grow outward, seriously retarding growth. Later, the young wasps emerge from the galls, and the cycle continues. Since 2009, the USDA has begun releasing them in test sites in Laredo and McAllen.

 

Since the Egyptians brought cats into their homes 4,000 years ago to eat rats, biological control projects have rested on a simple premise: If pests spread because they don’t have natural enemies, then introduce some. As the age of international travel began introducing new species willy-nilly, biologists began combing the globe for new, exotic predators to keep the invasives in check.

“We determine the pest’s point of origin,” said Adamczyk. “Then we observe the pest in its natural environment to see what natural enemies prey upon it. Then we try to reintroduce that natural enemy wherever the weed is located.”

Similarly, in 1870 U.S. Secretary of Agriculture Charles Valentine Riley launched the modern age’s first organized biological control project by sending a boatload of American predatory mites to France. The mites were supposed to eat the Phylloxera flies that were decimating the French grape crop. Riley’s attempt had approximately zero effect. The mites didn’t kill enough flies to save the vines. The only reason the French have a wine industry now is that they imported Phylloxera-resistant vines from Texas.

This debacle suggests that biocontrol’s underlying equation may be too simple. “The whole premise of biocontrol,” said Dean Pearson, a research ecologist with the Forest Service’s Rocky Mountain Research Station in Colorado, “is that things spread because they don’t have natural enemies. But that isn’t necessarily true. What keeps an invasive in check in its home ranges may not be another animal: It could be differences in climate, or in the soil chemistry, or in the bacteria in the soil. If you move them someplace else, they may spread for reasons that have nothing to do with what eats them.”

Biocontrol specialists, Pearson said, often don’t look at those other factors—they go straight for the “natural enemies.”

Pearson said he isn’t opposed to biocontrol, but thinks it needs to be used with extreme care because so much can go wrong. Most importantly, ecologists have to make sure that whatever natural enemy they import attacks only the target. Many of biological control’s most spectacular failures stemmed from carelessness about this risk. Sometimes, as with the cane toads the British brought to Australia in the 1930s to attack beetles, the new organism will devastate the environment it was supposed to save. The cane toad ignored the beetles, preferring native small mammals and amphibians. It has since driven many native Australian species extinct.

The scientists in McAllen have put a lot of time and money into making sure that they don’t repeat this sort of mistake. Part of this process involves finding an insect so specialized it can only live on one subspecies of Arundo cane. Among other things, the scientists looked at 16th-century shipping records to discover the cane had probably come from the port of Alicante, Spain. They compared DNA samples from South Texas Arundo to samples from eastern Spain. Living on the cane there were four species of insects that appeared to live nowhere else. Of these, the most promising was the Arundo wasp.

When the wasp eggs arrived from Europe, sealed in stalks of Arundo cane, agency scientists took samples of every plant in the Rio Grande Basin and put each in a glass case. They put wasps in to see what they did. “We had these biology students from Brownsville High School,” Adamczyk said, “and they would go into the sterile rooms, put on their iPods and record every time they landed on a non-target species, how long they stayed, what they did.”

He said the tests are so strict that if an insect eats a non-target plant even once, the USDA will likely ban its release. “There’s too much agriculture down here for us to release something we’re not sure of,” he said.

Pearson, the research ecologist, said you can’t ever be sure. “Biocontrol folks say, we have screening procedures to make sure introduced organisms won’t attack other organisms,” he said. “And it’s true, we’ve gotten very good at that. But there are other things that can go wrong.”

In 2008, Pearson published a paper showing how a “perfectly well-behaved” biocontrol agent wound up seriously changing the environment. In the 1970s, faced with European knapweed spreading over the Rocky Mountains, the USDA introduced the gall fly, a small fly that—like the Arundo wasp—lays eggs in the invasive plant. The USDA ran extensive trials to make sure the fly wouldn’t attack other plants in the region. They weren’t mistaken; it only laid its eggs in the weed.

But as the flies multiplied on the knapweed, their eggs multiplied as well. Native deer mice, which usually died out in the lean winters, suddenly had something to eat when everything else died. Their population exploded. The rodents carried hantavirus, and people in Rocky Mountain cities started getting sick. The flies, like Riley’s mites, haven’t stopped the target species from spreading. There’s still knapweed all over the Rockies.

Aside from attacking the wrong species, Pearson said, there are other ways a biological control agent can have unexpected effects. It may turn out that the invasive species takes on a valuable role in the ecosystem, like creating habitat for native species. Introduced control agents can also bring in new diseases: In the most dramatic example, amphibian populations all over the Americas and Australia were inadvertently devastated when species infected with chytridiomycosis, a lethal fungal infection, were introduced.

The South Texas Kika de la Garza facility has had successful biocontrol projects in the river. Until recently, many of the Rio Grande tributaries were fouled with water hyacinth, a South American ornamental plant said to have escaped from the 1884 New Orleans World’s Fair. The hyacinth had grown so thick that in places it covered the water entirely, slowing down flow and starving native plants of oxygen. To fight it, the USDA introduced an East Asian diving beetle that eats hyacinth roots; the plant population has since been reduced to manageable levels.

There’s no way of knowing now whether the Arundo wasp project will work. This is why the USDA is releasing the insects slowly, the better to track their effects. “The wasps have to survive,” Adamczyk said, ticking off the unknowns as we drove back from the river. “They have to not all get eaten. Then it becomes a question of whether they can keep the cane in check.”

Even if the introduction works, Adamczyk said, it’s not going to eliminate the cane. At best the wasps will be a control measure that, in conjunction with machetes and herbicide, will help keep the cane in check. The wasp-infested cane will grow slower and pull less water from the river. “Go to Alicante [Spain], and you’ll see the cane growing down by rivers. It grows all the way through Iran and into India. But it’s not everywhere—you don’t have it in massive stands like you do here. It’s just a normal plant.

“That’s what we’re trying to get to here.” 

 

Contributing writer Saul Elbein lives in Austin. His work has also appeared in The Jerusalem Post and the online magazines Tablet and Nerve.