A desert without a cactus is like a cat without whiskers and, in Texas, there is no cactus more iconic than the prickly pear. The plants, with their spiny pads and purple fruit, do more than complete the landscape; they support the desert ecosystem, provide food and shelter to animals, and form the basis of a multimillion dollar agricultural industry.
But the prickly pear could be lost thanks to an invasive moth.
First sighted on the Texas Gulf Coast in 2017, scientists have confirmed that Cactoblastis cactorum, also known as the cactus moth, is spreading. Its larva don’t simply eat prickly pear; they destroy it, often leaving dead plants and excrement in their wake.
The tale of how the moth got here is a centuries-spanning yarn of Mesoamerican empires, colonial intrigue and evolutionary biology. Researchers hope in that history they will find a lesson on how to stop the moth.
The story starts with a different cactus-eating bug, a much more useful one.
One day, thousands of years ago, someone in South America squished a bug. If you live around prickly pear cactuses you may have seen this type of bug. They are small pill-like insects called cochineal. They spend their lives sucking the juice from cactus pads, and, if you squish them, they produce a brilliant red color.
This fact was not lost on that original bug squisher. From this act, a red dye industry was born and spread throughout Mesoamerica. The Aztecs, and others, cultivated prickly pear cactuses to selectively breed cochineal bugs and create fine red dyes for painting, clothing and textiles.
Colin Morrison, a PhD student at UT’s Department of Integrative Biology, says cochineal dye production was a “market advantage” for those who mastered it.
“People from all over South and Central America traded with them to get this,” he says.
When the Spanish Conquistadors arrived in 1519 they took note. Hernan Cortez wrote the king of Spain about the vibrant colors of fabric and paint sold in Aztec markets.
Cochineal dye was superior to any red dye known to Europe. It was desired by royalty for fine clothes (the English “Red Coats” can thank the bug for their moniker) and by painters for its striking color and versatility.
Spain and Portugal, by virtue of their brutal conquests, had a monopoly on the dye. By the mid-1500s they were transporting 150,000 pounds of crushed cochineal bugs to Europe every year. It was the second most profitable commodity Spain extracted from the Americas after silver, and other colonial powers wanted a piece of the action.
British privateers raided Spanish ships carrying cochineal bugs. The English Navy Capt. Arthur Phillip snuck cochineal-infested prickly pear cactuses out of Brazil and transplanted them in Australia. A French botanist smuggled them to Haiti. These efforts failed to create sustainable cochineal populations, but they succeeded in spreading the prickly pear cactus throughout the world.
A Cactus Problem, A Mothy Solution
Mexico’s war of independence in 1821 spelled the end of Spain’s monopoly on cochineal dye. About three decadeslater, the invention of synthetic dyes made the cultivation of prickly pear cactuses and cochineal insects obsolete.
But the colonial competition over who controlled the cochineal left a strange legacy: prickly pear cactuses were everywhere. The British had brought them to South Africa, India and Australia. The French to the Caribbean.
“The cactus started to become very invasive in many areas, especially in South Africa and Australia” says UT’s Morrison. “All this rangeland in those countries was totally dominated by cactus because there is no natural predators of the cactus at those places.”
If the cactus was spreading because it had no natural predators, early 20th century naturalists had an answer: introduce a natural predator.
“They went back to South America and they did some classic ecological studies into what insects were specializing on the cactus,” Morrison says. “They found the cactus moth” in Argentina.
Morrison says the moth larva is voracious.
“It’s like you’re looking through a sheet of thin paper because they’ve eaten everything out of the inside of that cactus.” he says “They push the poop out through these holes that they’ve dug in the side of the cactus to get it out.”
It may be gross, but it was the perfect thing to fight the spread of invasive prickly pear cactuses. By the mid-1900s the moths had been introduced to Australia, India, South Africa and Hawaii among other places.
“This still is considered probably one of the most effective insect bio controls ever done at large scale,” Morrison says.
Except for one thing.
A Moth Hurricane
Morrison says the moths were introduced to the Caribbean sometime in the 1940s or ’50s.
“At that point, it was just a matter of time for the cactus moths to disperse, going north from island to island to island to the point where it finally got to Florida in ’89,” he says.
The cactus moth devastated cactuses in Florida. But the impact to the environment was muted because cactuses don’t play as large a role in that state’s tropical ecosystem. From Florida the moth spread to neighboring states.
Texas biologists watched and waited.
Colin Morrison, PhD student in integrative biology at UT Austin
The first cactus moth was spotted in Texas by a citizen scientist on the Gulf Coast in 2017. Researchers believe it may have been brought there by Hurricane Harvey.
“It’s becoming more appreciated that large-scale weather events can move insects huge distances, way farther than they’d be able to go themselves,” Morrison says. “They get blown up into the wind and dropped off somewhere.”
Since 2017, researchers have confirmed reproducing populations of the months on the Gulf Coast. The moth has no natural predators in the Southwest. A paper Morrison and others published in October forecasts that their spread could decimate native prickly pears.
“Imagine going to South Texas today and there’s this cactus,” Morrison says, “and you go back next year. And there’s absolutely none.”
But Texas could just be the start. From here, Morrison says the moths could spread and wipe out cactuses in Mexico and the American Southwest.
Prickly pear species in those regions, he says, “are pretty similar to the ones that they’re eating here in Texas.”
Biologists say the loss of native prickly pear cactuses in the U.S. and Mexico would have a devastating effect on the desert ecosystem. It could also destroy regional cuisine.
Cooked, raw or pickled, prickly pear pads, known as nopalitos, are a staple in Mexican, Mexican-American and Tejano cooking.
In Mexico, nopalito exports brought in around $16 million in 2017. The farming of the cactus also provides food and income to many low-income communities. The United Nations also expects nopalitos’ importance as a food source to grow as climate change turns more land into desert.
The threat of the bug is so great in Mexico that Robert Plowes, a research scientist at UT Brackenridge Field Laboratory, says the Mexican government even funded efforts to remove cactuses in the U.S. to stop the moth’s spread.
“I think it’s probably the first time in history that Mexican funding has been sent to the U.S. federal government in an attempt to help support the dispersal of the moths toward Mexico,” says Plowes, who studies the cactus moth with Morrison.
While unsuccessful in stopping the moth, he says, the project bought scientists time to figure out how to confront the insect.
Now they think they have a solution. But, as history has shown, it may be a risky one.
Meet The Cactus Moth Wasp
Apanteles opuntiarum is a wasp in Argentina that eats cactus moths. A plan in the works would bring it to Texas to do just that.
The irony of this solution is not lost on Morrison.
“In the same way that people went to South America to find the cactus moth to control the prickly pear,” he says, “people have gone back to South America to figure out is there any natural predators that are specializing essentially on eating cactus moth caterpillars.”
Robert Plowes, a research scientist at UT Brackenridge Field Laboratory
But, just as bringing the cactus moth from Argentina eventually came to threaten the desert Southwest, introducing the wasp could also backfire. In a worst-case-scenario, the wasp could spread unchecked, wiping out populations of native insects and throwing the environment into further disarray.
“You really need to be sure, if you’re going to use one of these parasitic wasps, that it’s going to target the thing you want and not other species,” Plowes says.
To do that, researchers at UT have been working with the USDA and the Florida Department of Agriculture to determine if the wasp might end up feasting on other native insects. Plowes says they do this in a lab, by collecting the larva of native Texas bugs and putting them in the same enclosure as the wasps.
The wasps “are given the opportunity to lay eggs in the Texas larvae, and then that is tracked and observed carefully,” Plowes says. “So far, I think signs are positive that this wasp isn’t going to disrupt the native species.”
The problem, researchers say, is that every day they spend testing the wasp is a day the cactus moth keeps spreading through Texas and, eventually, beyond. That’s why time is of the essence.
“We’d like to know how much time we have,” Plowes says, “and how much time before they get to Mexico.”