Newly detected moth threatens prickly pear cactus in Texas

Texas is now the front line of an ecological battle pitting a devastating invasive moth against the state’s ubiquitous prickly pear cactus.

Already, the South American cactus moth has laid waste to the cactus population in Florida, having come ashore there roughly 30 years ago. Now field biologists with the University of Texas have spotted it in South Texas and are rushing to bring the moth under control.

If they can’t, they say, a plant iconic to the Texas frontier and crucial for pollinators and wildlife could be wiped out. The westward migration of the moth has alarmed Mexican biologists partnering with Texas counterparts — south of the border, the prickly pear is a key species for human consumption and agricultural production.

The moth, named Cactoblastis cactorum, is “out of the box in Texas,” UT integrative biology professor Larry Gilbert reported Feb. 20 after he and a graduate student discovered a moth infestation in December among prickly pear cactuses at the Mad Island Wildlife Management Area, on the Gulf Coast. And in February, the cactus month was detected in Columbus, about 90 miles southeast of Austin.

Gilbert brought some of the moths and infected cactus pads back to the Brackenridge Field Laboratory in Austin, where they are under quarantine in an air-locked chamber.

Five infected prickly pear pads collected at Mad Island produced almost 200 moth pupae, according to Gilbert. Then, a little over a week ago, roughly 50 tiny caterpillars hatched and bored into cactus pads, essentially eating the cactus inside out.

Once burrowed into a cactus pad, the moth larvae “feed gregariously,” according to a description prepared by the University of Florida entomology department, until “the food supply is exhausted.”

Gilbert told the American-Statesman his research group is partnering with the U.S. Department of Agriculture to hasten the introduction of wasps, native to the moth’s Latin American stomping grounds, as a biological control before the moth causes further devastation. The wasp lays eggs in the cactus moth’s larvae — “parasitizing” and killing them.

Prickly pear has long been the bane of ranchers, many of whom consider it a nuisance if not properly managed.

When other forage is not available, sheep and goats have a tendency to eat large amounts of prickly pear fruit.

“Animals may pass this feeding behavior along from generation to generation, creating problems even when desirable forage is present,” Texas A&M University researchers have written. “When animals are consuming the (fruit) they lose weight. If they also eat the pads for a prolonged period, the small spines ... cause ulcerations and infection of lips, tongue, gums, palate and gastrointestinal tract.”

In Australia the moth was successfully introduced in the early 20th century to control a prickly pear infestation there. The moth also has been used to control prickly pear in Hawaii, India, South Africa and the Caribbean.

But Gilbert and others say that the experience in Florida, where the moth apparently was unintentionally introduced, possibly by cargo shipment from the Caribbean, is a cautionary tale.

First detected in Florida in 1989, its spread was swift. A letter written to the Florida Division of Plant Industry in 1990 described the level of damage by cactoblastis in the Florida Keys as “widespread and severe … with cacti being reduced to rotting masses.” Today it is found across Florida and has been detected as far north as South Carolina.

Stephen Hight, a research entomologist with the U.S. Agriculture Research Service in Tallahassee, said that sometimes when plants are attacked they’re not wiped out altogether but diminished in size.

The plants “can get waist-high, and it’s really common to be knee-high,” he told the Statesman. “They’re whittled down to be two to three pads tall, with fewer arms. And some have been just wiped out.”

“They can recover to some extent, but they don’t become the large plants they were before,” he said.

That’s a critical change on the open range, says David Hillis, an integrative biologist at UT.

The prickly pear cactus serves not only as food for white-tailed deer and javelinas, but also as a refuge spot for quail and other fauna.

And in parts of Mexico it’s a human dietary staple: Young pads, called nopalitos, are harvested early before their tissue has hardened or their spines have been produced. Pads are often canned or pickled.

“If (the cactus moth) moves south, it becomes an issue of economic devastation and human suffering,” Hillis said.

Scientists have worried for at least 15 years about the moth’s spread westward.

The moths were reported for the first time in Texas during the summer of 2017.

The U.S. Department of Agriculture is supplying the UT researchers with pheromone traps that will help determine the expansion more precisely by trapping males looking for females, Gilbert said.

But a four-year bio-control program to introduce the Argentinean wasp to Texas will cost roughly $300,000 annually, Gilbert estimates.

Rob Plowes, a UT research scientist who works with Gilbert, said there is no state funding for invasive species control, and federal agencies are overwhelmed with the number of invasive plants and animals now on these shores. Instead, the scientists, who already have money from the Lee and Ramona Bass Foundation, say they will seek private money for their project.

“These things are voracious feeders and voracious breeders and will eat any cactus in their way,” Plowes said.

There is no known satisfactory method of chemical control of the cactus moth.

“There is time but not much,” Gilbert wrote in a Feb. 20 Facebook post widely shared among biologists and plant specialists. “We won't know how much we miss it until it's gone.”