Native to South America, the cactus moth (Cactoblastis cactorum), which has been described as “voracious,” has arrived and is spreading in Texas. The moth threatens to devastate prickly pear (Opuntia spp., Cactaceae), which is the state cactus of Texas, and possibly other cactus species in the Opuntioideae subfamily and may have consequences for entire desert ecosystems. These cacti are ecological keystone species and an important source of income for people, especially in Mexico. The possibility that the moth may spread into Mexico and further west in the United States is a serious concern.^1-3

Researchers at the University of Texas at Austin (UT) are planning to introduce a parasitoid wasp species to control the moth (see “Wasp Versus Moth” section), but they have been hindered by the COVID-19 pandemic, so the moth may continue to spread unchecked, at least until the researchers’ work can resume.

In December 2019, at least a year after initial reports of the moth’s arrival in Texas, UT integrative biology professor Larry Gilbert, PhD, and graduate student Colin Morrison found the moth among prickly pear cacti at the Mad Island Wildlife Management Area on the Gulf Coast of Texas. And, in February 2020, the moth was seen in Columbus, Texas, about 90 miles southeast of Austin. Five infected cactus pads (cladodes) that Gilbert brought to UT’s Brackenridge Field Laboratory reportedly produced almost 200 moth larvae.¹

The cactus moth’s larvae eat the cladodes from the inside out and can destroy entire stands of cacti, causing them to collapse in rotting heaps (see “Controlling Cactoblastis cactorum” section). In some places where prickly pear is considered an invasive pest, the moth has been used to control the cacti. The moth was introduced intentionally to Australia in 1925 to control a dense infestation of non-native prickly pear there and reportedly cleared 25 million hectares (61.8 million acres) of O. stricta, demonstrating its destructive potential. Later, the moth was introduced to other places with prickly pear problems, including the Caribbean in 1957.^1-4

“If prickly pear is native, you would not want this moth,” said Rob Plowes, PhD, UT research scientist who is working with Gilbert (oral communication, June 9, 2020). “But, if prickly pear is a pest, maybe you would want to bring in the moth. In many places in Africa, prickly pear has become established and is a huge threat to native ecosystems and agriculture. Ecologists have tried releasing Cactoblastis, but it has failed to establish for whatever reasons, and prickly pear continues its march in Africa. So, establishing the moth there might be beneficial, and we would support that effort. But its arrival in Texas is just about the worst-case scenario.”

Opuntia species, which number approximately 200, can prevent or reverse desertification and promote reforestation in arid areas.² Along with Cylindropuntia cacti, they also provide food and nesting sites for wildlife and contribute to soil stability.³ According to Stiling (2002), “Opuntia is used by a whole community of organisms,” including mammals, birds, and at least 109 insect species in Mexico. For example, Opuntia is a food source for javelinas, a group of hoofed mammals.⁵

Gilbert remembers a multiyear drought when he was growing up in southern Texas in the 1950s. “After three or four years without rain, there was nothing, and prickly pear was the only thing that cows were surviving on,” he said. “People would burn the thorns off with a blowtorch so the cows could eat it. And then deer and all sorts of wildlife would start eating the pads before they were even cool. So, I grew up seeing prickly pear as a way to sustain wildlife and ranching in a drought-prone, semiarid ecosystem.”

In North America, prickly pear has been used and cultivated since pre-Columbian times. Remains of prickly pear fiber in human coprolites (fossilized feces) from the Lower Pecos region of West Texas suggest the cactus was a main food source for the area’s inhabitants around 6000 BCE.⁶ A 2,000-year-old tattoo implement from Utah consists of plant materials including two cactus spines that compare favorably to prickly pear spines and is the oldest known tattooing artifact from western North America (see image).⁷

In the 16th century, Spanish Franciscan friar Bernardino de Sahagún wrote that Native Americans lived for many years partly because they ate uncooked foods like prickly pear fruits and pads.⁸ Also in the 16th century, Spanish explorer Álvar Núñez Cabeza de Vaca, the first European to explore present-day Texas, reportedly noted: “there are many kinds of tunas [Opuntia fruits] and among them some are very good, although to me they all seemed so and hunger never gave me time to choose.”⁹

Opuntia ficus-indica is one of the most important cultivated prickly pear species and is vulnerable to the cactus moth. Its fruits, which mature in early fall, contain antioxidant pigments called betalains¹⁰ and are valuable for their high-water content. Because of their vitamin C content, the fresh fruits have been used to treat scurvy, a disease caused by vitamin C deficiency.¹¹ The cladodes, called nopales, often are canned or pickled¹ and are a good source of calcium and other minerals, pectin, proteins, and vitamins.¹² A 2014 study suggests that O. ficus-indica can reduce postprandial blood glucose and increase antioxidant activity in healthy people and patients with type 2 diabetes.¹³

In a 2010 study, Recovera® (Finzelberg GmbH & Co. KG, sister company of Martin Bauer Group; Andernach, Germany), a proprietary extract of O. ficus-indica cladodes and fruits, showed blood sugar-lowering effects in pre-diabetic men and women. This provides some support for the traditional use of the plant to treat type 2 diabetes, which is one of the leading causes of death in Mexico.¹⁴ Recovera also has been shown to support muscle function and recovery in various ways, including by modulating insulin response, stimulating nutrient uptake in cells, and boosting glycogen resynthesis, which restores muscles’ ability to turn carbohydrates into energy.^15,16

In Mexico, O. ficus-indica is among the most economically important plants, along with blue agave (Agave tequilana, Asparagaceae) and maize (Zea mays, Poaceae). The cactus is a host plant for the cochineal insect Dactylopius coccus, a source of the red dye carmine. Carmine-dyed wool and cotton are important in Mexican folk art, and cochineal was the second-most valuable export behind silver for much of Mexico’s history.¹⁷ One hectare of O. ficus-indica used for carmine production can generate an estimated $15,000.¹⁸ The cactus moth jeopardizes this source of income. Because of its importance for food, carmine production, and more, prickly pear is depicted below the eagle on the Mexican flag and shield.¹⁷

However, some people view prickly pear negatively, maybe partly “because they’ve gotten tangled up with it, and it’s not a pleasant sensation,” Plowes said, but also some land-use practices exacerbate the density of prickly pear. When bulldozers or chain plows run through prickly pear habitats, the cladodes are fragmented and can regrow as individual plants, he said. So, instead of small patches of prickly pear here and there, this can create a landscape dense with prickly pear below mesquite (Prosopis spp., Fabaceae) trees, making the land unusable for grazing, etc.

Plus, animals can lose weight from eating prickly pear fruits and develop ulcerations and infections from the spines when eating the cladodes.¹ These and possibly other factors may lead some people to welcome the cactus moth. In fact, according to Gilbert, there were reports that some people tried to introduce the moth into Texas in the early 2000s, but he and Plowes think that is “misguided” and urge that “education is needed here.”

Wasp Versus Moth

The UT researchers emphasize the potential benefits of using biological control agents to address problems like the cactus moth. To control the moth in Texas, they plan to introduce a wasp species, Apanteles opuntiarum, which is native to the same areas of South America as the moth. The United States Department of Agriculture (USDA) research unit also has been testing the wasp to allow its release.

This tiny wasp is a co-evolved natural enemy of the moth and locates the moth by scent, according to Plowes. The wasp enters the cactus pad and injects its eggs into the moth larvae, parasitizing them, he said. The UT researchers hope that, after the wasp is introduced, it will spread with the moth and keep the moth controlled, but they do not expect the wasp to eradicate the moth.

Unlike chemical controls, biological controls like the wasp are organic and do not leave chemical residues, Plowes said. So, the prickly pear fruits and cladodes would still be safe for humans and animals to eat. According to some sources, no satisfactory method of chemical control is available for the cactus moth.^1,3*

A good biological control agent should meet several criteria, Plowes said. It should be target-specific, which often means it has a close evolutionary connection with the target and needs to ignore most everything else in the landscape. “We don’t just want a predator that takes a bite and moves on,” he said. “It also has to be able to exert some sort of control. We don’t just want it to be a nuisance, like fleas on a dog.”

To be impactful, the control needs to overcome the defenses of the target. “There is an evolutionary argument as to why natural enemies are effective,” Plowes said. Then, it should be sustainable. “We don’t want to spend the rest of our professional lives breeding wasps and releasing them every year,” he added. “We would like to help establish the wasp and let it continue on its own.”

Partly because this wasp species is likely to be highly specialized, Plowes and Gilbert think it is unlikely that it will have any negative effects for other organisms in the environment. In laboratory tests overseen by the USDA, the wasp has been exposed to native Texas moths that are closely related to the invasive cactus moth. In all the testing so far, the wasp has not affected any of the non-target species, which indicates that it is an appropriate biological control, Plowes said.

“The worst outcome would be if [introducing the wasp] does not work,” Gilbert said. “These kinds of insects are often like antibodies. They are so specific. It is like getting a vaccine that is very specific to one virus. That is the way these insect parasitoids are. They are generally very target-specific, which is why we focus on them.”

Florida, where the invasive cactus moth was introduced previously, has its own native species of cactus moths with their own parasitoids, but these parasitoids do not seem to affect the invasive Cactoblastis moth, Plowes noted. “In a sense, this is a reciprocal test that shows how specific these parasitoids are,” he said. “You can go to Argentina, where Cactoblastis is native, find a particular parasitoid that attacks it there, and then bring it here and demonstrate that that specificity continues.”

Plowes and Gilbert are using pheromone traps to monitor the spread of the moth in Texas.¹ The traps help determine the limits of the moth invasion, where the invasion is most dense, what times of year the moth is present in specific areas, and more. “It is very much like testing the human population to see where COVID-19 is present,” Gilbert said. The pheromone traps will help determine ideal locations for releasing the wasps, how many wasps need to be released, and when the wasps should be released.

They hope to start releasing the wasp by summer 2021, but that depends partly on the COVID-19 pandemic and if they have enough funding. Gilbert estimates that a four-year program to introduce the wasp will cost about $300,000 per year, and no state funds are available. They already have some funding from the Lee and Ramona Bass Foundation, a collaborative ConTex grant, and a Texas Ecolabs award but are seeking additional funding.¹

“We have to do this before the moth gets totally out of the box and gets into Mexico,” Gilbert said. “If we let this go [unaddressed for] two or three years, we are in big trouble.”

If the problem is not addressed soon, Plowes said, then it could create a situation like in Australia, where the moth decimated millions of acres of prickly pear. “We do not intend to lose 60 million acres of prickly pear [in North America],” he said. “At that point, it would not be necessary to introduce the wasp, because the moth will just crater on its own. We do not want to see it get to that point.”

This problem requires international solutions, according to Plowes. “The Mexican government has recognized the huge risk this moth poses and previously sent funding to the US to stop the spread of the moth toward Mexico,” he said. “We have collaborators in Mexico and Argentina. I think there is a consensus that introducing this wasp is the first phase of control. But, to introduce the wasp, we need to have permits from the US and Mexico, because the wasp could potentially spread to Mexico.”

Sergio Sánchez-Peña, PhD, a professor of entomology at Universidad Autónoma Agraria Antonio Narro in Saltillo, Mexico, is working with Plowes and Gilbert. “My role is to detect beneficial insects that attack caterpillars that are related to the cactus moth and have similar feeding habits,” Sánchez-Peña wrote (email, July 24, 2020). “These caterpillars already exist in Mexico, but they are not nearly as destructive as the invasive cactus moth. The objective is to investigate whether those beneficial insects that kill similar caterpillars will also attack the cactus moth if it arrives.

“Cacti are essential in the diets of wild mammals, birds, and reptiles, for example,” Sánchez-Peña added. “We want people to support biologically clean and sound options for controlling the cactus moth. This problem is also critical for Mexico because cactus pads are a basic component of the daily diet. Biological control by beneficial organisms is the best option against the cactus moth, because these organisms propagate themselves. It is important that the United States and Mexico work together to approach ecologically sound control options for this significant pest.”

Controlling Cactoblastis cactorum

Female C. cactorum moths lay eggs on the cactus cladodes. About 75 eggs on average, and up to 140 or more, are stacked in a chain called an egg stick, which often resembles a cactus spine. After hatching, the larvae, which are orange to red with black bands, tunnel into the cladodes and consume most of the inside. After one cladode is eaten or decayed, the larvae may move to the next cladode.^2,3,19

As they feed, the larvae push frass (debris or excrement produced by insects) and mucilage out, creating a noticeable “ooze.” The cladodes may become yellow, translucent, and hollow. The larvae’s burrowing can also cause bacterial infection, which speeds up destruction of cladodes. Fully developed larvae leave the plant and spin silk cocoons, often in the debris of rotting cladodes and other nearby sites. The adult moths are seen rarely because they are active before dawn.^2,3,19

The female prefers to lay eggs on some Opuntia species, such as O. engelmannii varieties engelmannii and linguiformis. However, the female’s preferences are generally not a good indicator of the cactus species on which the larvae perform the best. In one study, O. streptacantha was one of the best hosts for larvae survival and development but not one of the most preferred by females. This mismatch may partly be explained by the new association between the cactus moth and North American Opuntia species. The moth likely has not had time to adapt to its new cactus hosts. The larvae do not fare well on Opuntia species with tough cladodes, such as O. engelmannii var. lindheimeri and O. macrocentra, because the larvae apparently have difficulty burrowing into them.^3,20

The moth was first detected in southern Florida on Big Pine Key in 1989. It may have dispersed naturally across the Caribbean or been introduced unintentionally on horticultural prickly pear cacti imported into Florida.² Since then, it has spread along the US Atlantic and Gulf coasts and can be found as far north as Charleston, South Carolina. Although the moth spreads more quickly along coasts, inland spread is also occurring.³

According to Stiling (2002), at least 31 Opuntia species in the US and 56 species in Mexico are “likely to be attacked by Cactoblastis.”⁵ According to Stiling et al (2004), Mexico has more than three million hectares (7.4 million acres) of wild Opuntia (an area roughly the same size as Belgium), with 56 species in the subgenus Platyopuntia, which may be attacked by the cactus moth. Thirty-eight of those species are endemic. And at least 250,000 hectares (617,763 acres) of Opuntia are cultivated for food for humans and livestock, fuel, and other purposes.²¹ In Mexico, these plants are valued at more than $80 million annually.³

In Florida, the cactus moth threatens prickly pear species including O. cubensis, O. stricta, and O. humifusa, as well as rare species including O. spinosissima, O. triacantha, and O. corallicola.³ Endemic to the Florida Keys, O. corallicola reportedly is one of the rarest plants in North America. Only one population of this species with 13 mature cacti was known to exist when the moth arrived in Florida. In 1990, the cactus moth infested this population and killed one cactus, but the Nature Conservancy’s efforts have allowed the population to persist.²²

Manual removal of cactus moth egg sticks from cacti has helped control the moth in some commercial O. ficus-indica stands. According to Stiling (2002), this may be effective on Opuntia plantations in Mexico, if the moth arrives there, but it is impractical for wild cacti. Previously, in Florida, cages were used to protect prickly pear from the moth. However, cages prevent cross-pollination and may topple over during storms, which may damage or kill the cacti inside, so they are no longer used.⁵

Besides the wasp A. opuntiarum, which the UT researchers plan to release in Texas, other natural enemies of the cactus moth include the insect species Apanteles alexanderi, Brachymeria cactoblastidis, Epicoronimyia mundelli, Phyticiplex doddi, and P. eremnus. But these other insect species may also harm other non-target moth species.^3,23

Mahr (2001) suggests that “biological control of C. cactorum would, on the surface, seem to be an ironic but logical solution.”²⁴ And Pemberton and Cordo (2001) wrote: “the primary risk of employing biological control is the reduction of the many North American cactus moths, some of which probably regulate native Opuntia that can be weedy…. The relative benefits and risks of biological control need to be carefully assessed prior to any operational biological control programs.”²⁵

References

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