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The cactus moth (Cactoblastis) is good at what it does: Destroy prickly pear cactuses.
In fact, it’s so good that the moth has been used from Australia to South Africa as a biocontrol agent to remove the plants from agricultural lands, where the cactuses are seen as nuisances.
But now the cactus moth is doing its job where it isn’t wanted, destroying cactuses from the Caribbean to Alabama. And just last month, the moth was discovered on Isla Mujeres, Mexico — 10 miles offshore from Cancun — perhaps blown there by storm winds.
The moth’s arrival in Isla Mujeres has raised fears that the insect could quickly jump to the cactus-rich Mexican mainland and even devastate the ecosystems of the American Southwest within a few years.
Can the moth be stopped? Nature.org spoke with John M. Randall, director of The Nature Conservancy’s Global Invasive Species Initiative, about detection and control efforts now underway, how the Conservancy has helped lead these efforts, and what more needs to be done.
nature.org: Where has moth infestation so far been the greatest worldwide, and what’s the damage like?
John Randall: Where the moth has been most devastating to cactus populations is probably Australia — but there it was intended to do that.
Where it’s been doing the most damage where it wasn’t intentionally introduced is in the United States — particularly in Florida. It’s attacked a number of native prickly pear cactus species there, including some that are rare. The first place where the moth was identified was actually on a Conservancy preserve in the 1990s, on Torchwood Hammock on one of the Florida Keys. It was hammering a really rare cactus there, and so the alarms went up. And since then it’s spread pretty widely.
nature.org: How far has the moth spread past Florida?
John Randall: It’s not clear how quickly it’s moving through the Gulf States, although it was moving far more rapidly through Florida than was predicted — at 130-plus kilometers [about 100 miles] per year as of 2004. That’s a lot faster than the moth spread in Australia, where it had abundant sources of food. The rapid movement may have been linked to shipment of infected cactus plants for nurseries and maybe also for human food consumption.
The farthest extent west right now is on Dauphin Island, Alabama. It’s not yet been found it any sentinel sites on the border between Alabama and Mississippi. But we have a pincer movement now going on between the threat of the Mexican invasion and this invasion. The fear on the U.S. side is that, if the moth moves further west, particularly into Texas and beyond to Arizona and California, then we’ve got real problems because Texas has quite a few species of prickly pear, as does Arizona.
nature.org: What kind of damage could the moth do in the American Southwest?
John Randall: It could decimate the prickly pear population. It’s known to be doing pretty severe damage to populations in Florida that have not been directly protected—where folks are not caging the cactuses to protect them or picking off the egg sticks regularly. It seems to feed really aggressively on North American species.
nature.org: How would that affect the biodiversity of the region?
John Randall: There would be huge losses. You’d see the loss of a number of animal species, both vertebrates — birds in part — and invertebrates that depend on the cactuses. Key species of birds, most famously in the Sonoran Desert, depend on the cacti for food and nest sites. There are insects whose main source of food is the cactus’s flowers, the plant or the pads themselves. And those insects in turn are food for mammals and birds.
The cactuses also play an important role in holding soil in desert systems as well. We would likely see some problems with soil erosion, which can really be severe in the desert once you lose that fragile cover.
nature.org: Assuming the moth jumps from Isla Mujeres to the Mexican mainland, how long would it take to have a big impact on Mexico’s biodiversity and its economy?
John Randall: If it advances as quickly as it has in Florida, within a matter of years it’s likely to be in the Mexican desert ecosystems — both the Sonoran and the Chihuahuan. Mexico has 76 types of flat-bladed Opuntia (the group of prickly pear cactuses that the moth attacks), and 38 of them are found only in Mexico. So Mexico is really a center for prickly pear cactuses.
The cactus is also important agriculturally and symbolically to Mexico — it’s on the Mexican flag, where an eagle is depicted perched on a prickly pear cactus.
nature.org: What’s the Mexican government doing to stem the infestation on Isla Mujeres?
John Randall: They’re doing control operations — which means either cutting and burning the pads that are infested, killing whole infected plants and destroying them, or treating the plants with insecticide. In addition, they’re doing surveys on the mainland of the Yucatan Peninsula to determine if any have spread there. Thus far, they have not found any moths in the wild. They have found some in shipments into the country.
nature.org: What’s the Conservancy doing to address the invasion?
John Randall: The Conservancy has been involved for several years in trying to promote implementation of the USDA's sterile-male-release technique or other ways of stopping the advance of the moths, particularly to the west but also to the north. We’ve also helped sound the alarm about the problem and sought funds for control and prevention. And we're educating people to keep their eyes out for it and to discourage the movement of material that might be affected.
In Mexico, the Conservancy was trying to get more government attention and money to the problem before the discovery on Isla Mujeres was even made. The Mexican government has actually put money into the U.S. effort to prevent the moth from moving across the Mississippi River. They were actually giving the USDA money for the sterile moth release, and our folks were promoting that.
In the United States, we’ve been involved since at least 2001 on the policy side to promote full-scale implementation of a sterile-male-release technique. There has been research on the sterile-moth technique, and then a pilot was held back due to lack of funding. We believe it’s a very high priority.
nature.org: How does the sterile-male-release technique work?
John Randall: Essentially what you’re doing is flooding the mating market. The sterile males mate with females — and if there’s enough sterile males, they reduce the numbers of successful matings to a very low level.
The technique is the best thing we have right now. There’s been research into a biocontrol agent, but one hasn’t been found that would reduce or drive down the cactus moth populations to acceptable levels without harming native species of moths. And we found in Florida that putting cages around the cactuses was a problem because they prevented the pollinators from reaching the flowers. So that’s not a permanent solution.
nature.org: The moth is already in Florida and other Gulf U.S states. What should the United States be doing to stop the moth from spreading to the U.S. Southwest?
John Randall: We should be fully funding the sterile moth program. We should also be fully funding the cactus moth detection and monitoring network at Mississippi State University. And also funding the Mexican program to eradicate the moth on Isla Mujeres and prevent it from moving to mainland — because if they get it and the pincer arm moves up from the south, we’re going to get it anyway. The Mexicans were very farsighted in working with us to prevent the problem from getting to them — it would be wise for us to do the same.
Nature picture credits (top to bottom, left to right): Photo © iStock Photo (Prickly Pear Cactus); Photo © TNC (John Randall)
