It sounds like a horror movie: A hungry moth marches across the country, attaching itself to prickly pear cacti and feeding until the plant looks as if it has been destroyed by dynamite. With all the plants in a particular landscape devoured, the moth moves on to the next prickly pear landscape, the next, and so on.

Something like this is happening with the South American cactus moth. It was introduced to Australia in 1926 to reduce the number of prickly pear and make large areas of landscape suitable for agriculture again.

As a biological control agent, it worked extraordinarily well, and in the 1950s, the moth was introduced to Hawaii and Caribbean islands. But it has also spread to areas where it was never intended to be, including the United States.

The moth was first detected here, in the Florida Keys, in 1989. Since then, it has marched along Florida's Atlantic and Gulf coasts, reaching as far north as Charleston, S.C.

In early 2008, its western boundary was thought to be Mississippi's barrier islands. But last May, according to the United States Department of Agriculture, scientists discovered infested prickly pear in the bayous south of New Orleans.

That is particularly troubling, says Jerry Levitt, who runs Arizona's plant protection and quarantine program for the USDA in Phoenix.

"They thought they had stopped it at Mississippi," says Levitt. "This is the first major movement we've seen in the last few years, and it surprised a lot of people."

If the moth continues moving west, scientists fear it will find—in Texas and especially Southern Arizona—much more accommodating environments, rich in different species of prickly pear.

What might the moth do to Arizona? After a caution about the difficulty of speculating, Levitt says, "In places where you have a lot of prickly pear now, they could potentially die away." Places with extensive collections of the plant "could potentially be in big trouble."

Mike Wallace, who also works with USDA in Phoenix, worries about an infestation in a remote area that goes unnoticed, spreading so far that cost of eradication becomes prohibitive.

"Folks don't like using pesticides, so treatment options are narrow," says Wallace, a supervisory officer in plant protection and quarantine. "It could have a significant adverse effect on the biodiversity of the state."

The Nature Conservancy's Global Invasive Species Initiative notes that some Sonoran Desert birds depend on the prickly pear for food and nest sites. And some insects feed on the prickly pear flowers and pads, and those insects, in turn, are food for mammals and birds.

But one Arizona scientist urges caution about overreacting.

"We need to learn more about it biologically instead of saying, 'Oh, my God, it's going to cause the extinction of everything, and we have to eradicate it,' says Carl Olson, associate curator of UA's entomology research collection.

"I know I'm in the minority, but it's not here. It's a long way from here, and it hasn't moved all that much since 1989."

John Madsen, a professor in the Department of Plant and Soil Sciences at Mississippi State University, disagrees, saying the moth "has shown a fair amount of movement for a biological organism." He added that because it's an invasive insect that entered the country through Florida, most of the scientists studying the moth and its impact are in the Southeast.

This interest led Madsen and others at Mississippi State to create the Cactus Moth Detection and Monitoring Network. The group's Web site says the moth threatens "rangeland grazing, nursery plants and landscaping, desert ecosystems and biodiversity, and the economy of the Southwest and Mexico."

In Mexico, prickly pear is not only an icon, depicted on the country's flag; it's also a $50 million to $100 million industry in food and forage. In the Southwest, it is a $70 million industry; the ornamental prickly pear industry in Arizona totals $15 million.

Arizona is particularly vulnerable, because the Sonoran Desert has more than 100 species of plants that the moth can attack. They're mostly in the prickly pear family, the so-called padded opuntias. Chollas are an exception; they wouldn't be impacted, and neither would saguaros.

The moth spreads by flight; through storms that push it into new environments; and even when a family packs up and moves from one section of the country to another.

In 2008, the USDA enacted a domestic quarantine that stopped the shipment of prickly pear from infested states to noninfested states through the nursery trade. The USDA has an ongoing campaign to educate professional organizations, homeowners and governments.

The agency has placed traps equipped with pheromone lures along high-risk pathways, in nurseries, and in the environment around those nurseries. Scientists monitor them, removing sticky boards that capture moths and testing them for positives.

Homeowners can check their prickly pear for trouble. Wallace says the first thing to notice is if the pads have begun to degrade and ooze from holes. On closer inspection, they'll seem more translucent than normal, because the insides are being eaten out. When the moth lands on a prickly pear, it lays its eggs on the pad. The larvae then hatch, burrow inside the pad and eat it from the inside out, until the pad falls apart.

At present, Texas and New Mexico lie between Arizona and the cactus moth, and opinions vary on whether it will get here.

"Our hope is we'll be able to eradicate colonies of cactus moths before they get to Texas," says Madsen. "There's a good chance we can keep it from moving into the Southwestern U.S."

But Levitt and Wallace say they're operating on the assumption it will get here eventually.

"We're working on sterile-insect releases, on new pheromones, all kinds of things to eradicate it or limit its impact," Wallace says. "This is a fight against time, because we know it's moving. It can be a very destructive pest, and we need time to get caught up with our science and technology."