Invasive snail in Louisiana, spreading

From Delta Farm Press
Mar 3, 2009 10:24 AM, By David Bennett

With a humid climate, a plethora of connected waterways and cornucopia of flora and fauna, Louisiana — like Florida and Hawaii — seems particularly inviting to invasive species. The channeled apple snail has certainly found Louisiana to its liking and agriculture officials are hoping the state’s rice farmers don’t have another pest to contend with.

Native to the Amazon basin, the snail can grow to the size of a man’s fist. The snail’s appetite is voracious; it multiplies quickly and has become a major drain on Asian rice yields. Now one of the most important pests in Filipino and Indonesian rice production, it is also in the Dominican Republic, where it caused substantial rice yield losses the first three years after it was introduced.

The snail was found in Texas in 2001. Mo Way, a Texas A&M entomologist, says it hasn’t impacted the Texas rice crop. However, most of Texas’ crop is in a drilled-seeded system, “so there’s more of a delayed flood,” says Natalie Hummel, LSU AgCenter entomologist. “Rice doesn’t have water on it until it’s quite large.

“We don’t know what the potential for this snail is on Louisiana’s rice but we certainly have much more water-seeded acreage, much more which would seem susceptible. If snails get into rice, farmers would find areas of their field where plants are clipped just below the water surface.”

Fortunately, the snail hasn’t been found in a Louisiana rice field yet.

“When it happens, a grower will probably be draining a field and find one of the large shells. Anyone who finds one needs to contact us. There may be some things we can do to help.”

Hummel was recently in Plaquemines Parish near some ditches alongside citrus groves. “There were tons of the snails in the ditches — they were rolling over each other. The numbers were incredible.”

How did the snails get to Louisiana?

“Apparently through the pet trade,” says Michael Massimi, invasive species coordinator with the Barataria-Terrebonne National Estuary Program. Unlike in Asia, where the snails were brought for the escargot trade before escaping, the United States “imported them for pets, not food. They’re popular in aquariums because they’re large with a handsome shell. They’re fun to watch because they’re so active.”

There are several features that make the channeled apple snail a particularly formidable pest. One is an operculum, “a sort of bony plate that acts as a shell door,” says Massimi. “Most snails don’t have that — you can poke them out with a toothpick. You can’t do that with this species once they seal that door shut.”

If trapped on land or in poor water quality, the snails are capable of sealing the operculum shut and going dormant for months. “When conditions are right, they pop back out. If they’re buried in the mud, they have to dry out for a year, or more, before they’ll die.”

The snail also has both a gill and a lung. “You’ll sometimes see them crawl to the water surface and put out this long, snorkel-like apparatus that they draw air through. They do that when water quality gets poor. Even in ponds overgrown with algae where fish are dying, the snail has no problem.”

The snail was first discovered in Louisiana in 2006 in a drainage basin near Gretna. “They were fairly isolated for a while and we kept an eye on that area. But since then several other populations popped up that, it appears to me, didn’t come from the original find. It seems to be multiple releases.”

Someone got tired of them and tossed their aquarium contents?

“That’s so common. And it may not be a case of being tired of them. The person may love them — ‘hey, I want this in my backyard pond!’”

The snail reproduces in a manner “typical” for an aggressive, invasive species, says Massimi. Once mated, the females are capable of laying fertilized eggs for months. Each egg cluster holds 200 to 600 eggs.

The adult snails live underwater but the eggs are laid above it. Any kind of control must target both.

“It’s a difficult critter to deal with. The eggs are highly visible — they look like a chewed piece of pink bubblegum. The clusters are usually only a foot or two above the waterline and look like nothing else around here. When you see pink clumps, the snails are around.”

The snails’ impact in other states has been “highly variable. Someone with a wildlife agency in Florida told me he will drive down a road that has a pond on both sides. Both ponds will be infested with snails. On the left, the pond is denuded of vegetation, is murky and algae-filled — there is an obvious aesthetic impact. On the right, the pond has the same number of snails with no visual impact. It’s very strange and we haven’t parsed out why there are such differences.”

Regardless, Massimi expects Louisiana vegetation to take a big hit. He predicts most of the plants Louisianans are used to seeing around waterways and along bayous will be the first to go.

Interestingly, a lot of that vegetation is also invasive — elephant ear, parrot feather, water hyacinth. But “I must warn that these snails are not a solution to control that vegetation — because they’ll also take out the native species that are already under threat: cattails, duck potato, duckweed, and bull tongue. The snails are indiscriminant.”

With fewer plants, there will be murkier water, water that’s more easily stirred up with sediment. That means “we’ll have more algae-dominated water-bodies. That impacts not only the aesthetics but also wildlife habitat.”

There are a few predators of the snail. Crawfish will eat them when they’re small enough. But the snails grow quickly to outsize adult crawfish.

River otters do eat the snails. Raccoons will, as well, when they find them while foraging at the water’s edge. Unfortunately, the otter populations aren’t near the densities that will dent the snail invasion.

The wading birds — egrets and herons — will also eat the snails, if able. Young alligators will also eat them.

“But I don’t expect to get much help in controlling the snail numbers from native fauna.”

Massimi says there are few control options. “It’s a big problem. There is no snail-specific pesticide. Anything we’d put in the water to control them would affect, if not everything in the water, then at least the other invertebrates. Toxins we might use on one invertebrate will also harm another. And we don’t want to hurt crawfish and the like.”

In the case of an isolated pond, “sure, you could go take out the whole thing and then restock it. But in southern Louisiana that’s rare since all the water bodies are connected. That’s certainly true whenever there’s a heavy rain.”

There is ongoing research at the National Wetlands Research Center in Lafayette, La., on snail controls. Several new chemicals show promise. “One is a saponin, a naturally-occurring toxin that’s soap-like and breaks down membranes. Preliminarily, there is some evidence this toxin has some success against the snail.”

Of all the states trying to control this snail, Hawaii is the most advanced. When the snails came into Hawaii, the taro crop was decimated.

Hawaiians found their best control was a multi-pronged effort. “They used some carnivorous ducks and other predators, allowing fields to lie fallow for a year or two — drying out the ground, chemicals and everything else they could think of. That’s allowed them to get the snail to more manageable numbers in the higher taro production areas.”

Among other things, Florida uses volunteers to strap on boots and wade into waterways to knock snail eggs off and crush them, scoop up the adults with nets. This must be repeated every few months.

“They’ll never get them all but they are trying to keep numbers to a minimum. It is very labor-intensive, though. Volunteers wear out pretty quickly.”

The snails are edible and, by some accounts, are tasty. Massimi hasn’t had one, though, and doesn’t recommend them.

“Aside from the common diseases our native species carry like swimmer’s itch, this species of snail carries a particular parasite called rat lungworm. It’s dangerous and can be ingested if the snails aren’t properly cooked. If you eat this thing, make damned sure to cook it thoroughly.”