- What is the invasive island applesnails’ impact in Louisiana?
- How far north might it range and how can the snails be controlled?
The first island applesnail discovered in the wilds of Louisiana was found in a drainage ditch near Gretna. The initial 2006 find of the invasive, voracious, fist-sized snail has since led to expanding populations and worries about Louisiana’s environment and rice crop.
There are several features that make the South American snail a particularly formidable pest. One is an operculum -- a sort of bony plate that acts as a shell door. If trapped on land or in poor water quality, the snails are capable of sealing the operculum shut and going dormant for months.
The snail also has both a gill and a lung. When water quality drops, the snails often use a long, snorkel-like apparatus to reach above the water surface and draw air.
A complicating factor when considering control of the species: adult snails live underwater but the eggs are laid above it. Any kind of control must target both.
The snail’s life cycle begins with a highly visible, bright pink egg cluster laid above the waterline. Once mated, the females are capable of laying fertilized eggs for months. Each egg cluster holds 200 to 600 eggs.
For more, see here. Note: the story misidentifies the snails as “channeled apple snails.”
Among other work, Jacoby Carter, research ecologist at the USGS National Wetlands Research Center in Lafayette, Louisiana, is studying invasive species in south Louisiana. Carter spoke to Farm Press in November about the snails’ impact, how far north it might range and how the snails might be controlled. He also spoke about nutria, a large invasive rodent that has made Louisiana home.
On Carter’s research in Louisiana…
“My position here is research ecologist. My background is in doing population dynamic models.
“I’ve done a variety of research projects since coming to Louisiana from Massachusetts. Those include looking at the distribution of submerged aquatic vegetation (SAV) along the estuaries of the northern Gulf of Mexico coast from Florida through Texas.
“I’ve done population dynamic models for a variety of species. These models are where you take what you know about the animal’s or plant’s dynamics, put it into a computer and run simulations as to how their populations should behave.
“I’ve specialized in doing work on two invasive species: island apple snails and nutria. That’s involved a variety of field projects and computer modeling projects.”
Is the home range of nutria expanding or has it settled in?
“There are actually two questions there: one on range and one on population density. It’s my contention that nutria are pretty much already everywhere in the United States that they can be. As winters get warmer, the populations move into areas that were previously too cold for them.
“As for the size of a nutria population in a given area, we don’t have a good way of measuring that. Generally speaking, apparently nutria populations have been declining along the Gulf coast, at least in Louisiana, over the last 10 years.
“There are many reasons why that might be occurring. One thing that’s likely contributing is the program to control nutria populations run by the state. We’ve also had several hurricanes that effectively knocked back populations.
“There’s also some evidence that nutria populations may be related to alligator numbers and the presence of very large alligators. Large alligators mean fewer nutria. Smaller alligators don’t hit the nutria populations as hard.”
On chemical treatments for island applesnails…
“Currently, there are still no chemical treatments for apple snails that are approved. We don’t have the facilities or the expertise to get some of the treatments approved, which does require EPA involvement.
“The saponins are unlikely to work too well. We did find another chemical, Bayluscide – currently being used to control lampreys – that shows promise. Again, though, there’s not an EPA permit to use it in anything besides a commercial fish pond. That doesn’t get us far.
“Recently, we’ve been figuring out ways to estimate the population sizes from the number of egg masses we observe. We think there’s a correlation between size and number of egg masses and the number animals in an area. We’re beginning experiments to test that.
“We’re also interested in seeing if it’s possible to control apple snail populations by controlling their egg masses. So, we’re experimenting with a few methods.”
Would those include bio-controls?
“I’m not hopeful for bio-control on applesnails. We have experimented with a fish called a ‘shell cracker.’ The fish is native and, in theory, could be stocked in an area. However, they can only control the very small snails – not one that’s larger than, say, 4 or 5 millimeters in size.
“There are natural predators that will attack and kill apple snails. Those include otters, raccoons, perhaps eels. There’s evidence that alligators may eat snails.”
On the snails’ range…
“The fact is, in all the areas in Louisiana where we’ve noted apple snails in the last six years, or so, we’ve seen nothing but expansion of their ranges. When we did a simple climatic model based on cold hardiness maps, the area apple snails could occupy basically comprises the entire southern half of Louisiana. It never gets cold enough here to completely wipe them out.
“We expect their range expansion to continue. How that could affect rice farmers may depend on their management practices. My understanding is rice farmers in Texas with apple snails haven’t yet seen major impacts. It could be they’re maintaining law enough water levels in the fields that the snails don’t like to be there.
“Unless they’re laying eggs, the snails don’t like to be places where the top of their shells are above water.”
So, right now, what’s your advice to anyone who finds these snails in their ditches? Crush them all?
“At this point, the best advice is to crush the egg masses.
“Our experiments have shown that simply knocking the masses into the water isn’t necessarily an effective way to kill them. Depending on the age of the mass, a certain portion may actually still hatch out.”
On toxicity of the egg masses…
“The egg masses are bright pink for a reason. They contain toxins so any animal that does eat them will eventually become sick. There are two kinds of toxins, one being an enzymatic inhibitor. Once an animal eats enough of the enzymatic inhibitor it will no longer be able to process certain amino acids. That leads to sickness from malnutrition.
“The second toxin in the eggs is a neurotoxin that can cause nausea.
“Now, this information comes from a study done in Argentina on channeled apple snails – not the island apple snails that are here. However, there’s no reason to believe that both species, with very ‘showy’ egg masses, wouldn’t have similar types of poisons in their eggs.”
On the impact of the snails on Louisiana’s aquatic vegetation…
“We have tanks here and can go grab bunches of submerged aquatic vegetation from the pond in front of the wetlands center. We spin off all the loose water, weigh it and place it in two different tanks: one with apple snails, one without.
“The snails can go through 10 pounds of the vegetation in less than a week. When you look at the tanks side-by-side, the amount they can eat is striking.”
If you locate the snails…
“We’re very interested in knowing the rate at which the snails are spreading. I’d appreciate if people would contact me and report where they’ve seen them. It would be preferable if they’d provide a latitude and longitude.
“One thing I’ve noticed in the Houma area is the snails aren’t just in the streams and rivers. They’re in drainage ditches and spreading. They’re moving everywhere and it’s just a matter of time before we see a major impact on aquatic vegetation.”
Contact Jacoby at: email@example.com