I had a nice, long talk with Dr. Jacoby Carter (USGS) yesterday. He is a Research Ecologist at the prestigious National Wetlands Research Center in Lafayette, Louisiana. Not only is National Wetlands Research Center a global clearinghouse of scientific information on wetlands, but also the generator of technologies that aid managers of natural areas. Dr. Carter shares a sense of urgency regarding the control of the Island Apple Snail in natural areas and predicts “extensive distribution without early intervention.”
To say he and his research team have been “thinking outside of the box” would be an understatement. For instance, they conducted feeding trials of Red Swamp Crawfish (Procambarus clarkii), a beloved native of Louisiana (boiled), on juvenile Pomacea insularum (<14 days old). The crawfish readily ate the young snails, providing hope for a local, biological control agent.
They also tested the theory that simply knocking the snail eggs into the water will kill them, something I have relied upon. Unfortunately, according to Dr. Carter, it doesn’t work on the more mature eggs. “The closer to hatching, the more likely they are to survive,” he said. “Smash them, especially if they are turning white.”
Of course, the key to success in controlling the Island Apple Snail is finding a selective molluscicide. Dr. Carter’s team has focused on two candidates: Saponins (see previous post) and Niclosamide (“Bayluscide”). The saponin source is the Punk Tree or Tea Tree (Mellaleuca quinquenervia), a native of Australia and the scourge of the Florida Everglades. A Tea Tree product, called “Harvest Winner” (6% saponin), was tested on a combination of Island Apple Snails and Red Swamp Crawfish. At higher rates (15-30 ppm), the snails were killed, but the crawfish were not.
Niclosamide (“Bayluscide”) has been the pesticide of choice against snails that carry human schistosomiasas since the 1960’s. It is the only commercial molluscicide endorsed by the World Health Organization for that use. In the U.S., Niclosamide has been used on Zebra Mussels (Dreissena polymorpha) and, by the U.S. Fish & Wildlife Service, on the larvae of Sea Lamprey (Petromyzon marinus).
Dr. Carter likes the low mammalian toxicity of Niclosamide but is concerned about reported impacts on fish and amphibians. Again, he tested the combination of snails and crawfish, but this time with 1.3 ppm Niclosamide. Though none of the crawfish appeared adversely affected, 100% of the snails died with a 48 hour exposure. Finally, Dr. Carter’s team tried spraying Harvest Winner and Niclosamide on snail egg clusters, but there was no statistically significant reduction in hatching rates verses the control. Thanks for the interesting information, Dr. Carter!
I just read that scientists at Upper Midwest Environmental Sciences Center (USGS) have developed “a controlled-release formulation of Niclosamide that is designed to kill sea lamprey in the bottom sediments where they reside without impacting other non-target organisms.” Hmmm. Take a snail-infested pond; crush the egg clusters; throw in some Red Swamp Crawfish; season with slow-release Niclosamide; and, maybe, you have a recipe for success against the Island Apple Snail! Posted by Jess Van Dyke
Dr. Jacoby Carter
National Wetlands Research Center (USGS)