(09/15/16) BATON ROUGE, La. – AgCenter researchers are seeing increased benefits of controlling giant salvinia with salvinia weevils, experts said at a meeting of the Midsouth Aquatic Plant Management Society annual meeting on Sept. 14.

Giant salvinia is one of several major aquatic weeds in Louisiana, and “there are probably others already in the state that we’re not yet aware of,” said Rogers Leonard, associate vice president of the LSU AgCenter.

With 9,174 square miles of surface water in Louisiana, “eradication of any aquatic weed, although possible, is unlikely,” said Leonard, who is an entomologist. “We’re looking at management strategies, not eradication.”

Weed problems in waterways not only affect recreation but also real estate values, he said.

“In southwest Louisiana, aquatic weed infestations are affecting drainage and irrigation,” he said. “These pests are influencing the efficiency of agricultural production and have to be managed carefully to reduce the potential for losses in net profits.”

Legislation to control the movement of invasive plants and mechanical removal are options, Leonard said. Chemical control is not permanent and may affect non-target species.

“Biocontrol represents another opportunity to manage these pests,” he said. The differential selectivity of insects targeting specific plants allows this integrated strategy to be successful.

A weevil from South America that was introduced to control the invasive weed giant salvinia has proven effective in Louisiana, Leonard said. The AgCenter is using demonstration sites and research operations to produce an adequate supply of these insects to distribute across the state.

Giant salvinia was first discovered in Louisiana in 1998 at the Toledo Bend reservoir, said retired AgCenter weed scientist Dearl Sanders, who spent more than 15 years working to control the weed.

The AgCenter began producing salvinia weevils in ponds and transferred them to infested areas of the state. From 2007 to 2014, the AgCenter along with the Louisiana Department of Wildlife and Fisheries conducted 19 major harvests and distributed nearly 10 thousand tons of giant salvinia containing more than 5 million adult weevils, Sanders said.

Using biocontrols can be the most effective method of controlling pests like the giant salvinia. But a biological control organism is fragile, Sanders said. For example, widespread spraying for mosquitoes to control the Zika virus could affect salvinia weevils and other beneficial insects.

Louisiana’s water bodies are a combination of lakes, rivers, bayous, canals and backwater swamps where water flows and carries invasive plants like the giant salvinia, said Christopher Mudge, a research biologist with the U.S. Army Engineer Research and Development Center and an adjunct faculty member in the LSU AgCenter.

Using chemicals or mechanical methods almost never completely removes giant salvinia from a water body, said AgCenter entomologist Rodrigo Diaz.

“Salvinia regrows faster than it can be removed,” Diaz said. “When successful, biocontrol can be the most cost-effective method.”

Because one of the drawbacks of releasing the salvinia weevil is its tolerance to cold weather, researchers have been searching the world for a weevil population that can live through the colder winters of north Louisiana.

Diaz and graduate student Alana Russell have collected weevils from Argentina, Australia and Uruguay to identify a population that can withstand north Louisiana winter temperatures. Results so far suggest a population from Argentina has greater cold tolerance than a population from Henderson Swamp in south Louisiana, Diaz said.

The researchers will seek permission from the U.S. Department of Agriculture Animal and Plant Health Inspection Service for permission to release the Argentine population in Louisiana. It’s the same species that’s already been released, and approval is anticipated within two years, Diaz said.

The researchers are looking forward releasing the new weevil population in north Louisiana because of the large-scale biocontrol success in south Louisiana this year, Diaz said.

Salvinia weevil. Photo by Scott Bauer, USDA Agricultural Research Service