Under ideal conditions, a single giant salvinia can multiply to cover 40 square miles in only three months. Weevils that feed on the plant are one way to keep salvinia in check. At a test area in Creole (pictured), weevils destroyed a salvinia infestation (turning it brown) and allowed the waterway to reopen.
Since first being discovered in the Toledo Bend Reservoir during the late 1990s, giant salvinia (Salvinia molesta) has spread – most likely from plant fragments on boats and trailers moved from waterbody to waterbody – across coastal parishes. This aquatic weed can completely cover waterways robbing native plants, fish, insects and other species of sunlight and oxygen.
“For anyone who has to work in the marsh or use boat trails for transportation, giant salvinia is a problem,” said Louisiana Sea Grant and LSU AgCenter Marine Extension agent Kevin Savoie, who has been on the forefront of salvinia management for nearly 16 years. “It chokes native aquatic plants that are a food source for waterfowl and other species. Duck hunters and alligator harvesters struggle to get their boats through it.” Agents with the Louisiana Department of Wildlife and Fisheries (LDWF) say that there are areas where the salvinia is so thick it can support a man’s weight.
In Cameron Parish, LSU entomologist Rodrigo Diaz is conducting research on how to control the invasive weed. Herbicides often are used to manage the salvinia, but they have a financial as well as environmental cost. “A biological control method is needed,” said Diaz. That biological control could be a weevil Diaz is researching – Cyrtobagous salviniae. The weevil larva feed on the plant, stunting the salvinia’s growth and causing it to sink.
Since spring 2016, Diaz and Savoie have monitored the impact of the salvinia weevil at several Cameron Parish sites owned by the Miami Corporation. Several years ago, weevils from the LSU AgCenter nursery were introduced at specific locations on Miami Corportion property and now the objective is to quantify the weevil density and damage to giant salvinia.
“The weevils have been doing their job,” said Savoie. “As salvinia begins to grow and spread, the weevils feed and the salvinia disappears. The weevils also move with the growing stands of salvinia, reducing the biomass of the stand and its impact.”
“Conditions this year have been ideal,” said Diaz. “We’re looking for what triggers weevil population growth and how quickly the salvinia comes back. Once we know, we can tell resource managers how effective the weevils will be under certain conditions and how patient to be. This natural control will be able to save land owners money on herbicides and manpower.”
Funding Diaz’s research is LDWF. He can be reached at firstname.lastname@example.org. Savoie can be reached at email@example.com.
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