Giant salvinia is one of the most invasive aquatic vegetation species in the world. Native to Brazil, this free-floating fern can dramatically alter aquatic ecosystems and causes major ecological issues.
Scientists with the LSU AgCenter began working with Puerto Rican officials to help manage an infestation of giant salvinia on Lago Las Curias, which is a 12-acre lake. A homeowner living along Las Curias was doing research on efforts to control salvinia and came across efforts undertaken by the AgCenter.
This effort led to a collaboration between AgCenter scientists and researchers from a Puerto Rican university and governmental officials from the island territory. The first step involved designing a plan to combat the salvinia that would win approval from several governmental agencies, including the U.S. Department of Agriculture.
Part of the plan involved a biological control method that had been successful in Louisiana. A weevil that feeds on salvinia, eventually causing it to sink and die, was brought in from South America. The weevil was reared in small ponds, and they were then released in areas covered by the salvinia.
During the summer of 2019, the plan was presented and was eventually approved by all of the governing agencies. In September 2019, Charlie Wahl, a research associate in the Department of Entomology, arrived in Puerto Rico with the weevils. A small area was established to place these weevils in an area that would serve as the nursery. The idea was to rear a high concentration of weevils in the nursery, which would then be released into Las Curias.
Wahl worked with local officials to provide guidance in rearing the weevils and the proper technique for releasing the weevils. By releasing the weevils in a strategic manner, the maximum biological control could be achieved.
An assessment of the health of Lago Las Curias was also conducted prior to arrival of the weevils to provide some baseline information and to help measure the effectiveness of the weevils over time.
The assessment determined the extent of the salvinia was significant. It took the salvinia two years from first being reported to covering the entire lake. Wahl found in some areas the mats of salvinia were 28 centimeters thick. Small plants and shrubs were supported by the salvinia and actively growing.
The salvinia was also responsible for creating a lake nearly devoid of dissolved oxygen because no light was reaching below the surface and prevented the growth of any aquatic vegetation.
Rodrigo Diaz, an assistant professor in the Department of Entomology, believes the weevils will provide biological control. He believes the significance of the infestation will also require an integrated management plan involving the use of chemical and mechanical control measures.
Diaz points out eliminating nonpoint sources of pollution into the lake, especially nutrients such as nitrogen, ammonia and phosphate, is critical to the lake recovery.
On Dec. 15, 2019, graduate student Giovana Matos Franco traveled to Puerto Rico to examine the health of the weevils. Matos Franco confirmed the weevils were established and dispersing. She also assisted in training local residents and University of Puerto Rico students how to measure the densities of the salvinia weevils.
When the densities of the salvinia weevils reach a certain threshold, some weevils can be moved to help establish other colonies. This process can aid in helping to control the weed at a faster rate.
In January 2020, Diaz secured a grant from the USDA Animal Plant Health Inspection Service. This three-year grant will be used to help rear the weevil, release and monitor the impact of the weevil and help monitor the recovery of the ecological functions of the lake.
At the end of February 2020, it was determined the weevil had dispersed several feet from a release location and was creating localized damage to the salvinia.
Craig Gautreaux is a specialist with LSU AgCenter Communications.
(This article appears in the spring issue of Louisiana Agriculture magazine, which focuses on entomology.)
LSU entomology master’s student Lori Moshman displays a handful of giant salvinia during an LSU AgCenter workshop in 2017 on using salvinia weevils to control the invasive aquatic weed. Photo by Olivia McClure