Korey Pham, Christopher Mudge, Hannah Laville and Rodrigo Diaz

Giant salvinia, Salvinia molesta, is a free-floating aquatic fern native to Brazil. In 1998, it was first found in the Toledo Bend Reservoir along the Texas-Louisiana border. Since then, it has spread into freshwater bodies across the state and has become one of the worst aquatic weeds in Louisiana. Louisiana’s subtropical climate and network of connected wetlands are optimal for the spread and invasion of giant salvinia. Giant salvinia forms a dense mat that reduces recreation and navigation, decreases dissolved oxygen and increases acidity, resulting in the loss of submerged aquatic vegetation.

Louisiana’s wetlands provide critical ecological and economic benefits, including habitat for wildlife and hunting, fishing, boating and other recreational opportunities. Activities involving waterfowl contribute almost $1 billion to the national economy, and waterfowl are critical for nutrient cycling, which contributes to healthy waterways. Although coastal Louisiana is home to millions of wintering waterfowl, a shift in waterfowl habitat selection has been observed since the invasion of giant salvinia. They select habitats with open water and available carbohydrates, such as submerged aquatic vegetation and seeds. Giant salvinia negatively affects waterfowl habitat by altering water quality and reducing open water access.

Controlling giant salvinia with herbicides, lake drawdowns and harvesters can be costly for land managers. Since 2001, the salvinia weevil, Cyrtobagous salviniae, has been used in Louisiana to manage salvinia. This insect is host-specific and successful at reducing salvinia coverage. The LSU AgCenter rears and distributes weevils to state agencies and private landowners in Louisiana (Figure 1).

Despite the success of the program, there is no information on how to maximize weevil dispersal and improve establishment and impact. Currently, land managers receive weevil-infested giant salvinia and release them in one location. Land managers have not known whether releasing the weevil-infested giant salvinia in multiple places increases weevil population. Likewise, weevil dispersal through flying is not well understood. LSU AgCenter researchers designed two outdoor studies to examine these gaps of knowledge.

First, the researchers examined how the number of release locations affect giant salvinia. They conducted two trials at the LSU AgCenter Aquaculture Facility in Baton Rouge. Using stock tanks, 200 grams of weevil-infested salvinia were inoculated into giant salvinia in different release patterns (Figure 2). The weevil densities added in each trial were 41 and 150 individual weevils per kilogram of fresh salvinia, respectively. Mat thickness, biomass, percent coverage, plant damage and weevil population density were measured over five weeks.

Giant salvinia was significantly reduced as release points increased. The summer trial had little impact on the infestation, while the fall trial showed a larger impact. In fall, coverage was reduced by almost 30%, mat thickness by 40%, biomass by 20% and plant damage increased by 40% compared to the control. Results showed that increasing the number of release points leads to greater reduction of giant salvinia. Further research will test this hypothesis on a large scale using quarter-acre ponds.

Second, the researchers evaluated weevil impact and dispersal via flight on a quarter-acre pond located on LSU AgCenter property in St. Gabriel. In May 2021, the pond was completely covered with salvinia and divided into control (no weevils) and treated (inoculated with weevils) sections. Sticky traps were arranged on 10 sampling stations at different heights to track weevil flight and trapped adults were recorded every two weeks (Figure 3). Additionally, plant performance and weevil densities were recorded over six months.

Plant performance, weevil density and adults trapped were significantly different between the sampling stations. Compared to the control, the weevil-treated section had higher weevil density and trapped adults. The salvinia had reduced biomass, mat thickness, normalized difference vegetation index (NDVI) and percent coverage. It had also sunk after six months. This suggests that as weevils eat more of the plant, adult weevil flight increases and giant salvinia performance decreases. Future laboratory studies will evaluate the role of plant quality on flight.

In conclusion, these two studies will improve management strategies and release recommendations for state agencies and private landowners in Louisiana. Our findings indicate that increasing the number of release points and show that when weevil feeding increases, weevil dispersal increases. These results can help land managers clear waterbodies faster, hastening the return of wintering waterfowl, and restoring the sportsman’s paradise.

Korey Pham is a research associate in the Department of Entomology. Christopher Mudge is a research biologist with the U.S. Research and Development Center and an adjunct professor in the School of Plant, Environmental and Soil Sciences. Hannah Laville is a student worker, and Rodrigo Diaz is an associate professor, both in the Department of Entomology.

(This article appears in the winter 2022 issue of Louisiana Agriculture.)

Figure 1. The salvinia weevil feeds on a giant salvinia frond. Photo by Korey Pham

Figure 2. Outdoor salvinia weevil release study conducted in summer and fall of 2021. A. Stock tanks filled with giant salvinia and inoculated with weevils. Photo by Korey Pham. B. This diagram details the different weevil release patterns or treatments used in the study. At the top left is the control. The other tanks had one or more release points.

Figure 3A. a six-month outdoor study conducted in 2021 in St. Gabriel, Louisiana, of weevil dispersal via flight. Sticky traps were deployed at different heights to quantify adult weevil flight. Photo by Korey Pham

Figure 3B. A drone image of the pond divided in half shows the gradient of weevil feeding and dispersal in August 2021. Photo by Korey Pham