Giant Salvinia Control

Introduction

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Lake Raven is a 203-acre reservoir located in Huntsville State Park outside of Huntsville, Texas. The lake is managed by the Texas Parks and Wildlife Department (TPWD). Kayakers, swimmers, and anglers frequent the lake and are important for park revenue, but their visits also facilitate non-native plant introductions. Water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), hydrilla (Hydrilla verticillata), and giant salvinia (Salvinia molesta) are all invasive plants present in Lake Raven. Giant salvinia, a floating fern native to southern Brazil, is the largest invasive plant threat in Texas due to its ability to double in size in just seven to ten days, allowing it to quickly overtake a body of water. In 2015, TPWD received record funding for vegetation control, largely due to the fact that giant salvinia had taken over various water bodies in east Texas. Biologists from TPWD recognized the need to control giant salvinia in Lake Raven using an integrated pest management (IPM) approach. This approach allows biologists to utilize a combination of methods while minimizing risks to people and the environment. 

Key Issues Addressed

Giant salvinia is a fast-growing, thick, floating plant species that is known to quickly cover substantial bodies of water. Its growth is thick enough to reduce the amount of sunlight down the water column sufficient to prevent the growth of native submersed vegetation that is important to aquatic life and waterfowl. Furthermore, the dense mats of giant salvinia, which can grow to three feet thick, can reduce dissolved oxygen in the water below the amount needed to support native fish by limiting gas exchange between the atmosphere and the water. For these reasons, giant salvinia can disrupt the entire aquatic ecosystem. In 2015, biologists found giant salvinia in Lake Raven. The largest giant salvinia growth occurred in two arms of the reservoir, but rainfall events regularly flushed it into the main lake. In addition to the ecological consequences, giant salvinia has the potential to impair recreational use by blocking boat access and harming sportfish populations. Reduced public access would be detrimental to Huntsville State Park because the park relies on recreational use of the lake for revenue. In 2015, TPWD biologists began introducing giant salvinia control measures to prevent these consequences. 

Project Goals

• Control giant salvinia in Lake Raven using an IPM approach
• Provide education to recreationists in Huntsville State Park to reduce the introduction and spread of invasive plants 

Project Highlights

• Non-Native Aquatics Treatment Techniques:TPWD biologists used IPM tactics, combining chemical, physical, and biological techniques to control the growth of giant salvinia in Lake Raven. In 2015, they began treating giant salvinia using herbicides (a combination of Glyphosate, Flumioxazin, and surfactants), but wanted to avoid spraying the entire lake and killing native vegetation. By April 2020, biologists had contained giant salvinia to the upper arms of the reservoirs using booms, floating barriers that restrict the plant to one area. Once contained, the giant salvinia was more efficient to treat because biologists only had to spray the areas within the booms, not the whole lake. This method also reduced the amount of herbicide sprayed on native vegetation. In May of 2020, however, giant salvinia mats began showing up outside the contained areas and herbicide treatments were becoming costly. As a third strategy, biologists harvested salvinia weevils (Cyrtobagous salviniae), that feed exclusively on giant salvinia, from local reservoirs and released over 5,000 weevils onto the remaining giant salvinia at Lake Raven.
• Education and Outreach: To educate recreationists about aquatic invasive species invasive species
  An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.
  
  Learn more about invasive species , Huntsville State Park introduced the Clean Drain Dry campaign through signs and educational activities. The campaign emphasizes the importance of cleaning off plants, animals, and mud from equipment, draining water-containing devices, and drying items used in a water body before reuse. Additionally, TPWD is using their Protect the Lakes You Love campaign to include boaters and anglers in the fight against the further spread of giant salvinia and other non-native species. In this campaign, they utilized local highway billboards and gas station pump toppers to help inform the public. 

Lessons Learned

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Weevils for the Win: Research shows that 35 weevils per kilogram of giant salvinia is adequate for control. Four months after introducing the weevils in Lake Raven, there were 123 weevils per kilogram.

Salvinia weevils have been shown to be an effective means of controlling giant salvinia; however, it usually takes about two years to establish weevil populations and take control over giant salvinia. Giant salvinia has a lower temperature threshold than the weevils, which usually do not survive the winter. Managers have therefore utilized other treatment methods in conjunction with the use of weevils to achieve giant salvinia management goals. Lake Raven, however, is located far enough south that the weevils were able to survive the winter and establish populations quickly. Introducing the weevils before summer was important for their survival and population growth. The salvinia weevil treatment was a small effort that resulted in huge, lasting impacts. Additionally, in the winter of 2018 and 2021, unexpected freezes dramatically reduced the remaining giant salvinia. In early 2021, biologists had achieved 99.5% reduction of giant salvinia in Lake Raven. However, some giant salvinia remains and ongoing control is required.

Biologists faced a hurdle with attaching a boom. One boom became disconnected from a shoreline and released the giant salvinia it was containing. Biologists were able to secure the boom with larger cables and clamps to avoid this occurrence in the future.

Importantly, TPWD’s aquatic invasive species and fisheries biologists and state park employees supported the shared goal of controlling giant salvinia without impeding native aquatic life or hampering park use.

Biologists at TPWD noted the importance of thinking outside the box when managing invasive species because it is easy to get stuck in a rut and follow what others have done. If TPWD biologists had not opened their minds to other options, such as using floating booms and introducing the salvinia weevils, they would not have been so close to achieving success in a year’s time. This IPM strategy works well in small reservoirs or where giant salvinia can be contained. 

Next Steps

• Monitor Lake Raven every 4-5 weeks and remove giant salvinia when found
• Utilize a similar IPM approach for giant salvinia in other water bodies
• Continue education and outreach to prevent the spread of giant salvinia and other invasive plants

Funding Partners

• Texas Parks and Wildlife Department

Contact

• John Findeisen, Inland Fisheries Division-Aquatic Habitat Enhancement, Texas Parks and Wildlife Department: john.findeisen@tpwd.texas.gov

CART Lead Author

• Krystie Miner, Research Specialist, University of Arizona: kminer@arizona.edu

Suggested Citation

Miner, K., A., and Findeisen, J. (2021). “Giant Salvinia Control in Lake Raven.” CART. Retrieved from https://www.fws.gov/project/giant-salvinia-control.