Biological Control of Common Salvinia (Salvinia minima) at Jean Lafitte National Historical Park and Preserve, Louisiana.

Introduction and Background

       Non-native, floating aquatic plants are one of the most significant threats to the wetland habitats that comprise the Barataria Preserve of Jean Lafitte National Historical Park and Preserve (JELA). The 18,500 acre Preserve, which is predominantly (80%) wetlands, encompasses a myriad of diverse aquatic habitats, including globally unique freshwater floating marshes, native bottomland hardwood swamps, and more than 30 miles of natural bayous and waterways. These unique habitats form the northeast portion of the Barataria-Terrebonne estuary system (deemed nationally significant by the EPA in 1991), and are vital to native plant and wildlife communities.  Common salvinia is one of the most widespread exotic plant species found within the Preserve and currently covers more than 9,000 acres. Common salvinia has infested all park waterways and dominates throughout most bald cypress water tupelo swamps and freshwater ponds. Virtually all park aquatic habitats are at risk of invasion.

       Non-native species like salvinia are a consistent threat to the wetland habitats that the park is mandated to protect. Native to South America, common salvinia is a highly invasive floating fern that has caused severe ecological problems throughout the southeastern United States. Salvinia grows rapidly to cover the surface of calm or slow moving water, and spreads aggressively by vegetative fragments. Once established, salvinia forms a thick vegetative mat that can reduce the amount of sunlight available for submerged aquatic vegetation and displace many native plant species. Plant species of special concern include the state imperiled floating antler fern (Ceratopteris pteridoides) and the native duckweed (Lemna spp.), which is an important food source for several migratory waterfowl species. The floating mats also degrade water quality by reducing amounts of dissolved oxygen available for fish and aquatic organisms and provide harborages for disease-carrying organisms like mosquitoes. During the long, subtropical growing season in southern Louisiana, the thick mats of salvinia often become impenetrable, which impedes or eliminates recreational opportunities such as boating and fishing for the visiting public and severely handicap park law enforcement operations.

       JELA has utilized an aquatic weed harvester in the past to mechanically remove salvinia and other exotic floating aquatic plants from the Preserves waterways. This operation is time consuming, costly, and often ineffective because the floating plants are readily carried back into cleared areas by changing winds, tides, wildlife movements and recreational boating. Another inherent problem with mechanical removal is that the weed harvester can only be utilized in navigable waterways, leaving all forested swamp habitats infested and inaccessible. The remaining plants quickly colonize and re-infest cleared areas, creating an endless maintenance cycle throughout the growing season. Chemical control of salvinia is also problematic and often only a short-term solution, due to the plants rapid reproduction rate and ease of spread.

       Perhaps an even greater threat to the aquatic habitats of the Preserve is the potential invasion of giant salvinia (Salvinia molesta). Giant salvinia possesses characteristics similar to common salvinia, but has a larger leaf size and is considerably more invasive. Giant salvinia was classified by the U.S. Department of Agriculture as a Federal Noxious Weed in 1983, and has the ability to double in size each week in the field. Although this species has been banned in the United States for 20 years, it was recently discovered along the Louisiana-Texas border and has currently spread eastward to within 100 miles of the Barataria estuary. Giant salvinia has not yet been documented within the Preserve; however, the threat posed by this invasive species requires serious consideration to a proactive management approach.

       Biological control provides a sustainable alternative to managing the current salvinia infestation. The salvinia weevil has proven to be an effective and safe biological control agent in Florida, where it has become naturalized and is currently widespread on common salvinia. In Florida, common salvinia is not a permanent pest but builds up periodically followed by sharp reductions in density as weevil populations increase and suppress the plant.  Redeployment of this beneficial species from Florida will reunite the salvinia in the Preserve with one of its most important natural enemies and may achieve a permanent and sustainable suppression of the weed in its new range.