Giant salvinia (Salvinia molesta) is a free-floating aquatic plant that originates from Brazil. It is an invasive species in over 21 countries. Plants grow vegetatively, forming thick mats that interfere with natural ecological processes and aquatic industries including boating, waterfowl hunting, rice farming, and aquaculture.
Giant salvinia can be distinguished from its cousin, common salvinia (Salvinia minima) by the shape of its trichomes, or leaf-hairs. Giant salvinia’s leaf hairs (right) are closed at the tip, forming an “egg-beater” shape, whereas common salvinia’s leaf hairs (left) are branched at the tip. In Louisiana, giant salvinia is typically more aggressive than common salvinia. Giant salvinia is a larger plant that forms thicker, denser mats. Common salvinia can cover large areas but typically forms thinner mats and does not pose as much risk to boating traffic.
Giant salvinia’s “roots” are actually modified leaves that function as roots, absorbing nutrients from the water column and transporting them to other parts of the plant. These “roots” can get very long, depending on site variables such as water depth and nutrient availability.
Rhizomes are underwater stems that connect salvinia’s leaves to its “roots”, and connect individual plantlets to each other. When the rhizome breaks, each plant fragment can continue to reproduce asexually by forming new buds.
Giant salvinia can exist as primary, secondary, or tertiary growth. The primary and secondary stages are known as “colonizing” growth forms and have small, flat leaves that can float around easily. When these stages cover a water surface and begin to get crowded, the plant transitions to its tertiary, or “mat-forming” stage. This stage is marked by dense, vertical growth that is the most problematic to ecosystems and the most difficult to manage.
How did giant salvinia get here? It was likely introduced as an ornamental species as part of the aquatic plant trade. Humans continue to play a big role in the species’ dispersal by inadvertently transporting plant fragments from one waterbody to another on contaminated boating equipment. Natural events such as floods can wash plants downstream and lead to new infestations. Animals such as birds and alligators may also play a role in transporting plant fragments from place to place.
Giant salvinia is a tropical to subtropical plant, and as such it cannot survive winters experienced in the northern United States. It has been found as far north as Virginia, but is more aggressive in states bordering the Gulf of Mexico. Giant salvinia thrives in eutrophic (high nutrient) conditions and therefore is most problematic in areas subject to high nutrient run-off.
Thick plant mats prevent sunlight from reaching the water column, which can reduce dissolved oxygen levels. Native vegetation is suppressed by competition and displacement, and this affects native fauna such as fish, waterfowl, and macroinvertebrates. Decomposition of plant mats can affect the composition of bottom sediments and influence local food webs.
In addition to affecting recreational activities, giant salvinia impacts aquatic industries and can infest drinking water reservoirs. Chemical spraying of herbicides to reduce infestations is expensive and requires repeat applications, costing millions of dollars per year.
Hope for control
The salvinia weevil, Cyrtobagous salviniae, is a plant-feeding beetle from salvinia’s native range in South America. The weevil has been used successfully in biocontrol programs to control giant salvinia in Australia, South Africa, Sri Lanka, and several other countries. Salvinia weevils are a safe solution, as they do not eat crops or ornamental plants—in fact, they only eat salvinia. However, the weevils are not uniformly effective across salvinia’s range in the United States; released populations are subject to winter losses in northern parts of Louisiana and Texas. To find a solution, the LSU AgCenter mass-rears salvinia weevils and conducts research to help improve biological control of giant salvinia throughout its invaded range.