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A green mat-forming plant covers the open water in a marsh
Information icon Giant salvinia covering a pond in fresh marshes near Forked Island, 6-May-2016. Photo by Ronny Paille, USFWS.

Bio-control of giant salvinia in coastal Louisiana

Giant salvinia is an invasive floating fern from Brazil. The plant spreads vegetatively, from whole plants or plant fragments. Giant salvinia can double its surface acreage in less than one week. It has been spreading and causing problems in coastal Louisiana since 1989.

Giant salvinia tends to accumulate in small ponds or areas lacking water exchange. Once it covers the water’s surface, this floating plant will begin to stack up upon itself, and can extend 12 inches or more above the water surface. Under such conditions, oxygen recharge of underlying waters is greatly reduced. Salvinia mats on marsh surface can smother the native marsh vegetation.

A pond covered in green vegetation
Giant salvinia covering a pond in fresh marshes near Forked Island, May 2016. Photo by Ronny Paille, USFWS.

Salvinia infestations reduce habitat quality for many fresh marsh species of fish and wildlife, including the resident mottled duck and its critically important brood rearing habitat, wintering migratory waterfowl, rails, and others. Giant salvinia is more tolerant of salinity than water hyacinths. As a result, giant salvinia is found not only in fresh marshes, but is common in intermediate marshes and even low-salinity brackish marshes. In these intermediate and brackish marshes, salvinia also has negative effects on estuarine fisheries production as salvinia-related oxygen loss may render potentially high quality nursery habitat unusable.

The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) has evaluated biocontrol via the salvinia weevil which is native to South America. This weevil feeds exclusively on the leaves, buds, and rhizomes of salvinia species, and APHIS determined that the weevils are safe for release. This weevil has been successfully used for decades in other countries to control and eradicate giant salvinia.

a salvinia weevil. Photo by Scott Bauer, USDA Agricultural Research Service.
a salvinia weevil. Photo by Scott Bauer, USDA Agricultural Research Service.

To address the giant salvinia problem, the Louisiana Department of Wildlife and Fisheries (LDWF) began funding Louisiana State University (LSU) to propagate the salvinia weevil in 2007. However, in 2015, LSU temporarily lost use of the ponds where the weevil was being grown and weevil production ceased for several years.

Due to the increasing acreage of salvinia infestations within privately-owned coastal marshes, the Louisiana Ecological Services Office partnered with the Louisiana Coastal Protection and Restoration Authority to obtain $3.8 million in funding through the Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA) for LSU to propagate weevils and make them available free of charge to landowners for 20 years, and to fund LSU and the United States Geological Survey to monitor selected release sites.

A tiny white larva that looks like a maggot
Larva of the salvinia weevil. Photo by Peggy Greb, USDA Agricultural Research Service.

With this funding, LSU stablished two additional weevil rearing ponds dedicated to providing weevils for coastal landowners. In these ponds, salvinia is grown and then inoculated with weevils. The salvinia must be regularly fertilized, and fire ants around the perimeter of the ponds must be controlled because the ants will pray upon weevil larvae and adults. When weevil densities become high, but before the weevils kill the salvinia, LSU staff conduct a harvest. Landowners come with utility trailers and dozens of empty 20-gallon plastic totes to receive weevil-infested salvinia. The landowners then distribute this material within their salvinia infestations.

Plastic boom contains invasive plants to one end of a pond
Salvinia weevil propagation pond with boom to crowd salvinia into one end. Photo, Ronny Paille, USFWS.

Because the salvinia weevil is native to the tropical climates of Brazil, it is not adapted to cold temperatures. The weevils appear unable to survive typical winters in central and northern Louisiana, but they do survive in southern Louisiana. Temperatures less than 39 degrees Fahrenheit are fatal to eggs and larvae; hence, only adults overwinter in south Louisiana. However, after two rare ice-over events in the coastal marshes during January 2018, weevil populations in some coastal marshes were eliminated or nearly eliminated. Those icing events also temporarily reduced giant salvinia coverage. Unfortunately, however, it is well on its way to recovery given its tendency for rapid growth and spread.

Landowners strap down supplies on a truck trailer
Landowners strapping down totes after being filled with weevil-infested salvinia. Photo by Ronny Paille, USFWS.

Storms surges have occasionally flushed salvinia out of certain coastal marsh areas. Where this happens, weevil populations are eliminated along with the salvinia, and landowners must reintroduce the weevils when salviniais re-established. Heavy rainfall associated with Hurricane Harvey in 2017 caused a massive headwater flood on the Sabine River which pushed salvinia deep into downstream marsh areas that did not previously have a salvinia problem. These events demonstrate that weevil production and introduction will be continually needed to address changing environmental conditions. Additionally, a coordinated landscape level effort is needed to reduce and/or eliminate upstream salvinia sources that continue to release material to downstream swamps and marshes.

At their maximum size, adult salvinia weevils are no greater than two millimeters in length. During its six-to eight-month-long lifespan, a single weevil cannot do much damage. Modeling has shown that weevil populations must build up to very large numbers before their feeding damage has a detrimental effect on the salvinia. The more salvinia biomass present, the larger the weevil population must be to significantly damage the salvinia. To achieve these large weevil populations, five to six generations are needed which, in turn, requires a moderate to long annual growing season with a warm spring and fall to accommodate those generations before cold temperatures once again preclude reproduction.

Because large numbers of weevils are needed to kill giant salvinia, control may not be achieved in the year during which the weevils are released. Anecdotal information suggests that two to three years post-release may be needed to significantly reduce thick and extensive salvinia infestations. However, small infestations can be more quickly subdued when large numbers of weevils are introduced. As weevil populations increase and weevil feeding becomes increasingly damaging, the salvinia mat coloration goes from a healthy green, to green-brown, to reddish-brown, then black.

Three photos in series showing the gradual removal of giant salvinia
Time series photos showing clearing of weevil-damaged giant salvinia in the Big Burns marsh. Photos courtesy of Dr. Rodrigo Diaz, LSU Entomology Dept.

Contact

Ronny Paille, Fish and Wildlife Biologist
ronald_paille@fws.gov, (337) 291-3117

The mission of the U.S. Fish and Wildlife Service is working with others to conserve, protect, and enhance fish, wildlife, plants, and their habitats for the continuing benefit of the American people. For more information on our work and the people who can make it happen, visit fws.gov. Connect with the Service on Facebook, follow our tweets, watch the YouTube Channel and download photos from Flickr.

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