Service uses weevils to control invasive salvinia that threatens Louisiana coast
Giant salvinia is an invasive floating fern from Brazil that can double its surface acreage in less than one week in optimal conditions. It has been spreading and causing problems in coastal Louisiana since 1989.
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. Combined with the decay of shaded salvinia, this often results in depleted oxygen in affected areas. When this happens, the water quality is severely degraded. Marsh loss can occur where storms and/or high tides have deposited salvinia mats on the marsh surface, smothering marsh edge vegetation.
Salvinia infestations reduce habitat quality for many fresh marsh species of fish and wildlife, including the resident mottled duck and its critically important mottled duck brood rearing habitat, wintering migratory waterfowl, rails, and others. Giant salvinia is more tolerant of salinity than water hyacinths. As a result, 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.
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, 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 salvinia.
Due to the increasing acreage of salvinia infestations within privately-owned coastal marshes, the U.S. Fish and Wildlife Service’s 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). The funding allowed Louisiana State University (LSU) to propagate weevils and provide them for free to landowners for 20 years, and also to fund LSU and the U.S. Geological Survey to monitor selected sites for three years.
With this funding, LSU has established two additional 0.25-acre 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 as the ants will pray upon weevil larvae and adults. When weevil densities become high, but before the weevils kill the salvinia, LSU staff conducts a harvest. Landowners come with utility trailers and dozens of empty 20-gallon plastic totes to receive weevil-infested salvinia (Figure 4). The landowners then distribute this material within their salvinia infestations.
At their maximum size, adult salvinia weevils are no larger than two millimeters in length. 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 is 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. This requires a warm spring and fall growing season to accommodate those generations before cold temperatures in the fall once again preclude reproduction.
Because large numbers of weevils are needed to kill 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.
Ronny Paille, Civil Works and Coastal Restoration Biologist
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