Biological control of invasive floating fern leads to rapid recovery of ecological functions in coastal fresh water wetlands in Louisiana

Abstract

Despite the success of the salvinia weevil ( Cyrtobagous salviniae ) at controlling giant salvinia ( Salvinia molesta ), its impact on the timing of reduction of giant salvinia cover and recovery of submerged aquatic vegetation (SAV) and dissolved oxygen remains unknown. A two-year field study (2016 to 2017) was conducted in coastal wetlands in southwestern Louisiana to measure the impact of biological control of giant salvinia. Water temperature, dissolved oxygen, weevil densities, and giant salvinia and SAV cover were assessed at 36 sampling locations comprising canals, small ponds, and large ponds. Results showed that adult weevils were distributed across the landscape. In 2016 mean adult densities were 46.9 weevils kg (cid:2) 1 in canals, 42.5 weevils kg (cid:2) 1 in small ponds, and 38.7 weevils kg (cid:2) 1 in large ponds. In 2017 mean adult weevil densities were 28.2 kg (cid:2) 1 in canals, 12.3 kg (cid:2) 1 in small ponds, and 29.4 kg (cid:2) 1 in large ponds. Percent cover of SAV was zero at all three site types in July 2016 but increased by 29.4, 35.0, and 73.3% in small ponds, canals, and large ponds, respectively, from July 2016 to January 2017. Our models demonstrate that higher adult weevil densities in June lead to faster control of giant salvinia and SAV recovery, subsequently increasing dissolved oxygen levels. The models can be used to estimate months needed to control salvinia given weevil density, percent salvinia cover, and site type. Resource managers could apply this to inform about timing to control and guide management decisions.