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 À1 in canals, 42.5 weevils kg À1 in small ponds, and 38.7 weevils kg À1 in large ponds. In 2017 mean adult weevil densities were 28.2 kg À1 in canals, 12.3 kg À1 in small ponds, and 29.4 kg À1 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.

Limnological Response of Las Curias Reservoir, San Juan, Puerto Rico: Successful Management of the Invasive Aquatic Fern, Salvinia molesta

“…As we predicted, the warm temperatures and longer growing season of Puerto Rico allowed the weevil to establish, grow, and ultimately control salvinia. This success is similar to biological control programs implemented in tropical and subtropical regions of United States, South Africa, Senegal, and Australia [17,[46][47][48][49][50]. The air temperatures of Las Curias fluctuate between 23 and 30 • C during winter and summer, respectively, matching near perfect conditions for the weevil's adult feeding, dispersal (flight and walking), reproduction (mating, oviposition), and larval development [51][52][53][54].…”

“…This makes us wonder about a weevil density at which there is a point of no recovery of salvinia, or a putatively tipping point where the damage is so severe that salvinia collapsed and sunk. Future studies should aim to understand the relationship among weevil densities, nutrient load, and growth rates of salvinia [17]. Finally, sampling towards the end of the project (2022) revealed that weevils did not extirpate salvinia from the reservoir, but rather weevils and salvinia reached an equilibrium at lower densities.…”

“…Under ideal conditions, the plant has the ability to form extensive layers of vegetation across the surface of lakes, ponds, reservoirs, and swamps [13,14]. The expansion of salvinia in water bodies can limit the growth of native species and reduce light penetration and dissolved oxygen below the mats, affecting water quality and ecosystem structure and service [15][16][17]. Rapid rates of nutrient uptake combined with relatively slow rates of decomposition enable salvinia to tie up nutrients that could be used by other primary producers that contribute to complex food chains [11].…”