23 Sep 2021

Field Site Analysis of Giant Salvinia Nitrogen Content and Salvinia Weevil Density

Abstract: In 2012, a giant salvinia (Salvinia molesta Mitchell) biological control project was initiated in Louisiana. Although similar quantities of salvinia weevils (Cyrtobagous salviniae Calder and Sands) were released at all sites, weevil densities were highly variable among sites. Additionally, signs of plant nitrogen depletion (yellowing plants) were observed at some sites. Because it is well known that plant nutrition can affect the success of a biocontrol agent because of slowed development and/or reduced fecundity, the correlation between giant salvinia nitrogen content and Salvinia weevil density was investigated during the growing seasons of the second and fourth years. During 2013, weevils were reintroduced to sites, and the magnitude of adult weevil density increase varied by site. Giant salvinia nitrogen content varied among sites and sampling dates. Upper Big Break plants had greater nitrogen than all other sites during 75% of sampling dates. Additionally, adult and larval densities were significantly correlated to plant nitrogen content. During 2015, trends were less distinct and weevil densities and nitrogen content varied based on the interaction between sampling date and site, but a significant correlation was not detected. Results confirmed published reports of the importance of plant nitrogen content to salvinia weevil productivity. Additional studies are warranted to evaluate and understand the role of nitrogen at giant salvinia biocontrol field sites.