Living on the edge: Population dynamics of Cyrtobagous salviniae (Coleoptera: Curculionidae) in temperate Louisiana, USA

 
Fluctuations on winter temperatures result on drastic changes on the survival of biological control agents of tropical weeds. While the salvinia weevil, Cyrtobagous salviniae, provides successful control of giant salvinia in subtropical climates, its impact in temperate regions has been negligible. Our objectives were to document the phenology of weevil at northern edge of the salvinia invasion, and compare the compare overwintering densities and reproductive conditions of weevils between north and central Louisiana. Field surveys at Cross Lake in north Louisiana revealed that weevil densities went from 145 adults per kilo in December 2017 to 0 in March 2018 following severe cold fronts. Due to this drastic reduction, lake managers had to restock weevils in several locations. In 2020, we collected weekly salvinia samples in Cross Lake and St. Gabriel in central Louisiana, and recorded adults and larvae, and reproductive conditions of females. Results revealed that St. Gabriel had a mean of 20 weevils per kilogram of salvinia while Cross Lake had only 5. In addition, the occurrence of larvae in the spring was delayed by 50 days in Cross Lake versus St. Gabriel. Dissections revealed that the proportion of females carrying immature eggs was similar in both regions. Therefore, the poor performance of salvinia weevils in temperate Louisiana is not only due to direct to winter freezing temperatures, but also due to shorter growing season, lack of oviposition of overwintering adults, poor larvae survival in the spring, which overall leads to a low number of F1 adults.