Winter and spring conditions determine the production of the salvinia weevil mass rearing programme

Invasion from giant salvinia (Salvinia molesta Mitchell, Salviniaceae) blocks sunlight entering waterbodies, reducing submerged aquatic macrophytes, thus lowering dissolved oxygen and stressing aquatic organisms. Due to its rapid growth and ease of dispersal through fragmentation, giant salvinia can quickly become problematic and difficult to eradicate. Since invasion of giant salvinia is persistent, land managers need a cost-effective method of control. Biological control by the salvinia weevil (Cyrtobagous salviniae Calder and Sands, Coleoptera: Curculionidae) has proven successful in controlling giant salvinia, and at a fraction of the cost of chemical and mechanical control. To maintain a source of salvinia weevils for field releases, mass rearing in outdoor ponds by natural resource managers is common practice in tropical regions. Using number of freezing hours and degree-day calculator, we examined how winter and spring conditions impacted the production of weevils in mass rearing ponds across southern Louisiana, USA. We found that winter temperature determined adult weevil mortality while spring temperature dictated reproduction, larval appearance, and timing of pond harvest. During winters with <55 h of freezing temperatures, ponds maintained densities of at least 13 adult salvinia weevils kg⁻¹. Winter with >200 hours of freezing temperature reduced adult density in the ponds nearly extirpating the population. Larvae were found on samples on average at 89 degree-days which began accumulating in January 1st. This study suggested that winter and spring conditions are critical for mass rearing operations and population growth of C. salviniae.