Water-in-oil emulsions that improve the storage and delivery of the biolarvacide Lagenidium giganteum

Abstract

Lagenidium giganteum is an effective biological control agent for mosquitoes with limited use due to poor survival and contamination during storage. Invert (water-in-oil) emulsions using crop oils were investigated for formulating L. giganteum mycelium for improved shelf life and delivery. Cells formulated in a water-in-oil (W/O) emulsion were just as effective against larvae as those formulated in aqueous suspension. Cells formulated in the W/O emulsion and cell suspension settled during storage and formed clumps, which significantly reduced the efficacy of formulations. Hydrophobic silica nanoparticles were added to the W/O emulsion formulation for oil thickening. The addition of silica significantly reduced cell sedimentation and improved storage; thickened W/O emulsions with an initial cell density of 3900 CFU/mg applied at 0.5 mg/cm2 were greater than 95% effective at infecting mosquitoes after 12 weeks of storage at room temperature. Cell density reduction during storage was represented using first-order kinetics. Surface treatment of silica nanoparticles and oil refinement both had a significant effect on the first-order rate constant; as the hydrophobicity of the silica increased and level of oil refinement decreased, the rate constant increased. The percentage of water in the W/O emulsion and type of refined crop oil had no significant effect on the first-order rate constant. Cells formulated in the thickened W/O emulsion were less likely to settle when applied to water compared to cells in aqueous suspension, suggesting better cell distribution in an aqueous environment could be achieved when cells are applied in a W/O emulsion.