Design of formulations for improved biological control agent viability and sequestration during storage

Abstract

Water-in-oil (W/O) emulsions were designed for improved storage of the biolarvacide Lagenidium giganteum (Oömycetes: Lagenidiales). Hydrophobicly treated silica nanoparticles were investigated for stabilizing cells. Increasing the concentration of silica significantly reduced cell death rate, particularly in formulations with a high initial cell density. Experiments were also done with NaCl and L. giganteum formulated in the same emulsion to examine the ability of the oil to sequester cells during storage. Such sequestration would allow combined formulation, storage, and delivery of incompatible ingredients. Forming a mixed emulsion in which a subpopulation of aqueousphase droplets contained NaCl and the other contained cells was imperative for minimizing solute-associated toxicity. When L. giganteum was formulated in an emulsion simultaneously with a NaCl solution, cell viability and mosquito infection efficacy were significantly lower compared to the same ingredients prepared as a mixed emulsion, suggesting NaCl was toxic to cells and the oil acted as a barrier to solute transport between aqueous droplets. The death rate for cells stored in a mixed emulsion with NaCl added in a separate subpopulation of droplets was statistically equivalent to storage in a mixed emulsion without NaCl during the first 30 days of storage, indicating the oil continued to sequester the cell-associated droplets from droplets containing NaCl. After 30 days of storage, death rate of cells in mixed emulsions with added NaCl exceeded death rate of cells in mixed emulsions without NaCl. Particle size distribution measurements indicated the accelerated death was likely a result of droplets containing NaCl coalescing with cell-associated droplets. Examining properties of the oil-phase components such as surfactant and nanoparticle chemistry and concentration may lead to better emulsion design to reduce the rate of droplet coalescence and improve sequestration of cell-associated droplets in the emulsion.